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.
The Cassava Press and its Structure.—Mode of using it.—The Siamese Link.—An ingenious Robbery.—Muscles and their Mode of Action.—Human Arms and Steelyard.—Change of Direction.—The Human Hand and Wrist.—Story of a Carpenter.—The Pulley.—Reduction by Friction.—Past and present Engines.—Oiling Machines.—Treatment of the Sewing Machine.—Use of Paraffine.—Disuse of Machine hurtful.—Human Joints.—Synovia and its Value.—Disuse of Joints hurtful.—The Lazy-tongs and its Usefulness to Invalids.—Suggestions for Improvement.—Larva of the Dragonfly and its Mask.—Curious Mode of seizing Prey.—Proboscis of the Housefly, and Mode of using it.—The Apple-parer.—Squirrel and Nut.—Structure of Teeth.—Rock-splitting.—Powers of Ice.—How the Pebble-ridge is formed.—Splitting Stones by Moisture.—The Diamond Drill.—Ovipositor of the Gad-fly.—Curious Similitude of Structure.