"Now," said Mr. Gregg, "I have often told you I would explain to you some things about the mechanical powers, and this seems to be the most appropriate time to begin, as you have fresh in your minds the application of the lever as we used it to-day in raising and moving the big rock. I am glad to see that Fred grasped the idea so readily, for that encourages me to let him use his own judgment while doing this job.
"The lever is known to accomplished mechanics, as 'the first mechanical power', and Archimedes said of it, if he only had one long and strong enough, together with a suitable fulcrum, he could, alone, lift the earth from its place.
"This Archimedes was a celebrated Greek philosopher and mathematician, who lived from about 287 to 212 B. C. The discovery of the law of specific gravity, which I will some day tell you about, is attributed to him. I think George can tell you something about this great man, as I saw him and Jessie the other day reading Plutarch's 'Lives,' in which he is mentioned.
Fig. 4. Principle of lever and fulcrum
"A lever may be formed of any strong, stiff material, wood, iron, steel, or similar stuff, and it may be of any length, or dimensions, according to the purpose for which it is to be used. In theory, it is supposed to have no weight, and is simply figured as a straight line having neither breadth nor thickness. In practice, however, a lever may be a handspike, a pry, a crowbar, a fire poker, a windlass bar, or any other appliance or instrument that can be used for prying. While we may not know the proper name of the little steel tool the dentist employs when preparing one's teeth to receive the filling, by cleaning out the cavities, we are safe in calling it a small lever. When your mother stirs the fire in the grate, she makes a lever of the poker, and bars of the fireplace become fulcrums. The fulcrum is the fixed point on which the lever rests when in use. The force applied is called the power and the object to be acted upon is called the weight. The spaces from the power and the weight, respectively, to the fulcrum, are called the arms of the lever. There are three different ways of using the lever, according to the relative positions of power, weight, and fulcrum. This rough sketch I am drawing on the blackboard ([Fig. 4]) shows the lever being used to raise one end of a heavy stone. Suppose W is a big rock, C will be the fulcrum, B the end of the lever under the stone, and O the power. The weight thrown on the lever by the man at O, raises the stone so that it can be blocked up, the lever and fulcrum arranged for another lift, and the process repeated. This can be continued until the stone is raised to the height required, or until it is turned over. This method applies to the raising of any sort of weight, engine, boiler, heater, etc.
"In this sketch the distance from B to C shows the short arm of the lever, and the distance from C to O shows the length of the long arm.
