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"T O-DAY I will show you a couple of experiments we used to perform for our own amusement when I was of your age. You may like to repeat them yourselves at the first opportunity. They will teach you how it is that steam has become the mightiest toiler man has ever had in his service.
"We have seen how the vapor drawn up by the sun's heat forms clouds, which let fall rain and snow, the source of all the streams that flow on our planet. Let us try to understand by what device man has succeeded in harnessing the steam produced by the heating of water, so that he now commands a servant of unlimited strength and fit for all sorts of tasks.
"Take a small bottle, a vial hardly larger than an egg, and pour into it a little water, let us say about a spoonful; then stop the bottle with a good cork so as to make it air-tight. After you have done this put your bottle before the fire, on the warm ashes and not far from the red-hot coals. Then retire some distance, to be out of range of any possible spattering of hot water, and await results.
"The water heats, gives forth steam, and finally boils. All at once there comes a pop, and up goes the cork into the air, propelled by the thrust of the imprisoned steam. To make this water-pistol go off with the best effect, the stopper must fit exactly and let no steam escape prematurely. If allowed to escape gradually as it formed, in the manner familiar to you in the ordinary tea-kettle, the steam would accomplish nothing whatever.
"But the corkstopper, if it fits properly, completely closes the mouth of the bottle until the steam has acquired sufficient volume and force to drive it out with a sharp report. It might even offer too great resistance, and in that case the bottle itself would have to give way, shattered by the irresistible pressure of the steam.
"Therefore this experiment should be conducted with some caution; otherwise it might cause burns from scalding steam and wounds from broken glass. Keep at a safe distance, then, while the water is heating, but without allowing yourselves to be unduly alarmed, for the danger is really very slight, in fact almost negligible. Unless the cork be tied down it will be almost sure to yield before the bottle itself bursts, and there will be nothing to fear.
"But there is still another more convenient method of performing the experiment, and one that will excite no alarm even in the most timid. You are familiar with a certain form of penholder in two parts, one fitting into the other. Into the shorter section the pen is inserted, while the other, which is much longer, serves as handle and is grasped by the fingers. It also plays the part of case or sheath, being cylindrical in form and sheathing the pen when not in use. Well, now, take one of these little metal cylinders and pour into it a few drops of water; then cut a slice of potato or carrot, making it about as thick as your finger, and through it thrust the open end of your penholder. The sharp, circular edge of the orifice will cut a little round plug which will remain in the end of the holder as a stopper. Your water-pistol is now loaded and ready to be discharged.
"Hold the end opposite the stopper in the flame of a candle, taking care to use a pair of nippers or a split stick of wood for the purpose, to avoid burning your fingers. Soon you will hear a noise inside the penholder, indicating that the water is beginning to boil and steam is forming. Then the pistol will go off, sending the stopper to some distance, while a jet of steam escapes from the cylinder.
"Your little piece of artillery is soon reloaded. A few more drops of water, another potato plug, and there you have the gun ready for the second discharge. In this manner you may keep up as long as you please this miniature cannonade in which water takes the place of gunpowder and a harmless bit of potato serves as cannon-ball.
"After performing these experiments your inquiring mind will seek the cause of this power possessed by steam. This vapor of water is seen by us rising from drying linen with such tranquillity, such lack of all appearance of strength, that no one pays it any heed. From a boiling pot, again, we see it ascending as harmlessly as possible. But in the experiments with the bottle and the penholder it shows itself possessed of the explosive force of gunpowder. Whence does it derive this force?
"Steam may be likened to a spring which, if confined in too restricted a space and pressed down upon itself, exerts a repulsive force on obstacles opposing it, but is no sooner set free than it ceases to act. It is thus that we may conceive of the power of steam: free to expand at will, steam has no appreciable energy; but closely confined and receiving constant additions, it becomes more and more compressed and puts forth increasing efforts to escape, until at last every bond is burst, no matter how strong.
"If the bottle were left uncorked and the penholder unstopped, the steam generated in them would escape freely as fast as it formed. It could not accumulate and thus become a sort of compressed spring pushing against everything in its way. But with the cork or the potato plug doing its part the situation is entirely changed. Confined in too narrow a space, the steam accumulates until finally it gains strength enough to hurl to a distance, and with a loud report, the obstacle opposing it.
"Of course it is to be understood that this pressure of the steam is exerted not merely against the stopper of the bottle and the plug of the penholder; it is exerted with equal force against all parts of the interior of the bottle and of the penholder, but only the point of least resistance yields to the pressure. Less resistant than the glass of the bottle and the metal of the penholder, the cork stopper and the potato plug give way and are projected to a distance. If they offered sufficient resistance, the bottle and the penholder would burst.
"Perhaps I have not yet made sufficiently clear to you the power of steam pent up in a narrow space. There is a plaything dear to youngsters of your age, the elder-wood pop-gun, which will help to make the matter plain. You are cleverer than I, no doubt, in making and operating this toy. Never mind; for the benefit of any that may be uninformed I will describe this famous pop-gun and tell how it is operated.
"First you select from the hedge a suitable piece of elder-wood—a piece as large around as the neck of a bottle, very straight and even, and of about a span's length. The elder is unusually rich in pith, and this is easily removable; all you have to do is to push it out with a slender stick, whereupon you have a substantial and serviceable tube for the barrel of your gun. The next thing to do is to whittle out a ramrod with your jack-knife. This ramrod is simply a slender piece of wood that will fit into the tube and reach from one end to the other. In order to make it easier to operate, one extremity of the ramrod is left larger than the rest and serves as handle. That finishes the outfit, and the gun is ready to load.
"A wad of frayed tow from some old bit of cord that is untwisting and coming to pieces is chewed and rechewed and formed into a plug, which is then stuck into the pop-gun and pushed through to the farther end with the ramrod. There you have one outlet of the tube tightly sealed, so that nothing, not even air, can pass through. A second plug of tow, similar to the first, is next thrust into the free opening. Then, resting the handle of the ramrod against your chest, you force the rod itself into the gun-barrel until, with a loud pop, the forward plug is projected like a bullet. But projected by what?
"What was there, to begin with, in the pop-gun, between the two plugs? Nothing, you will doubtless reply. Nothing visible, certainly; but invisibility does not mean nothingness. There was air, which we cannot see, but which is none the less real matter. This air was confined between two substantial plugs of tow, which allowed no passage through at either end of the gun. At first the air occupied the entire length of the gun-barrel, a space quite sufficient and affording ample elbow-room.
"But now the ramrod comes into play. The rear plug is pushed toward the forward one so that the air between the two is compressed within an ever-shortening canal; and the more the space diminishes, the more this air strives to regain its former volume, just as a spring when pressed down tries to return to its original position. At last the moment arrives when the thrust of the imprisoned air ejects the forward plug, the only part of the gun that can give way.
"So it is that steam, crowded by the heat that generates it into a space too small for it, behaves exactly like the air forced by the ramrod into a shorter and shorter section of the pop-gun. In each instance there is the same striving for more room, the same violent pressure against opposing obstacles."