"This morning on my way to school," said Mr. Wilson, "I saw some brewer"s draymen unloading great heavy barrels from their wagon, by sliding them down a sort of slanting ladder to the ground. When they had got themto the ground they lowered them into the cellar in the same way. Besides this I saw them raise other barrels, first from the cellar to the ground, and then from the ground to the wagon, by rolling them up the same ladder. This slanting ladder is a machine-very simple, and at the same time very useful. By means of it the men were able to move those heavy bodies with ease from their awkwardpositions. We call it the inclined plane. Let us see what we can learn about it.
"Here is a smooth, polished board. We will lay it on the table, so that it rests in a horizontal position, and then place on it one or two objects, also having a smooth, polished surface. The objects, whatever they are, rest on this horizontal plane, showing no tendency to move.
"Now we will rest one end of the board on this block of wood. It is a plane still, but, instead of being horizontal, it slopes; it is, in fact, an inclined plane. I want you to notice how the same smooth objects act when we place them onthe board now. They, one and all, roll quickly down the slope. We will next change the smooth polished board for one made of rough deal, incline it at the same angle, and place the same smooth objects on it as before. They do not move as quickly now; some of them, indeed, do not move at all, but remain at rest on the slope.
"The rough inequalities on the surface cause a rubbing or friction between the objects and the board, and it is this friction that retards the movement of the objects, and actually brings some of them to a standstill. The samethings rolled rapidly down the smooth board, because there was less friction. Indeed, in considering the inclined plane, we must for the present think of the surface as being perfectly smooth, so as to cause no friction, although practically this is never quite true.
"Let us glance for a moment in another direction. The slope of a hill is, of course, an inclined plane. Now, suppose I set one of you boys the task of taking to the top of a hill a load too heavy for you to lift, how would you act? I think, if it were a round body that would roll, you would roll it up the hill; if it were something with a flat surface, such as a box, you would probably attach a rope to it, and drag it up. What kind of hill would present the easier task-one with a steep slope, or one with a gentleslope? You would find it much easier to roll or drag theload up the gentle slope than up the steep one.
"I want to show you now the reason for this, and to make clear to you what mechanical advantage the inclined plane gives. Let us return to our smooth, polished plane.
