That on the extreme left, with a number of radiations, represents a portion of a Madagascar palm, popularly called the Traveller’s Tree. Having very large leaves, arranged in the manner there shown, the Traveller’s Tree condenses the nightly dews, and allows them to trickle down into the hollows of the leaf-stems.

There the water remains, out of the reach of sunbeams or wind, and if a traveller happens to be thirsty, all he has to do is to pierce the base of one of these gigantic leaves, and out rushes a stream of the purest water, as is shown in the illustration.

Next to the Traveller’s Tree is shown one of those extraordinary vegetables called Pitcher-plants, from the strange conformation of the leaves. They inhabit Borneo, Siam, and other hot countries. In these remarkable plants some of the leaves are developed into suitable pitchers, with hinged lids, exactly like our hot-water jugs. They serve, however, a different office, and contain cold water which the plant has distilled from the dew.

As the monkeys are in the habit of resorting to these plants when thirsty, they are sometimes called Monkey-pots. There is an admirable account of the Pitcher-plants and their development in the Transactions of the Linnæan Society, vol. xxii. part iv. The scientific name of those plants is Nepenthes.

Below the vegetable comes a rather celebrated animal cistern, namely, a portion of one of the stomachs of a Camel.

It exactly corresponds with that part of an ox which butchers call “honey-comb tripe,” and consists of a multitude of cells, which can be closed or opened at will. When the camel takes in its provision of water, it can treat this portion of the stomach much as the hive bee treats the honey-bag, and fill its cells with water.

By degrees, when it finds the necessity for moisture, it can squeeze the water out of these receptacles into the digestive portion of the interior, and so can sustain life for a wonderfully long time under conditions which would kill any other animal. I may remark, by the way, that the amount which a camel can drink, and the length of time through which it can endure its desert life, have been much exaggerated. There is another point to be considered, namely, the curious resemblance between these cells and the honey-comb of the hive bee. Every one knows that honey, no matter how tightly closed, will crystallize and lose its best qualities if kept in jars, whereas if it be allowed to remain in the waxen comb, where it is divided into very small portions, it will remain good for years.

It is just the same with the cells of the camel’s stomach, they being able to preserve water in a pure state by distributing it among a number of small cells, which can be opened or closed at will.

Then we come to the various means of obtaining water.

Reference has already been made to the Filter, by which foul water can be made pure for human consumption, and we will therefore pass to another mode of obtaining pure water, namely, the Still.

In former days, if there were a failure of the supply of fresh water on board ship, the whole of the occupants must necessarily perish. Now, however, no such danger exists, as every well-furnished ship carries at least one Still, by means of which the sea-water can be made to abandon its salt, and to give out nothing but pure water fit for drinking.

Even in cases where no regular Still has been on board, an extemporised Still has been made from a kettle, a gun barrel, or piece of lead piping, or anything of a similar nature.

The principle of the Still is simple enough, and is shown by the diagram, rather than drawing, on the right hand of the illustration. There is a vessel in which liquid is boiled. From the upper part of it rises a tube through which the steam must pass as it is generated. The tube in question is generally of considerable length, and is coiled inside a vessel filled with cold water, rendered colder by ice, if possible.

As the steam passes through the cold tube condensation takes place, and it becomes liquid again, but deprived of its heavier particles, so that if sea-water be placed in the still, the salt is left in the vessel, and nothing but pure water passes through the tube. In dissecting-rooms a small still is almost invariably kept. Many preparations are of such a nature that the spirit in which they are placed becomes discoloured, and has to be repeatedly changed. Now, even methylated spirit is an expensive article, and therefore, instead of being thrown away, the discoloured spirit is placed in the still, and reproduced in a clean and transparent state.

Nature affords innumerable examples of distillation, the chief of which are the Dewdrops which have already been mentioned. During the daytime the air is full of moisture drawn by the sunbeams from ocean. We cannot see it, but it is there, and when the chill of night cools the various trees, herbage, and other such objects, the aërial moisture is condensed upon them, which is then known by the name of Dew.

On the left hand of the illustration are shown the tiny Dewdrops as hanging on the slight threads of a spider’s web, and collected in larger drops upon a leaf.

There are many other familiar examples of the principle of condensation, the commonest of which is the so-called steam as it pours from the spout of a kettle. In point of fact, it is not steam at all, but only water condensed into very small drops. At the orifice of the kettle it is quite invisible, but when it passes into the air, and is condensed, the tiny globules become visible. The same fact may be noticed in the Napier’s Coffee Machine, which has already been mentioned. When the water is boiling in the glass globe no steam is visible, though the upper portion of the globe is entirely filled by it. But, no sooner is the cork removed, and the steam allowed to escape, than it at once becomes visible as a white cloud, being, indeed, a miniature copy of the rain-clouds that float above us.

Then there is that mostly invisible passage of liquid through the multitudinous pores of the body, which is generally known as perspiration. It is invisible in warm weather, but on a cold day is as visible as a rain cloud.

The Turkish Bath affords a good example of this fact. Sometimes the hottest room attains a temperature of 250° or more, water boiling at 212°. When a bather goes into that room, he appears to have a perfectly dry skin, the moisture being in the form of invisible steam, and swept off as soon as it is generated.

But, if he passes at once into the cold room, he is so enveloped in vapour that for a few moments he is wrapped in it as in a cloud, and can scarcely be seen, the vapour having been condensed by the cold air.

A very familiar instance of this sudden condensation may be seen in the streets of London on any winter day. There may be a couple of omnibus horses, nearly at the end of their day’s work, and quite tired out. Suddenly they are pulled up by the driver, and as suddenly disappear for a moment or two, being concealed in a cloud of moisture proceeding from their bodies. Of course in a hot day there is more of the moisture, but the warmth of the atmosphere prevents it from condensation, and so it is not visible.

One valuable property of the system of evaporation and condensation is its cooling power. Thus it is that a person who is ill with fever tosses about with a burning skin until the pores of the body act, and allow the normal moisture to pass through them. Then the body cools by evaporation, and the patient begins to amend.

So it is that the bather can endure in the Turkish bath a heat so great that a glass of water, if held in the hand, would speedily boil, and a piece of meat be cooked in about the same period. But, if the air were not dry enough to carry off the perspiration, the bather would be scalded to death.

A most valuable adaptation of the principle is shown in the little glass machine for dispersing perfumes in the form of spray. In cases of headache it is almost invaluable, the spray cooling the heated forehead, like magic, and at the same time filling the room with the grateful perfume.

It has even a greater claim to human gratitude, as I can personally testify. I have the strongest objection to a surgeon’s knife, especially when I know, from sad experience, that he is going to make very free use of it. But, on the last occasion, I cared nothing for it, owing to the happy invention called Ether Spray.

The effects were remarkable. First, a delicious cooling of a spot raging with internal fires. Then it was rather colder than I liked. Then it was much colder than I liked. Then it became almost too cold to bear, reminding me of my childhood’s feet on the outside of the Birmingham coach in the depth of winter.

Suddenly all sensation ceased, and the skin became white as parchment. Out came the surgeon’s bistoury, and I looked at him with as calm composure as if he had been whittling a deal plank. There was absolutely no feeling whatever, the local nerves having been temporarily frozen, so great is the power of evaporation. If it ever be my lot again to endure cold steel, I shall have the ether spray.

On the extreme right of the illustration is seen the “Wet-bulb” Thermometer, which carries out the same principle, the thermometer being double, and one bulb being covered with a wet envelope, while the other is dry.

Below is one of the many inventions for making artificial ice, all of them depending on the cooling power of evaporation. Perhaps some of my readers may have seen molten iron poured over the human hand without doing the least harm, or mercury frozen in a red, or rather a white, hot vessel. Both these phenomena are due to the cooling power of evaporation, which is made to act with extreme rapidity, and so absorbs the heat until even mercury is rendered solid, and can be cast in a mould like a leaden bullet.

In the accompanying illustration we have an example of the Condensating principle as applied to the steam-engine, and popularly known as the “Low-pressure Engine.” In this case force is reconverted, so to speak, and, if a cubic inch of water has been converted by heat into a cubic foot of steam, creating a pressure in one direction, it can be reconverted by cold, and so produce a pressure in another direction.

It is owing to this fact that some parts of the world are always hot and always wet, Guiana being a striking example.

The wind blows over the ocean, absorbing moisture as a sponge does water. As it passes from the sea over the land, it is met by secondary mountain ranges, too low to arrest its progress altogether, and high enough to have their summits clothed in eternal snows. As soon, therefore, as the warm, water-laden winds pass over these mountains, the moisture is condensed by their frozen tips, and down rushes the rain in torrents.

Even in our own temperate land we can often trace the cause of a heavy rain to the presence of a lofty hill, or even an exceptionally tall spire. The moist climate of Oxford has been attributed by scientific men quite as much to its spires and towers as to its low-lying situation.

Now we come to the various modes of extracting the water which is laid up within the earth, and which only slowly ascends to the surface when drawn up by the heat of the sun.

Water is everywhere, but the depths at which it is found are vastly different. For example, at one house in which I lived it was not possible to dig for three feet without coming to water. In another, no water was found within some two hundred feet, and, as I several times relieved the old gardener of the task of drawing the water for the day’s consumption, I have reason to remember the depth.

The pail, rope, and winch which were in use at that time—and may be still, to the sorrow of the gardener—are but a sort of semi-savage way of procuring water from the depths of the earth. It is a well-known fact that under certain conditions water always finds its own level, minus the friction of the channel through which it passes. On this principle all fountains are made. Those, for example, at the Crystal Palace, which fling their waters to such a height, are fed from tanks on the summit of the two great water towers. And, were it not for the friction of the water in the tubes, and that of the air, the fountains would rise as high as the tanks from which they are fed.

Such is the case with springs, especially with those of an intermittent character, in which latter instance the rushing of the water is exactly coincident with the filling of the hidden tank which supplies it.

The modern Hydrant system, which bids fair to supersede the cumbrous machinery of fire-engines, even when worked by steam, is based on the same principle. The water-tanks are placed at such a height that, when a hose is attached, and the tap turned, the water can be thrown over the roof of the highest building. Such hydrants have been attached to Canterbury Cathedral since the fire which so nearly consumed that magnificent and venerable building.

A very remarkable use has been made of this power of water in mining operations. Most of my readers know that in gold mines the metal is chiefly found scattered among quartz, one of the hardest of the minerals. The usual plan has been to dig out the quartz, pound it to powder with specially devised machines called “stamps,” to pass the powder through mercury, which amalgamated with the gold, and gave it up again on being heated to a certain temperature.

Now a different mode of mining is brought into operation, the pickaxe, spade, and stamps, with all their expensive machinery, being abandoned, and water made to do the duty of all three, some ingenious individual having noticed the effect which water has on the hardest rock.

Such, for example, is the case with those wonderful Victoria Falls of Africa, where the rushing water has cut its sinuous channel through so many hundreds of yards of rock. Such, also, is the case with the more celebrated, but not so wonderful, Falls of Niagara, which have been gradually working their way backwards, having worn away the rocks over which they fall, and which are shown to be many miles away from the spot where the river first discharged itself over the cliff.

In fact, it is well known that the Falls are receding at a definite rate annually, and that the rate has been calculated with scientific accuracy. The cliffs of our own coasts-say of Margate or Ramsgate—crumble away with equally calculable speed.

In the hydraulic mining system large tanks are erected, at least two hundred feet above the level of the mine. From these tanks proceed pipes, terminated by hose, just like those of our ordinary fire-engines. The miners, instead of using pickaxe or crowbar, simply direct the streams of water against the solid rock. Their effect is tremendous. They tear it to powder, and carry it down the wooden troughs called “flumes,” in which the mercury is so arranged that not a single atom of quartz rock can pass without having its gold extracted.

The following graphic account of Hydraulic Mining at Nevada is taken from Mr. J. K. Lord’s “Naturalist in British Columbia:”—

“Near Nevada are the famed Hydraulic washings. The gold is disseminated through terraces of shingle conglomerates, often three hundred feet in thickness. These terraces are actually washed entirely off the face of the country by propelling jets of water against them, forced by pressure through a nozzle.

“To accomplish this, the water is brought in canals, tunnels, and wooden aqueducts, often forty miles away from the ‘draft.’ This supply of water the miners rent.

“As we near the washing spot, in every direction immense hose, made of galvanized iron, and canvas tubes six feet round, coil in all directions over the ground like gigantic serpents, converging towards a gap, where they disappear.

“On reaching this gap, I look down into a basin or dry lake, three hundred feet below me. The hose hangs down this cliff of shingle, and following its course by a zigzag path, I reach a plateau of rock, from which the shingle has already been washed.

“A man stands at the end of each hose, that has for its head a brass nozzle. With the force of cannon-shot, water issues in a large jet from this tube, and propelled against the shingle, guided by the men, washes it away as easily as we could sweep a molehill from off the grass.

“The stream of water, bearing with it the materials washed from out the cliff, runs through wooden troughs called ‘flumes,’ floored with granite. These ‘flumes’ extend six miles. Men are stationed at regular distances to fork out the heavy stones.

“Throughout its entire length, transverse strips of wood dam back a tiny pond of mercury. These are called ruffles—gold-traps, in other words, that seize on the fine dust-gold distributed through the shingle. The flumes are cleaned about once a month, and the gold extracted from the mercury.

“I try with a powerful lens to detect gold amidst the material they are washing, but not a trace is discoverable, and yet it pays an immense profit to the gold-washers.”

There are two more modes of extracting water, which will be but cursorily mentioned.

The reader will remember that water finds its own level, and that the terrific power of hydraulic mining is owing to the fact that the water expends its force against the solid rock instead of ascending into the air.

It is now found that, even without artificial assistance, water has a habit of finding its own level, and that, if it be allowed its own course, it will contrive to find its way nearly to the highest point whence it derived its origin. On this principle are based the Artesian Wells, which, when they “strike water,” spurt it up in a torrent, as is the case with the now celebrated Norton Tubes, which are screwed down into the earth like hollow gimlets, and which always contrive to extract the water hidden beneath the surface of the earth.

The success of our army in Abyssinia was greatly owing to these Norton Tubes, which, being of small diameter and of peculiar make, could be screwed into the ground when the troops made a halt, unscrewed when they left the spot, and used again for the next halt.

Similarly, the French used the Artesian-well system with wonderful success in Northern Africa. Water is the chief necessity of life in that part of the world, and a nation who could cause pure cold water to spring out of the hot and thirsty sands was naturally looked upon as something more than human.

Yet the principle was exactly the same in both cases. Water is always latent somewhere beneath the surface of the earth, and, if a tube can be driven deep enough, the water will come up it.

The accompanying illustration shows the Artesian Well and Norton’s Tube, and their similitude in principle, the tube penetrating through various layers of soil, until it reaches the water which it seeks.

Then there is another way by which water can be made to force itself to a considerable height. Not being much of a mathematician, I do not recollect the exact proportional height to which a stream of water may raise itself, but if any one can secure a fall of some eight or ten feet, he can furnish his house with water by means of the “Ram,” a chart of which is shown in the illustration.

The principle of the Ram is, that the water is allowed to flow down a tube, when it meets with a valve. This valve is suddenly closed by the pressure, and the water is forced onwards by the shock. Much water escapes at each blow of the valve, but that does not signify.

The force of water thus suddenly stopped is hardly appreciated. Even in ordinary houses the sudden turning of a water-tap has been known to burst the pipe and deluge the house with water.

In Nature a similar effect is produced, called popularly the “Spout-hole.”

It is a hole or tunnel on the seashore, passing upwards from the level of the sea to the summit of the cliff.

When the waves are urged against the tunnel by the wind, the water is dashed into it. Being partially checked by the friction, which acts exactly like the water that is checked by the Ram, the wave hurls itself up the channel, and flies out in showers of spray, high above the level of the original wave which caused it.

In the illustration are shown the Water-ram with its globular valve, and the safety or escape valve of the waste water. On the left is shown one of the natural Spout-holes, with the water dashing through its tunnel into a mass of spray.

USEFUL ARTS.

CHAPTER XI.

AËROSTATICS.—WEIGHT OF AIR.—EXPANSION BY HEAT.

There were, however, two disadvantages about this plan. The first was the great danger of fire, which on one occasion did ignite a balloon when at a great height. The second was the perpetual labour required in keeping the fire alight. Straw burns very rapidly, and so the aëronaut had no opportunity of making those meteorologic observations in which consist almost the entire value of the balloon.

Then it was thought that hydrogen gas, being about fourteen times lighter than ordinary air, would answer the purpose, and such has proved to be the case. Formerly the gas was made at great expense from sulphuric acid and zinc, but it is now found that the common coal-gas is quite as efficient, very much cheaper, and fills the balloon much more rapidly.

The same principle, though not the same form, is found in Nature.

There are certain tiny spiders called Gossamers, which have a curious power of floating in the air. They have been seen on the tops of lofty spires, and they are sometimes so numerous that the air is full of their floating webs, and the ground is white with those that have descended.

Their mode of ascent is this. They climb to the top of some elevated object, if it be only a grass-blade. They then pour out a tuft of long, slender threads, which shortly begin to tend upwards. As soon as the Spider feels the pull, it crawls upon the web, and sails away into the air. The duration and height of the ascent depend much on the wind and character of the atmosphere.

The web ascends because it is for the time lighter than the atmosphere. But, as it gradually becomes laden with the moisture that more or less fills the air, it becomes heavier than the atmosphere, and gently sinks to the ground.

What may be the object of these aërial voyages no one knows. They may be for the purpose of capturing minute insects, or they may be for mere amusement. But in either case they are highly instructive, as showing the principle on which the balloon was framed.

The little Gossamer Spider is shown on the left hand of the illustration, clinging to its floating web. I believe that the Gossamer is not a single species of Spider, but that there are many species which deserve the name, being able to float in the air when they are small, but losing that capacity as they increase in size and weight.

Now we come to another branch of the same subject, namely, the safe descent from a great height by means of the Parachute.

On the right hand of the illustration is the ordinary Parachute as it appears when open and closed, in either case having somewhat the appearance of a large umbrella. It is hung to the balloon in its closed state, and when detached it falls rapidly for a yard or two with startling rapidity. The pressure of the air thus forces the ribs open, and gives sufficient assistance to the atmosphere to insure a gentle fall.

On one memorable occasion, when the late Albert Smith was in the car of a balloon upwards of a mile from the ground, the balloon burst. Fortunately it burst so completely, that the silk was driven into the closely meshed netting, and formed an extemporised parachute, which took the voyagers to the earth with safety, except some rather severe bruises.

Children often amuse themselves with miniature parachutes. They take a square piece of thin paper, tie threads to the four corners, and then bring the ends together, a cork taking the place of the car. They then launch it from a high window, and should there be a favourable breeze, it is wonderful how far it will be carried before it comes to the ground.

Once, when a boy of eleven, and consequently thoughtless, I set a chimney on fire by one of these Parachutes. I wished to see whether it would go up the chimney, and come out at the top. Unfortunately it was caught by a flame as it was launched, flew up in full blaze, and, as the chimney needed sweeping, the result was inevitable.

In the centre of the illustrations, and at the top, are two examples of a well-known natural Parachute called the Dandelion seed. The resemblance to the real Parachute is wonderful, the actual seed occupying the place of the car, and fulfilling the same office, i.e. keeping the seed upright until it reaches the ground.

When the tuft is closed, as is the case before the pretty ball of seeds bursts from the green envelope in which they had been confined during the process of development, its form bears the same startling resemblance to the Parachute.

Passing from the vegetable world, there will be seen three examples of Natural Parachutes. Several others will be mentioned, but we have no space for description or figure. It will be seen, however, that the one principle which characterizes them all is the exposure to the air of a flattened and large surface, in proportion to the size of the object.

Before beginning the description, however, I must mention that nearly all animal parachutes can to a certain extent guide their course, while neither the balloon, the gossamer, the parachute, nor the various winged seeds have the least power of guidance, but must follow every current of air in which they may happen to float.

The upper figure represents a Flying Squirrel.

There are many species of Flying Squirrel, but they all agree in one point. The skin of their sides is modified into a very thin fold, which extends as far as the feet.

It is very elastic, so that when it is not in use it falls into folds or wrinkles, and is hardly perceptible. But should the Squirrel wish to pass from one tree to another, without coming to the ground, it spreads its legs as widely as possible, so as to stretch the membrane into a wide, flat surface. It then boldly springs into the air, and sweeps upon its mark with a sort of skimming movement. Except that it does not revolve, it passes through the air much after the fashion of an oyster-shell when thrown horizontally.

Many mammalia are constructed after a similar fashion, such as the Colugo, or Flying Monkey, the Flying Mice, and the Flying Phalangists, or “Opossums,” as they are popularly called.

In the centre is the Flying Dragon, or small lizard, which very probably gave rise to the fabled Dragons in which our ancestors so devoutly believed. Indeed, on looking back at the old illustrated works on Natural History, there can be but little doubt on the subject.

In this creature, the ribs, instead of the legs, carry the flat and elastic membranes. When simply crawling on the branches, after the manner of tree-lizards, the ribs lie flat against the sides, and the membranes collapse, so that the shape of the body is little different from that of any crawling lizard.

But the ribs are movable at will, and, when the creature wishes to pass from one tree to another, it extends the ribs, stretches the membranes, and launches itself into the air, exactly as has been narrated of the Flying Squirrel.

The lowest figure represents a most extraordinary animal, called the Flying Frog. Only one specimen is believed to be known, and that was discovered in Borneo by Mr. Wallace.

Here we have an analogy with the bats of the present day and the pterodactyles of the past, namely, the elongation of the toes, and the stretching of a web between them. In the two latter animals, however, only the toes of the two fore-legs are elongated, whereas, with the Flying Frog, the elongation is found in both pairs of limbs. The ends of the toes are furnished with adhesive pads, like those of the tree-frogs, to which it is probably related.

By means of the four membranes, the creature is able to sweep through the air for some distance, and, indeed, this power was the reason why it was caught. It was seen to skim from one tree to another, and was immediately secured. Had it remained sticking on the tree, it would probably have escaped observation.

Weight of Air.

We have already noticed that hydrogen gas is fourteen times lighter than air, and infer necessarily that the weight of the atmosphere must be very considerable if so heavy an object as a balloon, with its car, instruments, sand-bags, and passengers, can rise and float in it.

We are not conscious of its weight, because it permeates us, and the pressure is neutralised. But, in fact, we live at the bottom of a vast ocean which we call the atmosphere; and as, on an average, there is a pressure of fifteen pounds on every square inch of surface, we have to sustain an almost incredible weight. Let, for example, any one measure the surface of his own hand, reduce it to square inches, add together fifteen pounds for every square inch, and he will then appreciate the weight of the atmospheric ocean in which we live. On an average, every human being endures a pressure of some ninety thousand pounds.

This ocean is in perpetual movement, sometimes violently, which we call storm; sometimes gently, which we call breeze; and sometimes very gently, which we call calm. There are air-spouts as well as water-spouts; and, in fact, the water-spout is nothing but a continuance of the air-spout, as is shown by the moving sand-columns of the desert. Whatever may be the character of the winds, as we call this movement, the air is never for a moment still; and, indeed, were it to be still for any time, the whole human race would perish.

How winds are caused we shall see by the aid of the diagram on the left-hand side of the illustration.

The original cause is the sun. His rays fall upon the earth, heating it, and so by radiation heating the air. Now, as has been remarked, heated air will cause a heavy balloon to float through ordinary air, and to carry up a considerable amount of dead weight besides; consequently the heated air must ascend, while cool and heavier air rushes in to take its place, and thus the currents are produced. Were the earth set straight upright, the currents would invariably run in one direction; but, as it is tilted on one side, the needful variety is obtained, and we find the winds blowing from all parts of the compass.

The principle, therefore, of all winds is, that heat expands, and therefore becomes lighter than air at an ordinary temperature.

Were it not that man has taken advantage of this principle, there could not be a deep mine in England. In any deep excavation, even though it be a well, foul air, mostly composed of carbonic acid gas, always collects, and, being much heavier than atmospheric air, lies at the bottom of the pit as surely as hydrogen would rise out of it. To breathe this air is as certain and as sudden death as to take prussic acid, and no mine can be worked as long as “choke-damp” is in it.

In coal mines there is an additional source of danger, namely, the coal gas, which is nearly identical with our coal gas of the streets, and takes fire when brought into contact with flame. To rid the mines of these gases, a simple, ingenious, and effectual remedy is used. A ventilating shaft is made, which reaches from the bottom to the mouth of the pit. At the bottom, diagonal shafts are made, entering the main shaft, as shown on the right hand of the illustration. One of these is connected with a furnace, and the other, or others, open into the mine.

The heat of the furnace rarefies the air in the shaft, causing it to rush upwards with great violence, and so, by creating a partial vacuum, to force the air in the shaft to follow it. The loss of air thus caused is supplied by fresh air from above, which, by the law already described, is obliged to take the place of that which was driven out. Thus a complete circulation of air is kept up, and a well-managed mine has a fresher atmosphere than many houses in which the windows are mostly kept shut, and the only ventilation is accomplished by occasionally open doors.

The “draught” of our domestic chimneys is owing to this principle, and the reason why factory chimneys are built of such enormous height is, that the column of heated air may be increased, and consequently that the draught may be stronger, and the heat of the furnace made fiercer.

The “Steam-blast,” by which the escape steam of engines is sent into the chimney, is another example of this principle, the steam taking the place of the hot air.

Further examples of the weight of the atmosphere are given in the illustration. That on the right represents the common Wheel Barometer, which marks the weight of the air by a hand moving in front of a dial. If the hand moves towards the right, the weight of the air is increasing; if to the left, it is decreasing.

There are certain words, such as Wet, Change, Fair, Dry, &c., on the face of the dial, but they are only conventional, the real test of the weather being the direction in which the hand moves. For example, if with a west wind the hand moves from Dry towards Fair, rain may be expected; whereas, if it should move from Wet to Change with an east wind, we may reasonably think that fine weather is coming.

The whole cause of this revolution of the hand may be found in the weight of the atmosphere.

It is found that a column of water thirty feet high, or a column of mercury thirty inches high, is exactly equal in weight to a column of air of the same diameter, but some forty odd miles high, so that the two columns precisely balance each other.

Suppose, then, the water or mercury to be placed in tubes closed at the top and open at the bottom, the water or mercury will exactly balance the air, and will not escape from the tubes. It necessarily follows that if the air be heavier than usual, it will force the liquid higher into the tubes, and, if it be lighter than usual, will allow them to fall lower. This is the principle of the Barometer.

The mechanism of the hand and dial is shown in the diagram which occupies the centre of the illustration. For convenience, sake the mercury column is mostly employed, but several Water Barometers, some thirty feet in length, have been constructed.

On the left hand is seen a boy engaged in sucking an egg. The plan employed is simple enough. A tolerably large hole is made at one end, and a very small one at the other. The yolk having been broken up by a long needle, or similar implement, the larger hole is placed to the lips, and, suction being used, the contents pass into the mouth.

Were it not for the hole at the end opposite the mouth, it would be impossible to extract the contents, but the air rushes through the aperture, and so forces out the contents of the egg.

Above is a representation of the way in which Sugar-cane is sucked. The reader probably knows that the Sugar-cane, like the wheat-stem, has knots at certain intervals, which divide the cane into a number of separate parts.

There is quite an art in sucking the Sugar-cane. If a joint be cut off, and the lips applied to the end, not a drop of the sweet juice would be extracted. But if a notch be cut close to the joint, as shown in the illustration, the air can gain access, and then the juice flows easily enough.

It has already been mentioned that air expands when heated. The same rule holds good when applied to other objects, such as the various liquids, metals, &c. A very familiar example of this fact is the “boiling over” of water, when the vessel has been filled too much to allow for the expansion of the heated liquid.

Advantage has been taken of this principle in the formation of the Thermometer, a word which signifies “heat-measurer.” Liquid of some kind is placed in an hermetically sealed tube, generally terminating with a bulb, and in proportion to the heat the liquid expands, and is forced up the tube.

Any liquid will answer to a certain extent, but, as water freezes at 32°, it would be useless for measuring degrees of cold below the freezing point. Coloured spirits of wine are used; but the very best liquid is mercury, which is a metal in a state of fusion.

This expansion by heat is so powerful in iron, that it is utilised in several ways.

Take, for example, wheel-making. The iron tire is made rather smaller than the wheel, and is then placed in a fire until it is red-hot. It then expands so much that it can be easily slipped over the wheel as it lies on the ground. Cold water is then dashed on it, and the tire contracts with tremendous force, binding the parts of the wheel firmly together.

In all buildings where iron is much used, such as iron bridges, iron beams, &c., it is necessary to make allowance at both ends, so as to permit the iron to expand on a hot day and contract on a cool one. Buildings formed of stone and iron were once thought to be safe in case of fire. They are now known to be just the contrary, the stone flying with the heat, and the iron expanding.

USEFUL ARTS.

CHAPTER XII.