The examinations were over, the class promotions were all made, and the boys (especially those of the upper classes) were looking forward with considerable eagerness to the new year"s course, on which they were about to embark. None were more eager and enthusiastic than our young friends the two cousins, whom we have watched with so much pride and pleasure year after year.
"There is just one thing more to do," said Mr. Wilson, "and then we can settle down to work. We must have the desk brought into our new room. Put your shoulder to it, Fred, and push it along. What! can"t you move it?" he said again, as he saw Fred exerting all his strength to no purpose. "One or two of you help him. That"s right; you can push it along easy now."Sure enough they did go easily-in fact a little too easily, as boys very frequently do; for Mr. Wilson was just in time to prevent them from running the desk into the glass front of the museum cupboard. This he did by pushing hard at the side of the desk, so as to change thedirection in which it was moving, and make it veer off sideways. Then by exerting all his strength, and pushingin the opposite direction to that in which the boys were pushing, he brought the desk to a standstill, and the cupboard was saved. It was all the work of a minute, and the boys scarcely knew what had happened till Mr. Wilson said sternly, " Boys, you should be more careful; I did not wish you to push like that." After they had got to work, however, he thought over the little incident, and determined to make use of it to introduce his first lesson in the new science course.
When the time for the lesson came round, he began by calling the attention of the class to the moving of the desk, and showed them that in order to move the heavy body at all it was necessary for them to make an effort- to put out their strength. "Whenever we lift a heavyweight, whenever we set a body in motion, whenever we stop a body that is already in motion, we are conscious of exerting some effort. The name which we give to this effort is force. We say that force is any cause which tends to move a body, to change the direction of its movement, or to arrest it when in motion. The particular kind of force which you used just now in moving the desk was your own bodily strength. This we call muscular force.
"Fred tried to move the desk alone, but he found his muscular force was not sufficient, and I called upon some of you to help him. Uniting your exertions with his, and pushing in the same direction, you were able to propel the desk along the floor, but as soon as I beganto push it at the side there was a change in the direction of its movement, and when I pushed in the opposite direction to you, it had the effect of bringing the desk to a standstill. I was exerting muscular force as well as you; that force, acting on a certain point of the desk, was able to change the course of its movement; when applied at another point it arrested its movement altogether.
"Man, in his primitive state, learned first to use this muscular force. In fact, the only force he employed was his own muscular force, and that of the animals he subdued. Savage nations of today know very little of any other forces beyond this.
"But civilized man gradually learned, and is learning still, that there are many wonderful forces existing around him, and his ingenuity teaches him how to utilize them. His observation soon taught him, for example, that wind is a force, and after a time he learned to turn this force to account in various ways. In like manner, things floating down a stream suggested to him the force of running water, and this became, through his ingenuity, another powerful working agent.
"In fact, science has taught man gradually to utilize these and other forces, such as gravity, cohesion, heat, steam, chemical force, magnetism, electricity. We group these all under the name of natural forces, i.e. the forces of nature.
"Let us go back once more to the work of moving the desk. When Fred tried to do it we might have noticedthree things: first, he applied force to a particular part of the desk; secondly, he pushed in a certain direction; and, thirdly, whenhis own muscular force was insufficient, he increased it by the help of more force, and so did the work. Inall our attempts to utilize a force, these same three things must be considered- (1) its point of application, i.e. the portion of matter on which it acts; (2) its direction; (3) its intensity or magnitude. In almost every instance a force can only be turned to account after we have alteredit in one or other, or all of these particulars, to suit our requirements.
"This will be readily understood by a glance at the construction and working of a windmill or a watermill. The essential part of either mill is the great millstone, which has to be set in motion. The force to accomplish this work is, in one case,the wind, in the other the running water. The wind-force acts on thesails, the water-force on the large
wheel; and the millstone cannot, in either case, move till the force is transmitted to itself.
"But not only must the force be transmitted to the stone, its direction must be changed. The sails of the one and the wheel of the other revolve vertically, the millstone horizontally. The whole contrivance of the mill is to effect these changes in the force, and so render it available for the work of grinding corn.
"We call any contrivance for transmitting a force from one point to another, or for altering the direction of movement, a machine."Lesson 02