Chapter XII.
I now propose to assist the young mechanic in special work, instead of continuing general directions. This will enable me to explain to him various lathe appliances, and other details of mechanical work hitherto passed by.
Of all models which boys (and very big boys too) are desirous to construct, the steam-engine holds the chief place, and deservedly so; for every boy calling himself mechanical, ought to know how this is made, and the general principles of its construction as well. However, I am aware, from experience, that many a youngster, who is even in possession of a model engine, is utterly ignorant of the cause of its motion; although it is a great delight to them to see the steam puffing out, and the wheel revolving “nineteen to the dozen,” as schoolboys say. Now, an engine is a very simple affair, and can be easily explained; and, as I wish my readers to work rationally, I shall show them what they have to do before I tell them how to do it.
Fig. 56.
A, Fig. 56, represents a cubical vessel of tin or any other substance. By cubical, I mean that all its sides are squares, and all exactly equal; each side in the present case is to be 1 inch wide and long, or a square inch. B is a similar vessel, 1 foot cube. It contains, therefore, 1728 cubic inches, or is 1728 times as large in capacity as the first. Now, if I were to fill the little vessel with water and tip it into the second, and put a lamp under it, the water would all soon boil away, as it is called. It would be converted into steam; and the quantity of steam it would produce would exactly fill the larger vessel, without exciting any particular pressure upon its sides.
Steam, thus allowed plenty of elbow room, is like a lazy boy; it will play and curl about very prettily, but will do no work. We must put some sort of pressure, therefore, upon it—confine it, and we shall soon see that, by struggling to escape, it will serve our purpose, and become a most obedient workman. We have, therefore, only to put double the quantity of water into our larger vessel, that is, two cubic inches. We will put on a cover tightly, adding a pipe through which to pour in the water. Soon we shall have the steam formed as before; but it has no longer room enough, and out it comes fizzing and roaring, very savage at having been shut up in so small a cage. And we can make it work too, for if we set up a little fan-wheel of tin right in its way, we shall see it spin round merrily enough; or if we cork the tube lightly, we shall find this cork soon come out with a bang. We have, in fact, already constructed a steam-engine and a steam-gun on a small scale. The pressure in this case is, indeed, not great, but what it is I must now try to explain.