The air or atmosphere, which surrounds us on all sides, exercises a pressure upon everything of 15 lbs. on every square inch of surface. If our little cubical inch box of tin had no air inside it, and no steam, but was absolutely empty, each side, and top, and bottom would have 15 lbs. pressure upon it; which would be evident if it were not very strong, for it would sink in on all sides directly, just as much as if you were to add a weight of 15 lbs. when it was full of air, as it would ordinarily be.
When I spoke of the larger box being exactly filled with steam from the evaporation of the cubic inch of water poured from the smaller box, I supposed it empty of air. The steam from that quantity of water, occupying the place of the air, would also be of the same pressure, 15 lbs. per square inch of surface; and as this only balances the pressure of the atmosphere, which would be, in such a case, pressing in on all sides, the steam would not show any pressure; just as, if you put equal weights into each scale of a balance, the beam of it would remain horizontal, neither scale showing to the outward senses that it had any pressure upon it. But in the second case, we have doubled the quantity of steam, but compelled it to occupy the same space; therefore we have now real, visible pressure of 15 lbs. upon each square inch; and if we again halve the space which the steam has to occupy, or double the quantity of water, we shall obtain a pressure of 30 lbs. beyond the pressure of the atmosphere.
Let us now disregard atmospheric pressure, and fit up such an apparatus as Fig. 56, D. Here we have first our small box, closed on all sides, and from it a small tube rising and entering into the bottom of a larger one, which is very smooth in the inside; in this is a round plate or disc, called a piston, which fits the tube nicely, but not so tight as to prevent it from moving up and down easily; and let a weight of 15 lbs. be laid upon it. Let us suppose this large tube or cylinder to be 1700 times larger than the cubic inch box, into which water is to be poured till full. Now we heat it as before, and when 212° of heat are attained by the water (which is its boiling-point) when it begins to be converted into steam, the piston will be seen to rise, and will gradually ascend, until quite at the top of the tube, because the steam required exactly that amount of room.
Now we have arrived at the same conclusion which we came to before; for you see that not only has the cubic inch of water become a cubic foot of steam (about 1700 to 1728 of its former volume), but it is supporting 15 lbs. weight, which represents that of the atmosphere, and if we could get rid of the latter, a solid weight of 15 lbs. would be thus supported. Now, still neglecting the atmospheric pressure, suppose instead of 15 lbs. we add another 15 lbs., making the weight 30 lbs., down goes our piston again, and stands at about half the height it did before. We have thus, as we had previously, a cubic foot of steam made to occupy half a cubic foot of space, giving a pressure (which is the same as supporting a weight) of 30 lbs.
I ought, perhaps, to add in this place, however, that under 30 lbs. pressure, or atmospheric weight and 15 lbs. additional, the water would not become steam at a temperature of 212°, but it would have to be made much hotter, until a thermometer placed in it would show 252°.
So far we have seen what a cubic inch of water will do when heated to a certain degree, and at first sight it may not seem a great deal. Far from being light work, however, this is actually equal to the work of raising a weight of 1 ton a foot high. Let us prove the fact. Suppose the tube or cylinder to be square instead of round, and that its surface is exactly 1 square inch, how can we give 1700 times the room which is occupied by the water? It is plain that the piston must rise 1700 inches in the 1-inch cylinder or tube, carrying with it, as before, its weight of 15 lbs.—that is, it has raised 15 lbs. 1700 inches, or about 142 feet. But this is the same as 15 times 142 feet raised 1 foot, which is 2130 lbs. raised 1 foot, very nearly a ton, the latter being 2240 lbs. So, after all, you see that our little cubic inch of water is a very good labourer, doing a great deal of work if we supply him with sufficient warmth.
Now this is exactly the principle of the ordinary steam-engine: we have a cylinder in which a piston is very nicely fitted, and we put this cylinder in connection with a boiler, the steam from which drives the piston from one end of the cylinder to the other. In the first engine that was made, the cylinder actually occupied the very position it does in our sketch; it was made to stand upon the top of the boiler, a tap being added in the short pipe below the cylinder, so that the steam could be admitted or shut off at pleasure. But it is plain that although our little engine has done some work, it has stopped at a certain point; there is the piston at the top, and it cannot go any farther; we must get it down again before it can repeat its labour.
How would you do this, boys? Push it down, eh? If you did, you would find it spring up again when you removed your hand, just as if there were underneath it a coiled steel spring; by which, however, you would learn practically what is meant by the elasticity of steam. Besides this, if you push it down, you become the workman, and the engine is only the passive recipient of your own labour. Try another plan; remove the lamp, and see the result—gradually, very gradually, the piston begins to descend.
Take a squirt or syringe, and squirt cold water against the apparatus. Presto! down it goes, now very quickly indeed, and is soon at the bottom of the cylinder. But we may as well try to get useful work done by the descent of the piston as well as by its ascent.
Set it up like Fig. 56, E. Here is a rod or beam, b a c, the middle of which is supported like that of a pair of scales. From one end we hang a scale, and place in it 15 lbs.; and as the piston sinks the weight is raised, and exactly the same work is done as before. Thus was the first engine constructed; but instead of the scale-pan and weight, a pump-rod was attached, and as the piston descended in the cylinder this rod was raised, and the water drawn from the well. This, however, was not called a steam-engine, because the work is not really the effect of the steam, which is only used to produce what is called a vacuum (i.e., an empty space, devoid of air) under the piston. In fact, the up-stroke of the piston was only partly caused by steam, and the rod of the pump was weighted, which helped to draw it up.