CHAPTER XI.

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

Ascent and Descent.--The Balloon and the Parachute.--Description of the Balloon.--The Montgolfier Balloon.--Causes of its Abandonment.--The Gas Balloon.--Hydrogen Gas and its Manufacture.--The Gossamer Spider.--Reasons of its Ascent and Descent.--Many Species of Gossamers.--Description of the Parachute.--Its Mode of Action.--A Balloon converted into a Parachute.--Toy Parachutes.--Natural Parachutes.--The Dandelion Seed and its Structure.--The Flying Squirrel.--The Flying Monkey.--Flying Mice and Flying Opossums.--The Flying Dragon and its Pseudo-wings.--The Flying Frog.--Weight of Air.--Pressure per Square Inch.--The Air Ocean and its Storms.--Principle of Air-currents.--The Sun, the Earth, and the Air.--Ventilation of Mines.--Choke-damp and Fire-damp.--The Air-shafts.--Chimneys of Factories.--The Steam-blast.--The Barometer, and Mode of its Construction.--Water and Mercury.--Sucking Eggs and Sugar-cane.--Expansion of Water and Metals by Heat.--The Thermometer.--Wheel-making.

AËROSTATICS.

We will begin this chapter with the only two modes at present known by which man can ascend from the earth or descend to it with safety, namely, the Balloon and the Parachute, the latter being generally attached to the former, and detachable at pleasure.

The Balloon is, in fact, as its name imports, a large, hollow, air-tight ball, filled with some substance lighter than ordinary air. The original Balloons by Montgolfier were filled with heated air exactly like our toy fire-balloons. Just as the supply of hot air is kept up in them by a sponge dipped in lighted spirits of wine, so in Montgolfier’s balloons the same object was attained by straw which was kept continually burning in a grate.

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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.