Friction by water and air.
Friction is not confined to solid substances. Any substance can make friction. Water can do it. The rocks over which it flows, or against which it dashes, are worn by its constant friction, just as the constant friction of passing feet in the course of years wears the stone steps of a building which is much frequented.
Air, too, makes friction. It is by friction that the air, moving along over the smooth water, raises it into waves; and it is the friction of the air, as it passes over a field of grain, that gives it the wavy motion which makes it so beautiful.
Earth moves round the sun without friction.
Wherever there is motion on the earth, it is lessened more or less by friction. Nothing moves without rubbing something, but this is not so with the earth as it goes around the sun. As it flies through space so swiftly, it rubs against nothing, not even against air, for the air, as I have told you, goes along with it.
Questions.—What does friction do? What is said about walking on ice? What about sleighing and sliding down hill? What is the contrivance for making heavily-loaded carts go down steep hills safely? How does the steam make the locomotive go? What is the difference between the driving-wheels and the small wheels? What comparison is made about these two kinds of wheels? How do the driving-wheels move the locomotive along? What is said about the rails being too smooth? How is the difficulty remedied? How is it after the locomotive is well agoing when the rails are slippery? What is the comparison about running on the ice? How do brakes operate in stopping the cars? What else is done when they want to stop the cars quickly? What is said about greasing and oiling wheels? What is said about the joints of machinery and the joints of our bodies? What is said about the friction of water on rocks? What about the friction of air? What is true of all motion on the earth? What is said about the earth as it goes around the sun?
CHAPTER XXXV.
CONCLUSION.
Very many things to be learned in this world.
I have thus, in the Three Parts of this book, described to you some of the wonderful things that are all around you upon the earth and in the water. But there are many more things than I have described. In this book you have only begun to learn what is in the world, and you could not learn all if you should study all your lifetime, and even if your life should be as long as Methuselah’s was. But I hope that you will go on to learn as much as you can. With your mind wide awake, you will see and hear, as you go about from day to day, a great many interesting things that I have not mentioned. I have told you about many things in plants; but if you look at different plants as you meet with them, you will soon see that you can learn much about them that you can not find any where in this book. So, also, if you watch animals, large and small, as you see them, you will find many more interesting things in them than I have told you. And the same is true of the subjects of the Third Part—air, water, light, etc. I have only opened to you a few of the leaves in the Book of Nature, and you can go on to open more of them for yourselves.