Uncle Paul took a glass vial, put a finger’s breadth of water into it, corked it tightly with a cork stopper, and then tied the cork with a piece of wire. The vial thus prepared was put on the ashes before the fire. Then he took Emile, Jules, and Claire, and drew them quickly into the garden, to see from a distance what would happen, without fear of being injured by the explosion. They waited a few minutes, then _boom!_ They ran up and found the vial broken into a thousand pieces scattered here and there with extreme violence. “The cause of the explosion and the bursting of the bottle was the steam, which, having no way of escape, accumulated and exerted against its prison walls a stronger and stronger pressure as the temperature rose. A time then came when the vial could no longer resist the pressure of the steam, and it burst to pieces. They call elastic force the pressure exerted by steam on the inside of pots that hold it prisoner. The greater the heat, the stronger the pressure. With heat enough it may acquire an irresistible power, capable of bursting, not only a glass bottle, but also the thickest, most solid pots of iron, bronze, or any other very resistant material. Is it necessary to say that under those conditions the explosion is terrific? The fragments of the pot are thrown with a violence comparable to that of a cannon-ball or a bursting bomb. Everything standing in the way is broken or knocked down. Powder does not produce more terrific results. What I have just shown you with the glass vial is also not without some danger. You can be blinded with this dangerous experiment, which it is well to see once under proper precautions, but which it would be imprudent for you to repeat. I forbid you all, understand, to heat water in a closed vial; it is a game that might cost you your eyesight. If you should disobey me on this point, good-by to stories; I would not keep you with me any longer.” “Don’t be afraid, Uncle,” Jules hastened to interpose; “we will be careful not to repeat the experiment; it is too dangerous.” “Now you know what makes the locomotive and a great many other machines move. In a strong boiler, tightly closed, steam is formed by the action of a hot furnace. This steam, of an enormous power, makes every effort to escape. It presses particularly on a piece placed for that purpose, which it chases before it. From that a movement results that sets everything going, as you will see in the case of the locomotive. To conclude, let us remember that in every steam engine the essential thing, the generator of the force, is a boiler, a closed pot that boils.” ------------------------------------------------------------------------ CHAPTER XLIX THE LOCOMOTIVE UNCLE PAUL showed his nephews the following picture, and explained it to them. [Illustration: An old-time Locomotive]