rushes up the pipe C, (fig. 6), forces the valve B open, and fills the barrel between A and B, and as the water above A keeps the valve A shut down, the water is lifted by the piston till it flows out at the spout D. As a pump is larger than a squirt, and the water raised by it is much heavier than that raised by a squirt, we could not readily pump by standing on it and pulling the piston-rod straight up by a ring at the top, and therefore we have to make use of a lever, in the shape of a pump-handle, which prizes up the piston when we throw our weight upon the piston and press the handle down.-Rev. J. Ridgway. THE STEAM ENGINE. AFTER our lesson on the pump we shall be able to understand a little about the steam engine. We must go back again to our old friend, the squirt, which you will remember I told you was made up of a cylinder, with a piston fitting tight into it, and worked up and down the cylinder by a piston-rod. You know that you work this piston up and down the cylinder by pulling the ring at the top with your finger, and then pressing it down; but we want now to see how we can work it up and down without touching it. You have seen a kettle, when the water in it boils, how the steam rushes out of the spout and makes the lid dance about; and the steam comes frothing out all round it. If we were to put a cork into the spout of a boiling kettle full of water, the steam would blow the lid quite off by the force of the steam. Suppose we cork up the spout, and, instead of the lid, fix a large cork bung into the top of the kettle, and then, through a hole bored in this bung, push down the nose of the squirt, what will happen? Why, the force of steam will push the piston of the squirt quite up to Fig. 1. the top (just as it would force the lid off), and if you do not pull it out quickly, it will either blow out the bung or burst the kettle. So you see what force steam has. But we want to get the piston pushed down again; and if we cut off the nose at the bottom of the squirt, and fix in a flat bottom with an opening at the side, B, we can let in the steam to raise the piston, A, and then, by putting a pipe into it at the top, C, admit steam above to push the piston down again. Now, this Fig. 2. would work very well once, but then the cylinder would be full of steam, and, as soon as we let fresh steam in, it would burst; so we must have some contrivance to let the steam out, as soon as it has pushed up the piston, and let it out again when it has thrust it down. This is managed without putting any more pipes into the cylinder, and we have only to carry on our drawing of the squirt in the last figure, to see how these two pipes are made so as to both let the steam in and let it out again. But as we have cut off the nose of the squirt we cannot push it through the bung at the top of the kettle, so we must put it at the side, and as we do not want the kettle spout we will cut it off, bung up the hole, and insert a pipe, D, into the lid so as to get the most steam, (as steam always rises to the top). A look at the figure above will explain how the steam is made to do its work, and then go off to play. We have our kettle on the fire and the squirt at its side. These are now the boiler and cylinder of a steam engine. As soon as the water boils, the steam rushes up the pipe D,. descends to B, in the direction of the arrows, and pushes up the piston, A, till it gets to the dotted line F. As soon as it gets there, a little rod at the top of the piston turns the stop-cock, E (which is on the same principle as a water tap), in the opposite direction (see fig. 4), and S. VI. R lets in the steam from the boiler above the piston at C, and pushes it down. As it descends, it forces the steam, that had raised it, downwards, and sends it up the pipe, B, and out of the "steam-escape," G. Again it turns the cock, E, which lets the steam run down the pipe, B, under the piston, which sends it up again, forcing the steam above it out at the pipe, C, and after going almost in a circle (see fig. 3), it escapes also by the pipe, G. If you will study each of these two figures, and follow the direction of the arrows, first in one and then the other, you will very easily understand how these two pipes first let the steam in, and then out again, by merely turning the stop-tap, E, after each admission of steam. Well, but what is the use of all this lifting up and forcing down of the piston? How does that make an engine draw a train of carriages, or turn the machinery in a factory? We shall soon see. Every time the piston goes up it moves the piston-rod up, and every time it goes down it moves the piston-rod down; so you see we have this rod constantly kept working backwards and forwards, or up and down. I daresay you have watched a knife-grinder, or a man at a turning lathe, working the wheel to grind the knives, or to turn the piece of wood that is to be cut. He turns the wheels above by working his foot up and down on a piece of wood (a treadle). Working his foot up and down is just what the steam Fig. 5. does, and the foot and the steam both work a straight piece of iron (a piston-rod) up and down. Let us see what it is like in a knife The grinding machine. B C Fig. 6. the foot pulls the crank down; and, letting it go, swings it up again by the impetus of the wheel, D, at the end of the axle. This wheel is called the flywheel, which in a factory is very large, so as to get a great impetus, and it is by she whirling round of this great wheel that all the machinery is kept working. The way in which the piston-rod works the crank will be seen by fig. 7. Attached to the end of the rod is a piece of iron, shaped like the letter T, the two little drop pieces falling from the top bar of the T being lengthened below the bottom of the centre line. These side pieces are attached, one to the fly-wheel, and the other to the crank of the axle, so that as it rises and falls it pulls up or presses down the wheel. But in a locomotive engine this T-piece is attached to a wheel on each side of the engine, so that it is working two wheels at once; but those used on a railroad have generally two cylinders, one to work the wheel on one side and the other that on the other. Now, if these worked exactly together, and they happened to start with the two pistons pulling the cranks straight at the same time, they would not turn the wheel at all. If you watch the knife-grinder you will see he gives the wheel a spin round with his hand before he begins to use his foot, just to start it; but we cannot do so with a steam engine on account of the weight, so the two pistons work alternately, and when one is at the weakest point, where the engine has no power to turn it, the other is at the strongest, and carries the other on with it. You will see this by the figure 8. When the crank, A, is in a straight line with the piston-rod, B, it cannot turn the wheel at all; but you will see the piston is at the bottom of the cylinder, and will be soon |