Frosty rails.
Now see how it is that the driving-wheels carry along the locomotive. They do it by their rubbing on the rails of the road. If the rails and the wheels were very smooth indeed, the locomotive would not get along so well. We sometimes see this in a frosty morning, when the rails are very slippery. With the rails so smooth, the wheels slip; and they slip back as readily as forward, just as it is with any one walking on the ice. They sometimes throw some sand on the rails when they are icy to give the locomotive a start, as people scatter sand and ashes on icy sidewalks that they may walk easily on them.
After the wheels of a locomotive are once well started on the frosty rails, they will go well enough. Indeed, it is sometimes rather difficult to stop them, because they slide along so easily, for the motion is partly sliding and partly rolling when the rails are so smooth. It is for the same reason that one can not stop easily when he is running on the ice. If he is running on the ground, he can stop very readily, because the ground is rough, and his feet rub upon it, and they do not slip as they do on the ice.
Operation of brakes.
The way that brakes, as they are called, stop a train of cars, I will explain to you. You know that the brakeman on each car turns around a ring-like thing as hard as he can when the signal is given to stop the cars. In doing this, he brings the brakes against the wheels of the cars, and the rubbing soon stops them.
When they want to stop the cars very quickly, they do another thing besides using the brakes. They manage to make the driving-wheels of the locomotive roll backward instead of forward. In this way the rubbing is backward on the rails; and as long as the locomotive is going forward, the wheels slide forward instead of rolling, as they commonly do.
Oiling machinery.
Joints of our bodies.
You see that sometimes we want friction, and sometimes the less we have of it the better. We want the friction of the driving-wheels of a locomotive on the track. But in the middle of these wheels, where they turn round on their axles, we want to have as little friction as possible. It is for this reason that all the wheels of the cars and of the locomotive are kept oiled at this part. So, also, we grease the wheels of carriages and oil the joints of machinery to lessen the friction. You will recollect that in the chapter on the bones, in Part Second, I told you that the joints of our bodies are tipped with a very smooth substance, and that they are kept oiled, so that there may be little friction in their motions.