Inventors at Work, with Chapters on Discovery

Modern manufacture in its designs gives us a kindred persistence of old forms in new things. For electric illumination we have bulbs which recall the shape of a candle-blaze, or surmount an old-fashioned candlestick; a gas-burner, popular for fifty years, repeats in milky porcelain the whole length of a candle. Gas-grates, in uncounted thousands throughout our cities every winter, offer us flames which flicker and leap over asbestos and clay molded into the semblance of maple or charcoal. Nor is the engineer himself, for all his sternness of discipline, quite free from prolonging the reign of the past, even at unwarrantable cost. When steel was first used for steam boilers there was a period of hesitation during which the metal was used unduly thick, as if to maintain the long familiar massiveness of iron structures. When automobiles were invented, they at first closely resembled common carriages. To-day, designers have departed from tradition, and provide us with horseless vehicles which respond to their new needs in ways wholly untrammeled by inherited ideas. In an automobile, driven by steam or gasoline, there must be due disposition of fuel, of machinery, of cooling apparatus, all so combined as to bring the center of gravity as low as may be best, affording ready access to any part needing lubrication, repair, or renewal; throughout there must be the minimum of dead weight, of friction, and of liability to derangement; all with means of easy, quick, and certain control. Why should these requirements be deferred to repeating the model of a carriage drawn by a horse? In Europe, to this hour, the railroad carriages are an imitation of the old road-coaches, horse carriages slightly modified. America, fortunately, from the first has had cars directly adapted to railroad exigencies, with a thoroughfare extending the whole length of a train, avoiding the box-like compartments which may give the lunatic or the murderer an opportunity to work his will.

Sometimes an inherited form taken to a new home proves to be faulty there, and is discarded. When Normandy sent forth its children to Canada, they built on the shores of the St. Lawrence just such high-pitched roofs as had sheltered them in Caen and Rouen. An example remains at Montreal in the roof of Notre Dame de Bonsecours. But in Montreal and Quebec the snowfall is much heavier than in Northern France, and the Norman roofs at intervals from December to March were wont to let loose their avalanches with an effect at times deadly. To-day, therefore, in French Canada many of the roofs, especially in towns and cities, are flat or nearly flat, while the best models quite reverse the old design. In breadths somewhat concave they catch the snow as in a basin, and allow it to melt slowly so as to run down a pipe through the center of the building.

Under our eyes, day by day, iron and steel are taking the place of stone and wood in architecture and engineering; yet the force of habit leads us to continue in metal many troublesome details which were imperative in the weak building materials of generations past. It was as recently as the autumn of 1903 that the first large American theater was opened having no columns to obstruct views of its stage. The architects of the New Amsterdam Theater, New York, simply by availing themselves of the strength of steel cantilevers have shown that henceforth all large auditoriums may be free from obstructions to a view of the stage, pulpit or platform. See facing page 118.

Modern architecture, in the judgment of an eminent critic, has not yet fully responded to its new materials and methods. Says Mr. Russell Sturgis, of New York, in “How to Judge Architecture”:—“Every important change in building, in the past, has been accomplished by a change in the method of design, so that even in the times of avowed revival there was seen no attempt to stick to the old way of designing while the new method of construction was adopted; now in the nineteenth century, and in what we have seen of the twentieth century, our great new systems of building have flourished and developed themselves without effect as yet upon our methods of design. We still put a simulacrum of a stone wall with stone window casings and pediments and cornices and great springing arches outside of thin, light, scientifically combined, carefully calculated metal—the appearance of a solid tower supported by a reality of slender props and bars.”

CHAPTER X
SIZE

Heavenly bodies large and small . . . The earth as sculptured a little at a time . . . The farmer as a divider . . . Dust and its dangers . . . Models may mislead . . . Big structures economical . . . Smallness of atoms . . . Advantages thereof . . . A comet may be more repelled by the sun’s light than attracted by his mass.

Buildings, carriages, structures of all kinds, whether reared by art or nature, often resemble one another in form while varying much in size. Differences of dimensions are of importance to the inventor and discoverer, and will be here briefly considered, beginning with a few of their obvious and elementary aspects.

Cinders Big and Little.

One frosty evening I sat with three young pupils in a room warmed by a grate-fire. Shaking out some small live coals, I bade the boys observe which of them turned black soonest. They were quick to see that the smallest did, but they were unable to tell why, until I broke a large glowing coal into a score of fragments, which almost at once turned black. Then one of them cried, “Why, smashing that coal gave it more surface!” This young scholar was studying the elements of astronomy that year, so I had him give us some account of how the planets differ from one another in size, how the moon compares with the earth in volume, and how vastly larger than any of its worlds is the sun. Explaining to him the fiery origin of the solar system, I shall not soon forget his delight—in which the others presently shared—when it burst upon him that because the moon is much smaller than the earth it must be much cooler; that indeed, it is like a small cinder compared with a large one. It was easy to advance from this to understanding why Jupiter, with eleven times the diameter of the earth, still glows faintly in the sky by its own light, and then to comprehending that the sun pours out its wealth of heat and light because the immensity of its bulk means a comparatively small surface to radiate from.

To make the law concerned in these examples definite and clear, I took eight blocks, each an inch cube, and had the boys tell me how much surface each had—six square inches. Building the eight blocks into one cube, they then counted the square inches of its surface—twenty-four: four times as many as those of each separate cube. With twenty-seven blocks built into a cube, that structure was found to have a surface of fifty-four square inches—nine times that of each component block. As the blocks underwent the building process, a portion of their surfaces came into contact, and thus hidden could not count in the outer surfaces of the large cubes. The outer surfaces of these large cubes I then painted white; when each was separated into its eight or twenty-seven blocks, we saw in unpainted wood how surfaces were increased by this separation into the original small cubes. Observation and comparison brought the boys to the rule involved in these simple experiments. They wrote: Solids of the same form vary in surface as the square, and in contents as the cube, of their like dimensions.

This elementary law I traced that year in a variety of illustrations presented in “A Class in Geometry,” published by A. S. Barnes & Co., New York. Our excursions, since extended, are here given as an example of the knitting value of a pervasive rule kept constantly in mind.

Earth Sculpture.

Our planet in diverse ways illustrates the law, just stated, of surfaces and volumes. Forces of unresting activity quietly transform the hills and plains, the sea coasts and lake shores of the world, and so gradually that in many cases detection proceeds only by noting the changes wrought in a century. For the most part these forces break up large masses into fragments, or slowly wear away the surfaces of rocks into dust. A lichen takes root on a granite ledge, and in a few years reduces the rock to powder. Rain always contains a little acid, so that in time flint itself is consumed, for all its hardness. Water soaking through soils to form underground streams has hollowed out vast caves, as notably in Virginia and Kentucky. Limestones and sandstones are of open texture, and take up much moisture into their pores; in cold weather this freezes, and in expansion wedges off thin flakes of stone. In the North one sees the ground strewn with such splinters when the warm April sun has melted the snow from beside a limestone fence. Watch the rills as they descend a hillside during a rainstorm and just afterward. They are dark with mud, and on steep declivities they carry down pebbles and bits of broken stone, building up valleys at the expense of high ground. Fed on a huge scale by such mud, the Mississippi River bears in suspension to the Gulf of Mexico a little more than a pound of solid matter in every cubic yard, a prime example of how the waters of the globe gain upon the land. The Falls of Niagara have retreated several miles from their original plunge; the carving of their channel has been wrought much less by the rushing waters than by their burden of abrading earth and sand. The ceaseless churning of water at the foot of the Falls cuts back into the rock, undermining its upper layers, so that ever and anon they break off from the brink of the cataract, with the effect that the stream steadily retires.

Throughout the ocean are strong currents to be constantly surveyed and charted on the mariner’s behalf. These currents transport fine mud, and organisms living and dead. Corals flourish best where such currents fetch an abundant supply of food, just as plants thrive best in rich, loose soil. Life in the sea just like life on land is thus dependent on forces which divide large masses into small, and distribute these small masses over wide areas, chiefly by water carriage.

Breaking Earth for Removal or Tilth.

Inventors have taken a hint from nature as she carries a burden of mud and pebbles in a rapid stream of water. A modern method of deepening a water course is to reduce to fine silt the surface of its bed, and then remove this silt with a powerful stream. Water in swift eddies both lifts and bears away not only clay, but stone and gravel when these are small enough. In placer-mining streams of water much more powerful are directed against hill-slopes of earth and stone, which disappear a great deal faster than by means of spades and shovels. One of our Northwestern railroads runs for some miles along the base of a steep ridge, from which at times heavy rains wash down masses of earth, sand and gravel to the track. A powerful steam pump forcing a stream through hose removes the obstructions from the line with amazing rapidity. Work a good deal commoner and vastly more important consists in taking a process begun by nature and carrying it many steps further, so as to break up masses of earth again and again. The plow, the harrow, the sharp-toothed cultivator, divide and subdivide the soil of farm and garden so as to offer rootlets new surfaces at which rain may be drunk in with its nourishing food. When a garden patch is to be fertilized by bones, these serve best when reduced to meal, so as to be quickly and widely absorbed.

Work of the Winds.

In earth-sculpture one of the busiest agents is the wind, especially as it seizes ocean waves and dashes them upon beach and cliff, grinding large stones to pieces, and reducing these at last to mere pebbles and sand. On land the gales take hold of sand and dust with effects even more telling: sand flung against the hardest quartz or granite will bring it to powder at last. Sand dunes, shifting under the stress of high winds, have spread desolation around Provincetown, Massachusetts, and in many another region once fertile enough. This process of nature immemorially old has been copied in modern invention, by the sandblast devised by the late General Tilghman of Philadelphia. In its simplest form, sand from a hopper falls in a narrow stream upon window panes, glassware and the like, to be roughened except where protected by a paper pattern. Had sandstone in lumps, as large as playing marbles, been dropped on the glass, there would have been harmful fracture; as each particle of sand weighs too little in proportion to its striking surface to do more than detach a tiny chip, we have a bombardment wholly useful.

Dimensions in Ignition.

Primitive man achieved an incomparable triumph when first he kindled fire by swiftly twirling one dry stick upon another, dropping the tiny sparks on finely divided tinder, quick to catch fire because it presented much surface to the air. Peat, a fuel common in many parts of the world, easily dug from bogs and marshes, can be readily dried if chopped into fragments and exposed to the wind in open sheds. Charcoal easily produced from wood of any kind, is often used to absorb harmful gases in boxes of preserved meats and in household refrigerators. Its effectiveness is due to its minute pores, presenting as they do a vast area of capillary attraction. Charcoal, of course, burns faster when powdered than when unbroken; and gunpowder, into which charcoal largely enters, is molded into cakes either big, if it is to burn somewhat slowly, or is pressed into fine grains, when an explosion all but instantaneous is desired.

Dust Common and Uncommon.

Common dust surrounds us always, entering the tiniest chink of wall and ceiling to show its path by a defacing mark. In dry seasons it abounds to a distressing degree, and accumulates rapidly at considerable heights from the ground. Observe a roof of the kind that slopes gradually toward the street, with a trough running along the cornice to carry off the rain or melted snow. When such a gutter is undisturbed for a few months it is clogged with mud due to the dust which has been lifted by winds to the roof, and swept by successive showers into the gutter. Dust particles, because they have so much surface for their mass, are readily caught up and borne to heights far exceeding those of the highest roofs. The terrific explosion of the volcano at Krakatoa, in the Sunda Strait of Java in 1883, shot more than four cubic miles of dust into the upper levels of the atmosphere, encircling the globe with particles which fell so slowly as for months to color the sunsets of New York and Canada, ten thousand miles away.

Inflammable Dust.

Wheat like other grain is combustible, hence as food it sustains bodily warmth. Under stress of necessity wheat, corn, and barley have been burned as fuel when coal and wood have been lacking. In the process of flour-making wheat is ground to a powder so fine that when its particles are diffused through the air of a mill, there is a liability to explosion because the inflammable dust comes so near to contact with the atmospheric oxygen that at any moment they may unite. At Minneapolis, frightful disasters were brought about in this way until specially devised machines removed the dust. In coal mines, too, coal may fill the air with a dust so fine that explosions take place, with serious loss of life. In Austria it has been found that the fineness of the dust has more to do with the violence of such explosions than has the chemical composition of the particles.

In mining, let us observe, the whole round of work consists in separations which bring masses from bigness to smallness, again and again. First of all the solid walls and floors are broken up by pick, or drill, or powder, or all together. Iron ores as hoisted to the surface of the earth are taken to breakers which crush them into pieces suitable for the blast furnace. When the ores carry gold, copper, lead, or tin, this crushing is followed by stamping to facilitate the final process by which metal is separated from worthless rock.

Dimensions in Woven Fabrics.

Spinning and weaving, remote as they are from mining, are equally subject to the law of surfaces and volumes. It is in furthering adhesion by giving their thread a multiplied surface that the spinner and weaver manufacture cloth at once strong and durable. The best linens and silks are spun in exceedingly fine threads; canvases and tweeds have threads comparatively coarse. From the cut edge of a piece of fine silk fabric it is hard to pull out a lengthwise thread; the task is easy with sailcloth.

The Dimensions of Models.

From observation let us turn to experiment as we further consider the law of size. Inventors, especially young inventors, are apt to underrate the difficulty of supplying an old want in a new and successful way. In their enthusiasm they may lose sight of principles which oppose their designs, as for instance, the rules which govern the plain facts of dimensions. Mr. James B. Eads, in planning his great bridge at St. Louis, chose three spans instead of one span. Why? For the simple reason that if built in one span the weight of the bridge would have been twenty-seven times that of a span one-third as long, while only nine times as strong, assuming that both structures had the same form. Two pieces of rubber will clearly exhibit the contrast in question. One piece is three feet long, one inch wide, one inch thick; the other piece is one foot long, and measures in width and thickness one-third of an inch. Placing each on supports at its ends we see how much more the longer strip sags than the shorter. The longer has twenty-seven times the mass of the other, but only nine times its strength. Many an inventor has ignored this elementary fact and built a model of a bridge, or roof, which has seemed excellent in the dimensions of a model, only to prove weak and worthless when executed in full working size.

Why Big Ships are Best.

We have glanced at a few cases of invention where it has been remembered that the larger a mass of given shape the less its surface as compared with its bulk. Let us note how this rule enters into the tasks of the shipbuilder. We take a narrow vial of clear glass, nearly fill it with white oil or glycerine, cork it, and shake it smartly. Holding the vial upright we observe that the largest bubbles of imprisoned air come first to the top of the liquid, because in comparison with bulk they have least surface to be resisted as they rise. For a parallel case we visit the docks of New York, and note a wide diversity of steamers. Here is the “Baltic,” of the White Star Line, with a length of 726 feet, and a displacement of 28,000 tons. Less than a mile away is a small steamer trading to Nova Scotia, having a length of but 260 feet, and a displacement of only 1,000 tons or so. We recognize at once why the quickest ships are always among the biggest. It is simply the case of bubbles small and great over again; the biggest vessels in proportion to size have least surface whereat to resist air and sea, so that they can run fastest between port and port. As with ships, so with their engines; economy rests with bigness; the largest engines have proportionately least surface at which to lose heat by radiation or by contact, or for resistance by friction as they move. Indeed in designing ocean steamers of the greyhound type it is imperative that the utmost possible dimensions be adopted. The “Mauretania” and the “Lusitania” just built for the Cunard Company, to be driven by steam turbines at 25 knots an hour, will each demand 70,000 horse-power. They are 790 feet in length over all, 88 feet in beam, 60¹⁄₂ feet in depth, with a displacement of 45,000 tons. Mr. William F. Durand, in his work on the resistance and propulsion of ships, considers three vessels less huge and swift than these Cunarders and able to cross the Atlantic in say seven days. The 5,000-ton ship could barely make the trip with no cargo at all, a 16,000-ton ship would be able to carry 3,000 tons of freight, while a 20,000-ton ship could carry 4,200 tons of cargo. Burdens of hull, machinery, and coal do not increase as rapidly as gross tonnage when the dimensions of a ship are enlarged.

Bigness Needs Strong Materials.

Now we begin to realize how great is the boon of cheap steel, much stronger than iron, of which ships and engines may be built bigger than at any earlier period. Steel of great strength has made feasible, too, the Eiffel Tower in Paris, nearly a thousand feet tall, the office-buildings of New York thirty stories in height, and steel will soon cross the St. Lawrence near Quebec with a single span of 1,800 feet. In 1904, at Schenectady, N. Y., the New York Central & Hudson River Railroad Company began comparisons between an electric locomotive of 201,000 pounds, shown opposite page 476, and a steam locomotive so huge that with its tender it weighed no less than 342,000 pounds. Steel, as the material of engines and tools of all sorts enables us to build in dimensions bolder than ever before; or, if old dimensions are not surpassed, we are free to employ velocities quite out of the question with iron.

It is a long time since adventurers first entrusted themselves to floating logs, afterward tied together as rafts, and slowly improved until they became boats moved by paddles or oars. Thus far little else than failure has attended the inventors who have sought to navigate the air as easily as river, lake or sea. A stride toward success was however distinctly taken when the strongest known alloys, those of steel and nickel, gave the aeronaut a stronger boiler, pound for pound, than he ever had before, with wings lighter in proportion to their power than those of earlier experiments. Let the burden of his apparatus be further reduced, and by one-half; then we may expect him to reign in the air as securely as the sea-gull. The original resource of the aeronaut, his balloon, suffers from a permanent disability. Air has but ¹⁄₇₇₀ the specific gravity of water, so that a balloon must be enormous to have any carrying capacity worth while. And what would become of a balloon, its rudder and ropes, if caught in a hurricane of eighty miles an hour?

A Store Continues the Lesson.

Let the aeronaut continue his wistful and envious gaze at the birds in the sky while we turn our attention to mother earth, there to note how every day trade surrounds us with further illustrations of the law of size, of the gains which may attend bigness. We enter a department store, displaying a varied stock of foods, clothing, shoes, furniture, and so on. As we cast our eyes about its counters, shelves, and floor we see cans of vegetables, fruit, and fish; jars of olives and vinegar; boxes of rice, soap and crackers; paper sacks of flour and meal. Outside the door are piled kegs, barrels, and packing cases. Plainly the cost of paper, glass, tin, and lumber for packages must levy a large tax on retailing. Once more is recalled our old lesson with the inch-cubes; the bigger a jar, box, or sack, the less material it needs in proportion to its capacity. Wholesale packers of merchandise save money as they form packages of the largest size. The contents of each box, crate, and sack tell the familiar story once again. The coffee is ground from the bean that it may be readily infused in the coffee-pot; wheat is reduced to flour, oats to fine meal, that they may be quickly cooked; sugar is crushed that it may rapidly dissolve in the tea cup. This very task began long ago with the mastication of food by the teeth, diminishing the size of morsels while moistening them for digestion before they reached the stomach.

Summer Holiday Notes.

During a visit to the country one summer, we observed new examples of our familiar rule. When we compared the dimensions of a small sectional cabin with those of a large house, we saw the principal reason why the cabin was hard to keep cool in July, and hard to keep warm in December. We noticed tasks which depended upon giving wood, cloth or other material as much surface as possible, whether new forms were like old ones or not. A neighboring sawmill was busy cutting up logs into thin boards; these were piled in open tiers, so that the drying winds might speedily finish their work. In the same way we noted a laundress spreading out by itself each table-cloth and apron fully to catch the wind, instead of leaving the linen as a solid heap in her basket, where only the edges would be dried. When the farm-hands went haymaking they followed the same rule; they tedded out their gavels to give them the utmost supply of sun and air; when all was as dry as a bone they reared a haycock of compact form so as to expose the least possible surface to rain and snow.

Dimensions Molecular.

So much for things to be observed in a country ramble, in a city store, or at the docks of a busy port. Apart from all such things is a world unseen, standing beneath the visible world, and equally worthy of study. Here knowledge is based upon inferences, upon what lawyers call circumstantial evidence. The chemist by means purely indirect studies the molecule and the atom, objects that far elude his microscope. A molecule is a part of a compound so small that it cannot be divided without becoming something simpler. Thus a sugar molecule is made up of carbon, hydrogen, and oxygen atoms; were these disjoined, the sugar, as such, would cease to be, just as a brick wall no longer exists when its bricks and their several slices of mortar are parted from one another as separate units. Small as molecules are they have not escaped the measuring rod of the physicist. Some years ago Lord Kelvin experimentally arrived at the estimate that the average molecule has a diameter of ¹⁄_760,000,000 inch. Such molecules when compared with masses of like form, and of a diameter of one inch; have 760,000,000 times as much surface. In the transmission of motion, with adhesion in play, surfaces count for much, as when a wheel in motion is brought into contact with a wheel at rest. Here may be an explanation of why electricity is conducted through a wire with a velocity far exceeding any speed we can mechanically impress upon the metal, because the molecules concerned have incomparably more surface than the wire as a mass.

Reservoirs of Energy.

By virtue, also, of its minuteness the molecule as a reservoir of energy can far excel a mass of visible dimensions. Let us compare two rotating spheres, one of them of seven times the radius of the other. We spin both at the same peripheral rate, and gradually increase this speed: which will be the first to break apart under centrifugal strain? The larger, and why? Because the cohesion of a sphere is in proportion to the area of its great circle, which varies as the square of its diameter, while centrifugal strain under swift rotation varies as the cube of that diameter, or as the volume of the sphere. From this it follows that we may safely spin our small sphere with a circumferential velocity seven times that given the large sphere; therefore as containers of energy small spheres are more effective than large, and this inversely as their diameters. Spheres, or bodies of any other form, if reduced in dimensions to ¹⁄_760,000,000th, would as reservoirs of energy gain 760,000,000-fold. Thus we open a door of explanation regarding the stupendous contrast between chemical energy and mechanical work. Chemical processes are exerted by molecules and atoms, mechanical work takes place among masses comparatively enormous in bulk. It may require a hundred blows from a ponderous steam hammer to raise the temperature of an iron bar ten degrees; that bar melts in ten seconds when plunged into a flame produced by a few ounces of hydrogen and oxygen gases.

Recent experiments by Professor Joseph J. Thomson point to the probability that the atom of the chemist while a unit, is in part built of electrons each but one-thousandth part the size of a hydrogen atom. An electron, by virtue of its infinitesimal minuteness, becomes able to hold proportionately much more energy than is possible to an atom moving as a whole. This brings us to some comprehension of the astonishing powers of radium, an element which maintains itself at a temperature 3° to 5° Centigrade higher than that of its surroundings, probably through the collision within each atom of its component parts.

Repulsion by Sound and Light.

Water-waves as they strike a shore or the sides of a basin exert a thrust, or a repelling action, which may easily be observed. That sound-waves act in similar fashion is proved by a little sound-mill devised in 1883 by Professor V. Dvorak, of the University of Agram in Austria. It consists of four vanes, each a small card slightly curved, mounted on a spindle. In a sounding-box nearby is a tuning-fork which may be struck through its stem F. A Helmholtz resonator has its wide opening turned toward this box, its narrow opening toward the mill. A stroke on the tuning-fork emits vibrations which send tiny jets of air against the sails of the mill, which accordingly rotate at a pace proportionate to the loudness of the sound.

Professor Ernest F. Nichols of Columbia University, New York, and Professor Gordon F. Hull of Dartmouth College, in the Journal of Astrophysics, Chicago, June, 1903, describe their apparatus for measuring the radiation pressure of light, a phenomenon analogous to that studied by Professor Dvorak in the field of sound. In the same number of that Journal they detail an experiment to show light exerting a driving action on very tenuous particles. They burned a puff ball of lycoperdon to charcoal spherules of about one-sixth the specific gravity of water. These spherules, with some fine emery sand, they placed in a glass tube shaped like an hour-glass; this tube was then exhausted of its gases until a mere fraction remained which could not be removed. With the sand and charcoal in its upper half the tube was held upright, while a beam of light twenty to forty times as strong as sunshine was thrown on the tube just below its neck. By tapping the glass a stream of sand and charcoal descended; the sand fell through the beam without deflection; the charcoal particles were driven away from the stream as they fell through the light. Part of this effect was due to the slight remnant of gas left in the tube which, warmed by the light, produced a motion resembling that of a Crookes’ radiometer; the remainder of the effect was caused by the drive or repulsion of the luminous beam. It is argued that this repulsion by light is probably one of the causes why the sun seems to drive away the tail of a comet, whose particles being extremely minute have much surface and little bulk, so that they are more repelled by the light of the sun than they are attracted by his mass. To approach cometary conditions in an experiment it would be necessary to intensify sunlight no less than 1,600-fold, because on the surface of the earth its own gravitation is 1,600 times greater than that which is there exerted by the sun.

A Law as a Binding Thread.

The law that a given shape when enlarged increases much more rapidly in volume than in surface has, in our brief survey, bound together a wide diversity of facts in astronomy, geology, geography, navigation, engineering, mechanics, physics, and chemistry. A good many times I have brought it before young folks as a means of linking together everyday observations and principles of sweeping comprehensiveness. Boys and girls are apt to think that there is a formidable barrier between science and common knowledge. No such barrier exists. The sun, his planets and their moons; the forces which carve mountains and valleys; the arts of shipbuilders, of designers of bridges, office-buildings, and lighthouses; the plans of the inventors of machinery; the rules discovered by investigators who pass from appearances to the underlying reality of molecule and atom, are all within the sway of the elementary law we have been studying. There is a gain in thus pursuing a connecting thread of classification, conferring order as it does on what might else be an assemblage of things collected at random. A law such as that of size links into unity, and fastens in the memory a vast array of observations and experiments which otherwise would have no associating tie, no common illumination.

CHAPTER XI
PROPERTIES

Food nourishes . . . Weapons and tools are strong and lasting . . . Clothing adorns and protects . . . Shelter must be durable . . . Properties modified by art . . . High utility of the bamboo . . . Basketry finds much to use . . . Aluminium, how produced and utilized . . . Unwelcome qualities turned to profit . . . Properties long worthless are now gainful . . . Properties may be created at need.

Materials are valued for their properties as well as their forms. We now pass to a rapid survey of properties as observed in gifts of nature, as modified by art, as turned to account in many ingenious ways, as studied by the investigators who would fain know in what particulars of ultimate form, size and motion, properties may really consist.

We go to market with a few different coins: one of them is worth a hundred times as much as another of about the same size, because gold is more beautiful than nickel, does not tarnish, may be hammered into leaves of extreme thinness, or unites with copper as an alloy which withstands abrasion for years after it leaves the mint. When we build a house we wish strength in its foundation and walls, so we pay a higher price for granite than for limestone; and choose for joists, floors and rafters well seasoned wood in preference to newly sawn lumber liable to warp and crack with heat in summer, with cold in winter. So with raiment: silk is preferred to cotton or wool because handsomer, stronger, more lasting. But food comes before shelter, raiment or any other need of mankind, and qualities of nourishment and palatability mark off nuts, fruits, grain and roots as suitable for food. In this regard all living creatures exercise discrimination under penalty of death.

Food.

A score of sparrows are flitting about a door-yard; strew a handful of crumbs on the gravel before them; at once the birds begin picking up the bread, leaving the gravel alone. They know crumbs, good to eat, from stone, not good to eat. The earliest races of men, immeasurably higher than birds in the scale of life, have eaten every herb, root, grass, and fruit they could find. Experiment here was as wide as the world, and bold enough in all conscience. In many cases new and delicious foods, thoroughly wholesome, were discovered. At other times, as when the juice of the poppy was swallowed, sleep was induced, with a hint for the escape from pain in artificial slumber. In less happy cases the new food was poisonous; yet even this quality was pressed into service. In Mendocino County, California, to this day, the Indians throw soap root and turkey mullein, both deadly, into the streams; the fish thus killed are eaten without harm. These same Indians make acorns and buckeye horse chestnuts into porridge and bread, pounding the seeds into a fine flour and washing out its astringent part with water. These and other aborigines use for food and industry many plants neglected by the white man, taking at times guidance from the lower animals. One of the early explorers of South Africa, Le Vaillant, says that the Hottentots and Bushmen would eat nothing that the baboons had left alone. Following their example he would submit to a tame baboon new plants for acceptance or rejection as food.

Weapons and Tools.

As with food so with other resources almost as vital. Long ago the savage learned that hickory makes good bows and arrows, that as a club it forms a stout and lasting weapon. He discovered, too, that in these qualities soft woods are inferior and the sumach altogether wanting. Thus, too, with the whole round of stones from which as a warrior or a craftsman he fashioned knives, chisels, arrowheads, axes; it was important that only tough and durable kinds should be employed. No lump of dry clay ever yet served as a hammer or an adze; happy were the tribes, such as those of ancient Britain, who had at hand goodly beds of flint from which a few well directed blows could furnish forth a whole armory of tools and weapons.

Properties Modified.

In the eating of foods simply as found, in the use of materials for clothing or building just as proffered by the hand of nature, much was learned as to their qualities; some were found good, others indifferent, still others bad. Then followed the art of modifying these qualities, so as to bring, let us say, a fibre or a thong from stiffness to pliability and so make it useful instead of almost worthless. The progress of man from downright savagery may be fairly reckoned by his advances in the power to change the qualities of foods, raiment, materials for shelter, tools, and weapons. These arts of modification go back very far. At first they may have consisted simply in taking advantage of the effects of time. In the very childhood of mankind it must have been noticed that fruit harsh and sour became mellow with keeping, just as now we know that a Baldwin apple harvested in October will be all the better for cellarage until Christmas, the ripening process continuing long after the apple has left its bough. Grains and seeds when newly gathered are usually soft and, at times, somewhat damp; exposed to the sun and dry air for a few days they become hard and remain sound for months or even years of careful storage. In warm weather among many Indian tribes such food was almost the only kind that remained eatable; all else went to swift decay, except in parched districts such as those of Arizona, so that roots, fruits, the flesh of birds, beasts, and fish had to be consumed speedily, a fact that goes far to account for the gluttony of the red man. His stomach was at first his sole warehouse; that filled, any surplus viands went to waste. In frosty weather this havoc ceased; as long as cold lasted there was no loss in his larder. A few communities, as at Luray, Virginia, or at Mammoth Cave, Kentucky, in their huge caverns had storehouses which would preserve food all the months of the twelve. In New Mexico and other arid regions the air is so dry that meat does not fall into decay. How it was discovered that smoke had equal virtue we know not. Probably the fact came out in observing the accidental exposure of a haunch of venison as the reek from a camp-fire sank into its fibres. Salt, too, was early ascertained to have great value in preserving food. Suppose a side of buffalo, or horse, to have fallen accidentally into brine in a pool or kettle, and stayed there long enough for saturation, its keeping sweet afterward would give a hint seizable by an intelligent housewife. Preservation by burial in silos began in times far remote, and was fully described by Pliny in the first century of the Christian era.

Properties in Clothing.

The skin just taken from a sheep, the hide when removed from an ox, are both as flexible as in life. But they soon stiffen so as to be uncomfortable when worn as garments. Wetting the pelt is but a transient resource; satisfactory, because lasting, is the effect of rubbing grease, fat, or oil into the texture of the hide. Peary in Greenland found that pelts in small pieces, and bird-skins, were softened by the Eskimo women chewing them for hours together.

Wetting was as notable an aid to handicraft of old as today. Boughs, roots, withes, osiers, or the stems of fibrous plants, when thoroughly saturated with water became so soft as to be easily worked, yielding strands, as in the case of hemp, separated from worthless pulp. Hence the basketmaker, the wattler, the builder, the potter, the weaver of rude nets and traps, long ago learned to wet their materials to make them plastic. Take now the reverse process of drying, which toughens wood, and the sinews used as primitive thread. Leaves when dried become hard and brittle of texture, hence the necessity that when woven and interlaced as roofs the work shall promptly follow upon gathering the material. In plaiting coarse mats and sails may have begun the textile art which to-day gives us the linens of Belfast, the silks of Lyons and Milan.

Cotton Strengthened and Beautified.

A good and serviceable imitation of silk is due to a simple and ingenious treatment of cotton. In 1845 John Mercer, a Lancashire calico printer, one day filtered a solution of caustic soda through a piece of cotton cloth. He noticed that the cloth, as it dried, was strangely altered; it had shrunk considerably both in length and breadth, had become stronger, with an increased attraction for dyes. This was the beginning of the mercerization which to-day produces cotton fabrics almost as strong and handsome as if silk. The cloth, preferably woven of long Sea Island staple, is immersed in a solution of caustic soda, and afterward washed in dilute sulphuric acid and in pure water. As it enters the caustic bath the cotton is pure cellulose, as it leaves the bath the fabric is hydrated cellulose, with new and valuable properties. The structural change in the fibre is decided. The original filament of cotton is a flattened tube, the sides of which are close together, leaving a central cavity which is enlarged at each edge of the surrounding tube. It is opaque and the surface is not smooth. The fibre has also a slight twist. The tube after treatment becomes rounded into cylindrical form; its cavity is lessened and the walls of its tube thicken; the surface becomes smooth and each fibre assumes a spiral form. Effects like these of mercerization are produced in paper as well as in cotton cloth, yielding vegetable parchment, a familiar covering for preserve jars and the like.

Properties in Building Materials.

Some sandstones, such as are common in Ohio and Indiana, soft when hewn in the quarry, soon harden on exposure to wind and weather; materials of this kind in early times afforded shelter more lasting than tents of boughs or hides. But the building art was to know a gift vastly more important when an artificial mud was blended of clay and water, with a steady improvement both in the strength and durability of the product. It was a golden day in the history of man when first a clayey paste was patted into a pot, a bowl, a kettle: then was laid the foundation of all that the potter, the brick maker, the tile molder have since accomplished. Another remarkable discovery, needing prolonged and faithful experiment, was reached when pottery was found to keep its form better when broken potsherds and bits of flint were mingled with its clay. A discovery of equal moment was that of mortar, probably approached in the daubing of mud or clay into chinks of stones, with the admixture first of one substance and then another until the right one was found, and the binder and the bound became of one and the same hardness. The Romans, a deliberate race, took two years in making a batch of mortar; that bond to-day protrudes from their walls as more resistant to the tooth of time than stone itself.

Flame and Electricity as Modifiers.

But if water did much to modify properties, flame did infinitely more. A block of blue limestone thrust into a fire was burned to whiteness, and became lime, which, mixed with water, proved a biting compound of slippery feel,—an alkali indeed. This same wonderful flame caused water wholly to disappear from a heated kettle; or could dissipate almost the whole of an ignited brand or lump of fat. By cooking a food, it gave a new relish to the poorest dish, banished from such a root as tapioca its poison, and when a yam was baked it remained eatable for a twelvemonth. Fire enabled man to melt metals as if they were wax, to soften iron or copper which a deftly swung hammer shaped as he willed. Here, too, opened the whole world of chemistry, one of its first gifts the power to take an ore worthless when unchanged, and gain from it a battle-axe, a knife, an arrowhead. Even in this day of electricity it is fire which the engineer must evoke to create acids, alkalis, sugars, alcohols, from substances as different from these as iron is from iron ore.

Electricity as a modifier of properties in turn throws flame into eclipse. Take an example: a strip of ferro-nickel is fast dissolving in an alkaline bath; attach one end of the metal to the negative pole of a battery or a dynamo, the other end to the positive pole; at once solution ceases and the metal begins to pick out kindred particles from the bath, adding them to itself. Electricity has completely reversed the wasting process; what was eaten away is now growing, what was a compound is now shaken into its elements, one of which rapidly increases in mass. Nothing in the empire of heat is as striking as this process—familiar in renewing the energy of a storage battery. Many a union or a parting impossible to fire is wrought instantly by the electric wave.

The Bamboo Rich in Utilities.

When Mr. Edison devised his electric lamp, his first successful filaments were fibres of bamboo; they glowed more brilliantly than anything else he could find, they were tenacious enough to withstand intense heat for weeks together. A single gift of nature, such as the bamboo, may be so many-sided that its applications greatly enrich human life. A task of interest would be to trace the vast indebtedness of modern science and art to carbon, iron, or silver, in their various forms. But the bamboo is cheaper and more abundant than any of these, so that it will be worth while to glance at the many wants it has satisfied, at the creations it has suggested to ingenuity. In Ceylon, India, China, Japan, the Malay archipelago, it is the chief item of natural wealth, the main resource for the principal arts of life. First of all it provides food. More than one case is recorded where its abundant seeds have staved off the horrors of famine; these seeds, too, are commonly fermented to produce a drink resembling beer. Many species of bamboo have shoots which when young and tender are a palatable and nourishing food. As a building material it is strong, durable and easily divided. Its sizes are various enough to provide a fishing-rod for a boy, or a column for a palace.

“To the Chinaman, as to the Japanese,” says Mr. Freeman-Mitford, in “The Bamboo Garden,” “the bamboo is of supreme value; indeed it may be said that there is not a necessity, a luxury, or a pleasure of his daily life to which it does not minister. It furnishes the framework of his house and thatches the roof over his head, while it supplies paper for his windows, awnings for his sheds, and blinds for his verandah. His beds, tables, chairs, cupboards, his thousand and one small articles of furniture are made of it. Shavings and shreds of bamboo stuff his pillows and mattresses. The retail dealer’s measure, the carpenter’s rule, the farmer’s waterwheel and irrigating pipes, cages for birds, crickets, and other pets, vessels of all kinds, from the richly lacquered flower-stands of the well-to-do gentleman down to the humblest utensils of the very poor, all come from the same source. The boatman’s raft, and the pole with which he punts it along; his ropes, his mat sails, and the ribs to which they are fastened; the palanquin in which the stately mandarin is borne to his office, the bride to her wedding, the coffin to the grave; the cruel instruments of the executioner, the beauty’s fan and parasol, the soldier’s spear, quiver, and arrows, the scribe’s pen, the student’s book, the artist’s brush and the favorite study for his sketch; the musician’s flute, the mouth-organ, plectrum, and a dozen various instruments of strange shapes and still stranger sounds—in the making of all these the bamboo is a first necessity. Plaiting and wickerwork of all kinds, from the coarsest baskets and matting down to the delicate filigree which encases porcelain, are all of bamboo fibre. The same material made into great hats like inverted baskets protects the coolie from the sun, while the laborers in the rice fields go about looking like animated haycocks in waterproof coats made of the dried leaves of the bamboo sewn together.”

Materials for Basketry.

In North America the Indians have had no such resource as the bamboo, but with tireless sagacity they have laid under contribution either for food or for the arts every gift of the soil. In seeking materials for basketry, for example, they have surveyed the length and breadth of the continent, testing in every plant the qualities of root, stem, bark, leaf, fruit, seed and gum, so far as these promised the fibres or the dyes for a basket, a wallet, a carrier. With all the instinct of scientific research they have sought materials strong, pliant, lasting and easily divided lengthwise for refined fabrics. In his work on “Indian Basketry” Mr. Otis T. Mason has a picture of a bam-shi-bu coiled basket, having a foundation of three shoots of Hind’s willow, sewn in the lighter portions with carefully prepared roots of kahum, a sedge; while its ornamental designs are executed in roots of a bulrush, the tsuwish. Often a basket, as in this case, is built of materials found miles apart, each requiring patient and skilful treatment at the artist’s hands.

A few trees, the cedar in particular, lend themselves to the needs of the basketmaker with a generous array of resources. Mats of large size made from its inner bark are common among the Indians of the Northern Pacific Coast. From the roots of the same tree hats are woven as well as vessels so close in texture as to be watertight. When the roots are boiled so as to be readily torn into fibres, these are formed into thread, either woven with whale-sinews or with kelp-thread as warp. Among the handsomest of all Indian baskets are those of the Pomo tribe, one of which is shown on page 109. The splints for their creamy groundwork are made from the rootstock of the Carex barbarae, which are dug from the earth with clam shells and sticks, a woman securing fifteen to twenty strands in a day. These she places in water over night to keep them flexible, and to soften the scaly bark which is afterward removed. To make a basket watertight the Indians of Oregon weave the inner bark of their maple with the utmost closeness. In other regions a simpler method is to apply as water-proofing the gum of the piñon, the resins of pines, or mineral asphalt. Equal diligence and sagacity mark the Indians as users of stone. The Shastas heat a stone of such quality that in cooling it splits into flakes for weapons and tools. They place an obsidian pebble on an anvil, and with an agate chisel divide it as they wish; all three being chosen from a vast diversity of stones which must have been tried and found, inferior.

Aluminium and Its Uses.

From Indian handicrafts, developed by aboriginal skill, patience and good taste to remarkable triumphs, let us turn to an achievement of a modern chemist who, calling electricity to his aid, bestowed a new metal upon industry, making possible new economies in a wide sisterhood of arts. Aluminium was discovered in 1828 by Wohler, a German chemist, who noted its lightness, toughness, and ductility. At the Centennial Exhibition at Philadelphia, in 1876, a surveyor’s transit built of aluminium was shown, but the metal at that time was six-fold the price of silver, so that the instrument for some years remained uncopied. Of course, engineers and mechanics were much interested in a metal only about one-third as heavy as brass or copper, of white lustre, and with as much as five-eighths the electrical conductivity of copper. All that hindered the extensive use of the metal was its high cost. If that cost could be lowered, at once copper, and even silver, would face a rival. After many unsuccessful because expensive processes for obtaining the metal had been devised, a method was found at once simple and inexpensive.

This method of separating aluminium from its compounds was devised by Charles M. Hall, while an undergraduate student at Oberlin College, Ohio. His success turned on his knowledge of the properties of related metallic compounds. He recognized the probable value of aluminium in the arts, could it be produced in large quantity at low cost. He believed that electrolysis would prove the most convenient, thorough and inexpensive method; but there was at that time no process known by which it could be applied to this element. His problem was to find a form of electrolyte rich in aluminium which should be comparatively easy to separate into its elements, and to discover a substance for the solvent which should prove a satisfactory bath. This latter substance must, furthermore, be a good conductor of electricity, must readily dissolve the proposed electrolyte, and must have a higher resistance to electrolytic disruption than the electrolyte. To discover the needed substances for electrolyte and solvent involved the examination of all available compounds of aluminium, the study of the various possible solvents for the compound selected, and the determination of electric conductivities. By virtue of rare familiarity with the chemistry and physics of the subject, with the properties of every substance concerned, the search was, after a time, rewarded with complete success. It was found that bauxite—the oxide of aluminium, alumina, in fact—is dissolved by molten cryolite, the double silicate of aluminium and sodium, and that the latter, while dissolving the bauxite freely and serving as an ideal solvent, also itself breaks up under the action of the electric current at a much higher voltage than alumina. So far as known, these are the only substances in nature which stand to each other in such relation as to permit the commercial production of the metal.

Aluminium as constructive material has disappointed some of its earlier advocates. It is difficult to work, gumming the teeth of files and resisting cutting and drilling tools by virtue of the very toughness which makes it desirable for tubes, columns, and the like. Its excellences, however, are manifold: the German army on investigation found that helmets of aluminium, as light as felt, turned the glancing impact of a bullet. For soldiers’ use it now forms not only helmets, but cooking vessels, cartridge cases, buttons, sword and bayonet scabbards. It gives the photographer as well as the surveyor instruments which unite strength with lightness. It has furthermore the quality which has long given value to the lithographic stone of Hohenlofen in Bavaria. Aluminium takes a sketch as perfectly as does the stone, with the inestimable advantages that the metal may be readily curved for a cylinder press, that it is compact and light in storage, while without the brittleness which has made stone so costly a servant to both artists and printers. To produce a deep color from stone it may be necessary to print one impression over another again and again; from aluminium a single impression is enough, as severe pressure may be safely applied.

Aluminium has so great an affinity for oxygen as to play a conspicuous part in the metallurgy of other metals. In the casting of iron, steel or brass, the addition to each ton of two to five pounds of aluminium greatly improves the product; the aluminium by combining with the occluded gases reduces the blowholes and renders the molten metal more fluid and therefore more homogeneous. A second use for aluminium turns on the same quality; it was devised by Dr. Goldschmidt for producing high temperatures, and is especially useful in welding steel rails and pipes. A mixture of iron oxide and aluminium finely divided is ignited by a magnesium ribbon; a very high temperature results as the aluminium combines with the oxygen derived from the iron oxide.

Aluminium by reason of its lightness occupies a large field in naval and military equipments, in motor-car construction, and the like, where the reduction of weight is of paramount importance. For cooking utensils the use of aluminium is constantly extending; the metal is a capital conductor of heat, is not liable to deteriorate in use, and gives rise, if dissolved, to harmless compounds. The chief objection to aluminium is its low tensile strength, which, for the cast metal is only 10,000 to 16,000 pounds per square inch. An improvement is effected by adding as an alloy a small quantity of some other metal, such as nickel or copper. When one part of aluminium is joined with nine parts of copper we have aluminium bronze, the strongest and handsomest of copper alloys, much resembling gold in its lustre.

Aluminium is finding acceptance as an electrical conductor. An installation of this kind in Canada unites Shawinigan Falls with Montreal, 84.3 miles distant. Three cables are employed, each composed of seven No. 7 wires. The total loss in the transmission of 8,000-horse power, at 50,000 volts at the generating station, is about eighteen per cent. Comparing equal conductors, in round numbers the cross-section of an aluminium cable is one-and-a-half times that of a copper cable, the weight being one-half and the tensile strength three-quarters. Everything considered when aluminium is 2¹⁄₁₀ the price of copper, the investor is equally served by both metals as conductors. This is true only where the conductors are bare. Where insulated cables are needed, the increased diameter of an aluminium conductor entails extra cost for insulating material.

Properties at First Unwelcome are Turned to Account.

At first the lightness and weakness of aluminium were much against it; these, as we have seen, were soon overcome by alloying the metal with copper or nickel. But by giving aluminium forms of utmost stiffness, by reinforcing these forms with steel wires, the metal is quite strong and rigid enough for cups, plates, cameras and other instruments for which lightness is most desirable. In many another case a material or a characteristic at first unwelcome has been turned to excellent account. Smokiness in a fuel is not a quality mentioned in its advertisements, and yet smokiness is just what is sought in the twigs, stubble, or coals set on fire to give plants a cloud protecting them from unseasonable frosts. It is astonishing how little fuel will serve in such cases, especially if the atmosphere is calm, so as not to carry the smoke where it is not needed. Many another instance might be given of a quality objectionable for one service and then turned to satisfying a new want. Sometimes, too, offensive qualities are most useful. Illuminating gas, as at first manufactured, had a distressing odor, which gave prompt and unmistakable notice of a leak. When water gas came into use, most harmful when inhaled, the chemists were puzzled to know how to give it an offensive smell; they found that a quality long complained of was really an advantage in disguise.

So in the electrical field, when an unsought quality has intruded itself, and proved unwelcome, the question has arisen, what service can we enlist it for? Not seldom the answer has been gainful in the extreme. Dr. Oliver J. Lodge tells us that a bad electrical contact was at one time regarded simply as a nuisance, because of the singularly uncertain and capricious character of the current transmitted by it. Professor Hughes observed its sensitiveness to sound-waves, and it became the microphone, which, duly modified, brought the telephone from the whisper of a curious toy to the full tones which ensured commercial success the world over. This same “bad” contact turns out to be sensitive to electric waves also, forming indeed nothing else than the coherer of the wireless telegraph.

Many an electrician has been perplexed and thwarted by the small bubbles of air which place themselves on a metallic surface immersed in an electric bath, interrupting the attack sought to be carried to a finish. Happily there is a task which these very bubbles perform as if they had been created for no other purpose, namely, the re-sharpening of files. First the dull and dirty files are placed for twelve hours in a fifteen to twenty per cent. solution of caustic soda; they are then cleaned with a scratch-brush and a five per cent. soda solution. Next they are placed in a bath of six parts of forty per cent. nitric acid, three parts sulphuric acid, and 100 parts water, each file being connected to a plate of carbon immersed close to it, by means of a copper plate connecting at the top all the carbons and the files. This produces a short-circuited battery generating gas at the surface of the files; the bubbles which adhere to the points of the files protect them from being eaten away, while the rest of the metal is being etched. Every five minutes the files are taken out and washed in water to remove the oxide which collects on their surfaces. When sufficiently etched they are placed in lime-water to remove any adherent acid, dried in sawdust to prevent rusting, and rubbed with a mixture of oil and turpentine. Indispensable in the whole process is the protection afforded by the bubbles of air.

Evil, Be Thou My Good.

For a long time its creation of sparks kept electrical machinery out of mines liable to fire-damp, which might be exploded by these sparks. In many other places they worked evils quite as serious, setting fire to shavings, cotton and such like. To-day these very sparks are applied to touching off the charges of gas and air in gas-engines of all types, whether stationary, or for automobiles and motor-boats. In another respect the automobile should be provided with a means of creating what is usually considered a nuisance, namely, a noise. Moving rapidly as it does on thick rubber tires, it gives no warning to hapless wayfarers. In Canadian cities, where in winter deep snow may muffle the tread of horses, every sleigh, under severe penalty, must be furnished with efficient bells.

Compensating Devices.

Sometimes an important property has unwelcome effects which, in particular cases, cannot be applied to advantage, and must be counterbalanced with as much care as possible. Many pieces of mechanism from the qualities of their materials are subject to deviations which must be compensated by introducing equal and opposite action. Tasks of this kind proceed upon an intimate acquaintance with the properties of substances common and uncommon. From the first making of clocks there was much trouble due to changes of temperature which affected the dimensions of pendulums, and consequently their rate of going. This difficulty is overcome by taking advantage of the fact that heat expands zinc about two-and-a-half times as much as it expands steel. Accordingly the two-second pendulum of the great clock at Westminster is built of a steel rod 179 inches in length, and a zinc tube, less massive, 126 inches long; they are joined at their lower ends only and are parallel. As temperatures vary, the fluctuations in length of the steel compensate those which occur in the zinc. Another mode of effecting the same purpose is to employ a cylinder partly filled with mercury; as this rises when warmed it exactly compensates for the lengthening by expansion of its supporting rod of steel.

Gravity, that universal force at which we have just glanced as it swings a pendulum, cannot be banished, but its downward push may be balanced by an equal upward thrust. In a remarkable feat Plateau poured oil into a blend of water and alcohol, adding alcohol until he produced a mixture having the same specific gravity as the oil—which now became a sphere, taking its place in the middle of the diluted spirits. He then introduced into the oil a vertical disc which he rotated; very soon spherules of oil separated themselves from the parent mass, and as satellites moved in the same direction as the primary sphere, because immersed as they were in the diluted alcohol, they shared the direction of its motion: the whole afforded a remarkable illustration of how nebulae may become planets, moons, and suns.

On somewhat the same principle as Plateau’s model are the liquid compasses for ships. Their needles are disposed within hollow metallic holders of the same specific gravity as the immersing liquid, in which therefore they move with perfect freedom on their sapphire bearings. Sometimes it is desired to use compass needles so poised that they will respond to the slightest magnetic influence. To this end one needle is placed above another, the north pole of the first over the south pole of the second; the astatic needle formed by this union is much more sensitive than a simple needle. The astatic needle, for all its ingenuity, is little used; of incomparably more importance is that other magnetic device, the telephone. No sooner had it entered into business than a serious fault was found with its messages; they arrived blurred and mingled with many sounds and noises, as if the conveying wire had caught every audibility of a neighborhood. The difficulty is remedied by using two conductors instead of one, and so arranging them that the currents induced on one conductor are exactly equal and opposite to those induced in the other.