"Y-y-yes—yes, sir."
"Then go ahead."
The people stood looking on, and quite ready to laugh at the poor boy's failure. Ah! she moves. The big wheels turn slowly, and the cold and silent engine rolled slowly backward. For an instant there was a laugh. She was going the wrong way. She moved faster and faster, and the laugh died away. Ah! she's slowing up. She has stopped. It's a failure. No. Tom could be seen turning the reversing bar. The engine gave one loud whistle, and started ahead. Faster and faster! On it came, and rushed past all the people, at twenty miles an hour. How the people cheered and cheered! It was wonderful. Tom was looking straight ahead, like a good engineer. The ladies waved their handkerchiefs, and the boys shouted until they were hoarse. Tom Stayboltt had won the prize.
The engine ran on about half a mile, stopped, and then came slowly back, and stopped just before the President's pretty daughters. Tom came to the window, and took off his hat and bowed politely to the ladies.
"How much pressure have you, Master Engineer?" said the President.
"T-t-twenty—twenty pounds, sir."
Then the school gave three cheers for Tom, and three more for the President, for every one said it was far better to do something than to write the best composition ever seen. Of course every one wished to know just how it was done, and to make it all clear, the President mounted a pile of sleepers, and told them the whole story.
"You all know that in a steam-engine is a boiler and a furnace, or fire-box. Water is put in the boiler, and a fire is made in the furnace precisely as in a tea-kettle on a stove. The water boils in the tea-kettle, and we see the steam escape. In the engine the steam is locked in, and can not escape, and very soon it becomes crowded, and if still kept locked in, it will burst the boiler. Before this can happen, the engineer opens a valve, and permits the steam to enter two oblong iron boxes, called the cylinders. Here it meets a piece of metal, called the piston, that fits the inside of the cylinder pretty closely. It can not get past, and so it pushes the piston away to the other end of the cylinder. As soon as this happens, the valves close of their own accord, and the steam escapes into the open air with a loud puff. Then the steam enters the other end of the cylinder, and drives the piston back again. In this manner the steam pushes the piston to and fro as it tries to escape from the boiler.
"Now there is a rod fastened to the piston, and passing through the end of the cylinder. Each cylinder has one, and these are connected by means of other rods with the great wheels of the locomotive. You now see that the piston, driven forward and backward, moves the wheels, and thus it is the escaping steam moves the engine. These rods you can see outside the engine; the piston and valves are inside, out of sight.
"Now the air is elastic, like steam, and it may be used in any engine in place of steam. If air is pumped into a tight box like a boiler, it may be locked up, or compressed, and if we were to go on pumping, we might burst the boiler with compressed air. Master Stayboltt knew all this, and he also knew that when an empty engine is dragged along the rails by another engine, as happened on our ride out here, the wheels will turn round, and these move the rods and the pistons, and each cylinder works like a pump. Instead of letting steam out, it pushes air back into the boiler, and very soon the boiler is full of elastic compressed air struggling to get out. Master Stayboltt, as soon as the train stopped, opened the valves, and the air rushed out the way it went in, making the pistons move, and the wheels turn round. Of course the air soon ran out, and the engine stopped. This made no difference to us, for Master Stayboltt clearly showed that he had learned his lessons well, and knew how to apply them."