"Our lesson this morning," said Mr. Wilson, "is to teach us the principle of the steam-engine, but before proceeding to examine the steam-engine itself, I mustdirect your thoughts back to the piston and the long- necked flask of our last experiment.
"Why did the piston rise in the neck of the flask?""As the water boiled in the flask, some of it was converted into steam, sir," said Fred. "It was the expansive force of the steam, at the moment of the change, that sent the piston up.""Why did the piston not fly out of the neck altogether?" "It would have done so, sir, but you cooled the flask.""What was the result of this cooling?"
"It condensed the steam into water, and, as the water did not occupy so much apace as the steam from which it was formed, a vacuum was leftin the flask, and the piston moved d o w n w a r d s a g a i n o w i n g t o t h e pressure of the air outside.""You have followed the experiment wel l, my lad," said Mr. Wilson . "We shall have no difficulty now in proceeding to examine the working of the steam-engine, for we shallfind that there is a remarkable resemblance in principlebetween it and our flask and piston.
"The steam-engine consists essentially of a cylinder, fitted with an air-tight piston, which works by the elastic force of the steam, up and down, or backwards and forwards, as the case may be. Here we have the principle of the flask and piston, except that in the steam-engine both the up movement and the down are brought about directly by the steam. The piston, you remember, moves downwards in the neck of the flask, not by the action of the steam at all, but by the pressure of the outer air after that steam has been condensed.
"Now let us find out how, in the steam-engine, both movements are brought about by the steam itself.
"The water is boiled in a great boiler, and the steam isled, by means of a pipe, from the boiler into the cylinder. But before it can reach the cylinder, it is first admitted into a small box-like chamber, called the valve-chest.
"In one side of this valve-chest are three holes, twoof them communicating respectively with the upper and lower parts of the cylinder. The third, which is exactly midway between the two others, is in communication with a chamber called the condenser.
"In front of the three apertures is a smooth, flat plate of steel, capable of moving up and down. It is known as the slide-valve, and is just long enough to cover the center hole and one of the others. When, therefore, one of these is closed the other is always open.
"If you look at this diagram of the engine, you will see that, as there are two apertures leading to the cylinder- one above, the other below, the piston-it would not do to have both open at once. If that were the case, the steam would force its way through both; there would be equal pressure on either side of the piston, and of course no movement could take place.
"Still keeping the diagram before us, let us imagine the slide-valve to be in position to cover, and so close, the lower of the two apertures leading to the cylinder. The upper one is open, and allows the steam to enter the cylinder above the piston. The elastic force of this steampresses upon the piston and moves it downwards.
"In the meantime the slide-valve has moved up, and isnow closing the upper aperture, leaving the lower one free for the steam to enter.
"When, therefore, the steam enters by the lower aperture, and by its elastic force presses upon the piston, it tends to force it upward.
"But there is already steam in the upper part of the cylinder above the piston. How can the piston move upwards in opposition to this? The fact is this steam in the upper part of the cylinder has already done its work in forcing the piston down; it is no longer wanted. It escapes through the centre of the three apertures, from the cylinder, into the condenser, at the very moment when that which enters through the lower aperture is exerting upward pressure on the piston. The piston, finding no opposition from above, rises by the elastic force of the steam in the lower part of the cylinder.
"The rising of the piston brings the slide-valve down again; the same movements are repeated, and so on perpetually. This is a deion of a very simple and somewhat primitive steam-engine, but whatever work an engine is meant to do, it is all accomplished by the two simple backward and forward movements of the piston in the cylinder, and they are both brought about by the elastic force of steam."