Inventors at Work, with Chapters on Discovery

Advantages of the Cantilever, Arch, and Bowstring Designs.

In most cases a bridge crosses a valley or a river in a place which permits the engineer to erect scaffolding to support his trusses until they can be united and become self-sustaining. In some places this course is denied; a river such as the Ohio or the Mississippi may have to be spanned at a point where the waters in a single day may rise forty feet, bearing along trees and timbers with destructive violence. As a rule the difficulty is met by employing cantilever spans which require no scaffolding for their construction. To understand their principle let us suppose that on opposite banks of a creek we roll out to meet each other the joists FG and HI, taking care that the parts over the water shall always be lighter than the parts on land. When the joists at last touch they are secured to each other as a continuous roadway. Or, while they are at a moderate distance apart they may be joined by a third timber laid across the gap from one to the other. In practice the simple principle thus illustrated is developed and varied in many ways, but in every application the one rule is that the trusses as they stretch out from the two sides of a pier shall balance each other, the shore ends being duly weighted down or safely anchored to solid rock. And thus, at length, we come to the wonderful bridge, six miles west of Quebec, whose channel span of 1,800 feet will be the longest ever reared. See illustration, page 29. From the cantilever arms, DA and BE, will be suspended the central truss, AB, of 675 feet. A cantilever span may be much longer than a simple truss because on a pier, as D of this bridge, a part, DA, of the whole span, DE, is balanced either, as in this case, by a shore span, CD, or by a corresponding part of the next span should that span not extend to the shore but pass from one pier to another.

The first cantilever bridge in America was designed by C. Shaler Smith for the Cincinnati Southern Railroad, to cross the Kentucky River; it was built in 1876-7.

Spanning the gorge of Niagara, close to the Falls, is an arch bridge of 840 feet in its central span, which, in its construction during 1898, followed the plan originated by James B. Eads in building the St. Louis bridge nearly thirty years before. As scaffolding was out of the question in both cases, each bridge was built out from its piers on the cantilever principle. An arch is sometimes disguised as a modified bowstring, as in the Burr design of 1804, a horizontal tie connecting the extremities of the arched rib and taking its thrust, dispensing with the abutments demanded by an arch. In the chords of such a pattern the strength comes as near to uniformity throughout as practical considerations permit, avoiding the losses of early days when one part of a bridge might be twice as strong as another. The bowstring was adopted for the great span of 542¹⁄₂ feet over the Ohio at Cincinnati built in 1888, and for the span of 546¹⁄₂ feet erected at Louisville in 1893. A bowstring 533 feet long, forming part of the Delaware river bridge of the Pennsylvania Railroad, built in 1896, in Philadelphia, is outlined on page 32. At Bonn, on the Rhine, there was completed in 1904 a bridge whose central span is a bowstring 616¹⁄₄ feet long.

Suspension Bridges and Continuous Girders.

If we take the design of an arch bridge and turn it upside down we have a contour such as that of the Williamsburg Suspension Bridge, opened in 1903 between Brooklyn and Manhattan, depicted on page 33. For the utmost length this is the only available span; it brings into play the tensile strength of wire, the strongest form that steel can take. A steel cable of suitable diameter, if it had to support only itself, might safely be three miles long. A suspension bridge has another advantage in employing an anchorage to bear strains which would break down a simple truss resting on piers. As first erected suspension bridges were liable to extreme and harmful vibration, in many cases being shaken to pieces by storms of no great violence. It was found that this vibration was checked and that safety was ensured by introducing stiffening trusses which, at the same time, benefited the bridge by distributing the load uniformly throughout the sustaining cables.

At Lachine, about eight miles west of Montreal, on the line of the Canadian Pacific Railroad, a remarkable bridge crosses the St. Lawrence river. Its design is that of a continuous girder of four spans, the two side spans being 269 feet each in length, and the two others each 408 feet. This type is discussed by Mr. Mansfield Merriman and Mr. Henry S. Jacoby in Part IV, page 30, of their work on Roofs and Bridges. One of the advantages presented is that deflection under live load is less, and stiffness greater than for simple, discontinuous girders, the harmful effect of oscillation being thus diminished. Furthermore, less material is required than for simple, discontinuous spans. Both these elements of gain are brought out in placing a strip of rubber, AD, upon four equidistant points of support, when we find that BC, the central third of the strip sags less than either AB or CD, the first or last third. Cutting off one-third of the whole strip we deprive the removed piece, at its surface of separation, of the cohesion which did much to keep the whole strip, before cutting, almost horizontal at that point. We take AB, our short removed piece of rubber, and lay it at its ends on two points of support; it now serves in a rough-and-ready way as a model of a simple truss, all by itself; its decided sag shows it much less rigid than when it formed a part of an unbroken and longer structure. Continuous girders despite their advantages are seldom employed; they are liable to serious difficulties; among these may be mentioned that changes, often unavoidable, of level in piers and abutments cause them to suffer great reversals of stress, always a source of danger; furthermore, variations of length due to changes of temperature are, of course, much greater and more troublesome to provide against than in the case of discontinuous girders.

Best Proportions for Spans: A Slight Upward Curve is Gainful. Pins or Rivets in Fastening.

Whether spans are long or short, engineers are fairly well agreed as to the best proportions for girders and panels. They consider that a girder should have about one-twelfth to one-tenth as much depth as span; and that the weight of a web should be about equal to that of its flanges. They usually give panels twice as much depth as length, with a tendency to increase the proportion of depth to length, in order to minimize the deflections and oscillations which shorten the life of a structure. For definite lengths of span, particular types of construction are preferred; usually for lengths of from 20 to 125 feet, plate girders are chosen; for spans of 125 to 150 feet riveted lattice trusses are built; for spans of 150 to 600 feet pin-connected trusses are employed. Here we reach the economical limit of a length for simple trusses; beyond 600 feet the engineer is obliged to have recourse either to a cantilever or a suspension bridge.

Whatever the breadth of the stream or the chasm over which he is to build a roadway, each case must be studied in the light of its special circumstances. There must be due regard to business as well as to engineering considerations; the designer will bear in mind that types of parts customarily turned out at great steel works are procurable in less time, and at less cost, than novel types requiring to be manufactured to order. Then, in speed of construction, he will remember that a pin-connected bridge can be built much faster than a riveted structure. Furthermore, every part must be vastly stronger than ordinary duty requires. Tempests and floods may suddenly arise; at any instant a derailment or a collision may create a strain of the utmost severity; and even under ordinary circumstances it must not be forgotten that train loads grow constantly heavier because economy lies that way.

One detail of bridge design is worth a moment’s attention. When a book-shelf is a thin board, quite straight as manufactured, it sags in the middle when fully burdened. This downward dip may be avoided by making the shelf at first with a slight curve which brings the middle a little higher than the ends. In bridge building a like curve, or camber, is given to each span so that when fully loaded it will be level or nearly so. In a span of 500 feet it is found that a rise of half a foot at the centre is sufficient. In suspension bridges, for the sake of strengthening the structure, the camber far exceeds this ratio.

In fastening together the parts of a bridge the usual American practice, already mentioned, is to employ pins which pass through eye bars. In England riveting is preferred, as shown in the figure of the lattice truss, page 36. This difference in methods arose through the use of materials which differed. In the construction of bridges the English engineer started with the flanged girder of cast or rolled iron, or some other form of stiff beam, and as bridges increased in size so as to require the framing of a truss, his whole effort was directed toward making that truss as much like the original flanged or box girder as possible. The American engineer, on the other hand, had at first little or no iron or steel to work with, and of necessity used wood. As the necessary bridges were of considerable span, the only feasible method was to pin together small pieces of wood so as to form a connected series of triangles. To make rigid joints in wood was impracticable, and indeed rigid joints were not desired, because the strength of wood is slight when strains are applied in any direction other than that of the fibres of the piece, and the pin joint insures just this line of action. As a rule a riveted bridge requires more metal than a pin-connected design, takes more time to build, but demands somewhat less skill. To provide for changes in length as a bridge is subjected to variations of temperature, friction rollers are used to support its extremities. In the first suspension bridge at Niagara Falls, built by Roebling, a little cement accidentally covered the friction rollers and prevented them from turning; fortunately the structure escaped the destruction to which it was thus exposed.

We have now taken a rapid survey of some of the methods by which the designer of bridges plans a structure which is at once safe and to the utmost extent economical of material. Step by step he has discovered how little steel he may use for designs all the bolder because his hand is so sparing of weight. His success began in adopting the girder, which we have seen to be in effect the working skeleton long concealed within the common joist; the cantilever span near Quebec, which compasses 1,800 feet in its flight, has been dissected out of preceding burden bearers in the same way. Its metal stands forth as so much sheer muscle kept to the most telling lines, unencumbered by a single pound of idle substance. A designer of such a fabric is an artist skilled in disengaging from masses of material every ounce that can be wisely removed. In some cases, as when Roebling linked together New York and Brooklyn, a bridge is created as much a thing of beauty as of use, as graceful as it is strong.[3]

CHAPTER IV
FORM—Continued. WEIGHT AND FRICTION DIMINISHED.

Why supports are made hollow . . . Advantages of the arch in buildings, bridges and dams . . . Tubes in manifold new services . . . Wheels more important than ever . . . Angles give way to curves.

Having glanced at methods by which forms, judiciously chosen, economize the materials of buildings and rails, of bridges diverse in type, we pass to further consideration of these and like shapes, to find that they effect a saving in material while they make feasible a new boldness of plan, and introduce new elements of beauty. We will also remark that judicious forms prevent waste of energy as structures are either set in motion, or serve to convey moving bodies. Incidentally we shall see that well chosen shapes insure a structure against undue hurt and harm.

Hollow Columns and Tubes.

In lofty structures, the box girder is frequently employed as a column or a beam because it has even greater rigidity than the I-beam; usually it has four sides, but it may have eight, sixteen, or more, and thus step by step we come to a hollow cylindrical column which has, indeed, the best form that can be bestowed on supporting material. Chinese builders learned its economy on the distant day when they adopted the bamboo for their walls and roofs. Comparison with a solid stick of timber of like weight and substance will show that an equal length of bamboo is decidedly preferable. The inner half of a round solid stick does comparatively little in holding up a burden; to remove that half is therefore as gainful as to strip from a joist the timber surrounding its working skeleton. At first the journals or axles of engines and large machines, as well as the axles of railroad cars and the shafts of steamships were solid; to-day, in a proportion which steadily increases, they are hollow. The advantage of this form comes out when we take two cylinders of rubber, alike in length and weight, one solid, the other hollow. Supporting both at their ends, the hollow form sags less than the solid form, proving itself to be the more rigid of the two.

With like advantage seamless tubing is adopted for a broad variety of purposes. It builds bicycles and sulkies which far out-speed vehicles of solid frames; it is worked up into elevator cages, mangle rolls, pneumatic tools, fishing-rods, magazine-rifle tubes, inking rollers, farm machinery, poles, masts and much else where strength and lightness are to be united. Steel tubing is readily bent into any needed contour, even when of considerable diameter. Mr. Egbert P. Watson has pointed out its availability for highway bridges of about forty feet span, no professional bridge-builders being needed for their construction. Near Saxonville, Massachusetts, a pipe-arch bridge, eighty feet long, provides a roadway across the Sudbury River, while carrying within its pipe a stream which forms part of the Boston water system. A bridge of similar form, 200 feet long, spans Rock Creek in the City of Washington. The Eads bridge crossing the Mississippi, at St. Louis, employs for each span eight steel tubes of nine inches exterior diameter. Tubes large and small have been strengthened by adopting the model of an old-fashioned fire-lighter, or spill, a bit of paper rolled spirally as a hollow tube. Blow sharply into it and you but tighten its joints. In like manner tubes and pipes of metal are all the tighter when their seams are spiral instead of longitudinal. An eager quest for combined strength and lightness in the bicycle has ended in the choice of tubes spirally welded.

Arches.

When builders of old began to rear masonry they repeated in stone or brick the forms they had constructed in wood. Accordingly the lintels of their doors and windows were flat. It was a remarkable step in advance when the arch was invented, probably by a bricklayer, spanning widths impossible to horizontal structures. A flat course of stone or brick presses downward only; an arch presses sidewise as well as downward. It is this sidewise thrust, calling into play a new resource, that gives the arch its structural advantage. In modern masonry the boldest arch is that of the bridge at Plauen, Germany, with its span of 295¹⁄₄ feet. Of pointed arches the chief sustain the walls of Gothic cathedrals; it was to counteract the outward thrust of these arches that external buttresses were reared, either solid, as at St. Remy in Rheims, or flying, as at Notre Dame in Paris. The Saracenic arch, offering more than half of a circle, is not so strong as the Roman arch, but it has a grace of its own, fully revealed in the Alhambra, and in the incomparable mosque at Cordova. A chain of small links, a watch-chain, for example, freely hanging between two points of support strikes out a catenary curve; this Galileo suggested as the outline for an arch in equilibrium; it is adopted for suspension bridges.

“The arch,” says Mr. William P. P. Longfellow in “The Column and the Arch,” “was the great constructive factor in the architecture of the Roman Empire; it added enormously to the builder’s resources in planning, and to his means of architectural effect. It gave him the means of spanning wide openings, and when expanded into the vault, of covering great spaces; it habituated him to curved lines and surfaces. Helped by it, and spurred by the new wants of the complex Roman civilization, he enlarged the scale of his buildings and greatly increased the intricacy of their plans. He used his new combinations with a boldness and fertility of invention that have been the wonder of the world from that age to ours, constructing on a scale that dwarfed everything that had gone before except the colossal buildings of Egypt. Under a new stimulus, and with new means of effect, Roman building greatly outstripped that of the Greeks in extent, in variety, and magnificence.”

An arch built on its side, with its convexity upstream, and its ends braced against rocky banks, serves admirably as a dam. It has in many cases withstood floods much higher than those expected by its designers. Such dams must not be too long, or what is saved in thickness is more than lost in length. Arches inverted are used in many places as gulleys for drainage. Near Bristol, in England, they anchor the cables of the Clifton Suspension Bridge, at a depth of eighty-two feet below the surface of the ground. Many tunnels finished in masonry have outlines which are two arches united, the lower arch being inverted. The Cloaca Maxima, the famous sewer at Rome, is of this pattern; it is twenty-six feet high, sixteen feet broad, and is now in its twenty-fifth century of service.

Circles and Other Curves.

From arches, built of parts of circles, let us pass to the circle itself, and glance at the use of tubes of circular section as we begin to consider how resistances to motion may be minimized. The use of the bamboo not only for building, but for the carriage of water, began in the remote past. As structural material it was light and strong as we have noticed; laid upon the ground it was a ready-made water pipe of excellent form. When trees were hollowed out to convey water, when clay was modeled into tubes, the hollow cylindrical shape of the bamboo was in the mind of the Asiatic artisan, to be faithfully copied. That form has descended to all modern piping for water, steam, and gas, because the best that a pipe can take. No other shape has, proportionately to capacity, so little surface for friction inside or rust outside. A locking-bar water pipe, devised by Mephan Ferguson, of Perth, Australia, is made of two plates of equal width, curved into semi-circles which are pressed at their ends into channel bars of soft steel. As the locking-bars and joints are opposite each other, their joints can be tightly closed by a simple machine which exerts pressure in a straight line. This construction may be used not only for pipes, but for hydraulic cylinders, air receivers, mud and steam drums, tubular boilers and boiler shells where high pressures are to be withstood.

A steam boiler or other vessel under severe internal strains had best be spherical if equality to resistance is particularly desired. Usually a cylindrical shape is much more convenient, and no other is given to simple steam boilers or to the tubes of water-tube or fire-tube boilers. Tubes comparatively narrow, are readily manufactured without seam, so that they may be quite safe though thin; large boilers of plates riveted together, must be built of thick metal. It was estimated by Mr. F. Reuleaux, the eminent engineer, that if such boilers could be made in one continuous piece of metal by the Mannesmann process, so successful in tube-making, an economy in weight of at least one third would be feasible.

In water-tube boilers a gainful departure from the circular form in a detail of their design is worthy of notice. In order that their tubes may be kept sound and clean they are rendered accessible by hand-holes which pierce the front and back of the boiler. Usually these hand-holes and their covers are round, a form which makes it necessary to put the cover outside the boiler where even a good joint, well stayed, may leak or give way under a pressure which tends to force apart the cover and its seat. In the Erie City boiler the covers are elliptical; they are readily passed through the hand-holes so as to rest not on the outside, but on the inside, of the boiler, where the steam pressure makes their joints all the tighter. A further advantage is that each elliptical plate is large enough to give access to two tubes instead of one, lessening the lines of juncture along which leakage may occur.

Wheels.

It was a memorable day when first a round log or stick was thrust under a burden, easing its motion and leading to the wheel by piecemeal improvements. A section cut off from the end of a round log is to-day the wheel for ox-carts in China and India. In its crudest form a roller enables a man to drag a load instead of carrying it, and he can readily drag much more than he can carry. Wheelwrights of old soon found that a wheel need not be solid, that strong spokes, a sound rim, and a metal tire embody the utmost strength and lightness. Roller and ball bearings much extend the benefits of simple wheels; they lessen friction in the best typewriters, bicycles, and elevators; in wagons, carriages, and automobiles roller bearings are so helpful that their use should be universal. Of notable efficiency is the Hyatt bearing, formed by winding a steel strip into a spiral roller. This device has a flexibility which enables it to conform to irregularities of motion much better than can a solid cylinder.

For machinery the wheel is indispensable. The hand does its work chiefly in moving to and fro, as in sawing and whittling. Machines outdo manual toil by moving swiftly and continuously in a circle: instead of the smoothing iron we have the mangle, boards are planed by rotary knives, timber is divided by circular saws, and the steam turbine is displacing the steam engine which every moment has to check the momentum of huge reciprocating masses. Noteworthy in this regard is the perfecting press which prints a newspaper from a continuous roll, as contrasted with the old machine which demanded for each impression a distinct series of to and fro movements. The Harris Rotary Press for job printing is of like model. It feeds itself with 6,500 sheets an hour, printing from a stereotype or an electrotype curved upon its cylinder. The lathe, simple enough a century ago, has been developed into machines of great complexity, power, and variety, all with the original rotary mandrel as their essential feature. Milling machines, steadily gaining more and more importance, employ rotary cutters which dispense with the manual chipping and filing of former days.

Angles Replaced by Curves.

Wood as commonly hewn, sawn, and planed; bricks as usually molded; stone as it leaves an ordinary hammer, all have flat sides and square edges. Hence it has been easiest to build walls and floors which meet at right angles, and to leave sharp corners on outer walls, windows, doorways, and chimneys. This is being changed for the better; in staircases the boards on which we tread and those which join them together now meet in smooth curves; so do the walls of rooms as they reach ceilings and floors, conducing to ease and thoroughness in sweeping and cleansing. In outer walls, in doorways and windows, similar curves reduce liability to hurt and harm. A wagon wheel easily knocks pieces from an angle of brickwork; it makes little impression on bricks retiring from the street line in a sweeping curve, as in the Madison Square Garden, New York. Factory chimneys have long been built round instead of square; to-day in the best designs the ducts to a chimney are also freely curved. In blast furnaces this is the rule for every part of the structure, ensuring gain in strength, lessening resistance to the flow of gases, and thus saving much fuel. When waterpipes varying in diameter are joined, the junction should be a gradual curve, otherwise retarding eddies will arise, wasting a good deal of energy; the same precaution is advisable in laying pipes for steam or gas. The elbows of pipes for gas, steam or water exert the least possible friction when given the utmost feasible radius. All the various parts of heavy guns are curved, since any sharpness of angle at a joint brings in a hazard of rupture under the tremendous strains of explosion.

Embossing and stamping machines may either decorate a sheet of note paper or make a tub from a plate of steel. Whatever their size these machines have the edges of their dies nicely rounded, so as to avoid tearing the material they fashion. To ensure the utmost strength in the machines themselves they are contoured in ample curves. In hydraulic presses, subjected to strains vastly greater, the same shaping is imperative, otherwise a cylinder may part abruptly with disastrous effect. So, too, in the manufacture of magnets and electro-magnets, their terminals are well rounded to ensure the closest possible approach to uniformity of field and of working effect.

A glance at a warship discovers her varied use of curves in defence; to deflect assailing shot and shell, her plates are given bulging lines, her turrets are built in spherical contours, and her casemates are convex throughout. On much the same principle fortifications are rendered bomb-proof, or rather bomb-shedding; while outworks are so inclined that bombs fall to distances at which they do little or no harm. As in war so in peace; there is gain in building breakwaters with an easy curve; to give their masonry and timbers a perpendicular face would be to invite damage, whereas a flowing contour like that of a shelving beach, slows down an advancing breaker and checks its shock. In rearing lighthouses to bear the brunt of ocean storms the outline of a breakwater is repeated to the utmost degree feasible. Often, however, the base supporting a lighthouse is too small in area for such an outline to be possible.

CHAPTER V
FORM—Continued. SHIPS

Ships have their resistances separately studied . . . This leads to improvements of form either for speed or for carrying capacity . . . Experiments with models in basins . . . The Viking ship, a thousand years old, of admirable design . . . Clipper ships and modern steamers. Judgment in design.

Forms of Ships Adapted to Special Resistances.

In giving form to a ship a designer has a three-fold aim,—strength, carrying capacity and speed. Strength is a matter of interior build as much as of external walls; it is conferred by girders, stays and stiffeners which we have already considered, so that we may here pass to the general form of the hull, which decides how much freight a ship may carry, and, to a certain extent, how fast she may run. A ship is the supreme example of form adapted to minimize resistance to motion; its lesson in that regard will be the chief theme of this chapter. Until the close of the eighteenth century the resistance to the progress of a ship was regarded as a single, uncompounded element, plainly enough varying with the vessel’s speed and size. It was Marc Beaufoy, who first in 1793 in London, pointed out that a ship’s resistance has two distinct components; first, friction of the shell or skin with the water through which the vessel moves, dependent upon the area of that skin; second, resistance due to the formation of waves as the ship advances, dependent upon the speed of the vessel and the shape of her hull. Other resistances have since been detected, but these two are much the most important of all; each varies independently of the other as one ship differs from another in form, or as in the same ship one speed is compared with another. To take a simple case: a ship’s model of a certain form, of perfectly clean skin, is towed at various speeds and the pull of the tow-line is noted; then the same model with its skin roughened and covered with marine growths is towed at the same speeds, and much greater pulls are observed in the tow-line. The wetted surface is the same in the two series of experiments, the speeds correspond throughout, and the increase of resistance due to a roughening of surface can only mean that the friction between the water and the submerged skin has increased. Next we take a model of certain form and definite size, and a second model having the same area of wetted surface but a different form; we tow both models at the same speed to find that one requires a decidedly stronger pull than the other. This difference cannot be due to frictional resistance of surface, for this is the same in both models, therefore it must be due to the increased resistance offered by the water as it is pushed aside, a resistance measurable in the created waves. Mr. Edmund Froude, an eminent English authority, says:

“For a ship A, of the ocean mail steamer type, 300 feet long and 31¹⁄₂ feet beam and 2,634 tons displacement, going at 13 knots an hour, the skin resistance is 5.8 tons, and the wave resistance 3.2 tons, making a total of 9 tons. At 14 knots the skin resistance is but little increased, namely 6.6 tons; while the wave resistance is nearly double, namely, 6.15 tons. Mark how great, relatively to the skin resistance, is the wave resistance at the moderate speed of 14 knots for a ship of this size and of 2,634 tons weight or displacement. In the case of another ship B, 300 feet long, 46.3 feet beam, and 3,626 tons displacement—a broader and larger ship with no parallel middle body, but with fine lines swelling out gradually—the wave resistance is much more favorable.[4] At 13 knots the skin resistance is rather more than in the case of the other ship, being 6.95 tons as against 5.8 tons; while the wave resistance is only 2.45 tons as against 3.2 tons. At 14 knots there is a very remarkable result in this broader ship with its fine lines, all entrance and run and no parallel middle body:—at 14 knots the skin resistance is 8 tons as against 6.6 tons in ship A, while the wave resistance is only 3.15 tons as compared with 6.15 tons. The two resistances added together are for B only 11.15 tons, while for A, a smaller ship, they amount to 12.75 tons.”

Experimental Basins.

These figures show that a designer must bear in mind the speed at which this ship is to run; they prove that he may choose one form to minimize friction, or another form if he particularly wishes to bring wave-making resistance to the lowest possible point. Forms of these two kinds are readily studied when represented in models 12 to 20 feet in length towed through tanks built for the purpose. Experiments of this kind were undertaken as long ago as 1770, in the Paris Military School; the methods then inaugurated and copied in London at the Greenland Docks were greatly improved by Mr. William Froude in a tank which he constructed at Torquay in England, in 1870. His modes of investigation, duly adopted by the British Admiralty, and after his death continued by his son, Mr. Edmund Froude, have created a new era in ship design. To-day in Europe and America there are eleven such tanks as Mr. Froude’s, all larger than his and more elaborate in their appliances. In addition to learning the behavior of models diverse in type, Mr. Froude worked out the rules which subsist between the performance of a model and that of a ship of like form; these he brought to proof in 1871 when he towed Her Majesty’s Ship Greyhound, and verified his estimates in towing its model. The rules concerned, known as those of mechanical similitude, are given in detail by Professor Cecil H. Peabody in his “Naval Architecture,” page 410. While experiments become more and more valuable as one refinement succeeds another, there is always much well worth knowing to be learned from the actual behavior of a vessel as she takes her way through a canal, a shallow river, or the storm-beaten stretches of the sea.

The experimental tank of the United States Navy at Washington, is 470 feet long, 44 broad, and 14¹⁄₂ deep; it is arranged for models 20 feet in length. See the page opposite. The towing carriage is a bridge spanning the tank just above the water; it is a riveted steel girder. The towing mechanism, of massive proportions, is driven by four electric motors of abundant power. A double set of brakes brings the carriage gradually and quietly to rest from a high speed. A self-acting recorder measures both speed and resistance. Ship builders may have models built by the Bureau in charge, that of Construction and Repair of the United States Navy Department, and have these models towed at any desired speeds, paying simple cost.

It was in 1880 that the lessons of towing experiments with models began to be adopted in practice. As a result the forms of steamers have been greatly improved. Originally their lines were taken from those of sailing vessels but, as dimensions grew bolder and speeds were increased, it became clear that steamers demanded wholly different lines of their own. These lines, fortunately, may be plainly disclosed in experiments with a model, because a steamer usually runs on an even keel, in which position a model is easily driven through a tank. A sailing vessel, on the contrary, is nearly always heeled over by the wind so that it seldom runs on an even keel; tank experiments, therefore, avail but little for the improvement of its lines. Even were the model inclined at various angles in one test after another, sails must be omitted, with their influence on steering, their lifting and burying effects, often extreme.

A Viking Ship a Thousand Years Old.

A thousand years ago the Vikings of Norway roved the seas in boats of a form which is admired to-day. To those hardy adventurers swiftness and seaworthiness meant nothing less than life and victory, their eyes perforce were keen to note what craft sped fastest through the water, what new curves kept waves from coming aboard. Perchance as they refined upon keel and rib they took golden hints from the shapes of gulls and fish. To be sure, long before science was dreamt of, they had to work by rule-of-thumb, but the thumb was joined to brains that did honor to human nature. On page 56 is illustrated the Viking Ship unearthed early in 1880 at Godstad, near Sandefjord in Norway, in a mound where, according to tradition, a king and his treasure had been buried. It is the most complete and the best preserved vessel of ancient date in existence. It is fully described and pictured in “The Viking Ship,” by Mr. N. Nicolaysen, a work published in 1882 by Mr. Albert Cammermeyer, Christiania. Mr. Nicolaysen regards the vessel as having been built about A. D. 900, for use in war by the great chieftain whose tomb it became. The ship was 65 feet, 10 inches long, on the keel; with an extreme length over all of 78 feet, 1 inch; amidships it was 16 feet, 9 inches; its depth amidships from the top of the bulwarks to the keel was 3 feet, 11¹⁄₄ inches. The material throughout was pine. The helm, a plank shaped like a broad oar, was fastened to the side of the vessel. In accordance with the number of its oars and shields this ship must have had a crew of sixty-four, besides these came the steersman, the chieftain and probably a few more of his companions, making a total, in all likelihood, of seventy to be carried by her. Says Mr. Nicolaysen: “In the opinion of experts this must be deemed a masterpiece of its kind, not to be surpassed by aught which the shipbuilding craft of the present age could produce. Doubtless, in the ratio of our present ideas, this is rather a boat than a ship; nevertheless in its symmetrical proportions, and the eminent beauty of its lines, is exhibited a perfection never attained until after a long and dreary period of clumsy unshapeliness, when it was once more revived in the clipper-built craft of the nineteenth century.”[5]