Now what is it that makes steam so powerful? To understand this, look at a locomotive when it is standing still, with its boiler full of steam. A valve is opened, and out rushes the steam, spreading itself, and turning into a cloud of fog. It is this trying to spread itself that makes the steam so powerful. If the valve were not opened the boiler might explode; for, as the steam is not used as it is while the locomotive is going, there would be more and more of it in the boiler, for it is making all the time. The force with which it rushes out when the valve is opened shows how much power it exerts in trying to spread itself.
What makes the lid of a tea-kettle rattle.
You see the same thing in the rattling of the lid of a tea-kettle when the water is boiling in it. The steam which is made has not room in the kettle to spread itself. It gets out, therefore, wherever it can. It blows out at the nose; and if the water boils very briskly, it can not get out fast enough at the nose, and so it keeps lifting the lid and puffing out there.
Bursting of boilers.
When the steam is shut up very tightly, as it is in the boiler of a steam-engine, it has very great power, and the more steam there is thus shut up the greater is the power. Men are sometimes careless about this, and get so much steam made in the boiler that it bursts. This is just as the roasted chestnut is burst by the steam and heated air that are in it. The boiler bears the pressure of the steam as long as it can. This pressure is made by the steam’s trying to spread itself, or by its expansive force, as it is expressed. After a while, the steam being made all the time, and being crowded together, as we may say, the boiler all at once gives way with a loud noise. The noise is caused in the same way as the pop of the roasted chestnut. It is the sudden shaking that the escaping steam gives to the air.
Safety valves.
There is always a safety-valve to a steam-engine. This is commonly kept shut by a weight which is upon it. But when there comes to be a great deal of steam in the boiler, it has expansive power enough to raise the valve, and so some of the steam escapes. This prevents the boiler from bursting, and hence the valve is called a safety-valve. Now, if there happen to be a weak place in the boiler, and the weight on the valve is heavier than it should be, the weak place will be apt to give way rather than the valve, and an explosion results. Many a boiler is burst in this way.
Steam compared to powder.
I have told you about another way in which boilers are burst in the chapter on Powder. It is this. The boiler is carelessly left to get nearly empty, and the fire therefore makes it very hot. Then, when more water is let into it, a great deal of steam is made all at once. This exerts its expansive force with such violence that the boiler gives way. You can understand how this is if you see a little water dropped upon red-hot iron. A great cloud of steam arises, spreading itself in the air, and you can see that if this were pent up it would make a strong pressure in trying to get free.
Boy melting lead.