Mr Barlow then led Tommy into the house, and showed him a stick of about four feet long, with a scale hung at each end. "Now," said he, "if you place this stick over the back of a chair, so that it may rest exactly upon the middle, you see the two scales will just balance each other. So, if I put into each of them an equal weight, they will still remain suspended. In this method we weigh every thing which is bought, only, for the greater convenience, the beam of the scale, which is the same thing as this stick, is generally hung up to something else by its middle. But let us now move the stick, and see what will be the consequence." Mr Barlow then pushed the stick along in such a manner, that when it rested upon the back of the chair, there were three feet of it on one side, and only one on the other. That side which was longest instantly came to the ground as heaviest. "You see," said Mr Barlow, "if we would now balance them, we must put a greater weight on the shortest side; so he kept adding weights, till Tommy found that one pound on the longest side would exactly balance three on the shortest; for, as much as the longer side exceeded the shorter in length, so much did the weight which was hung at that end require to exceed that on the longest side."
"This," said Mr Barlow, "is what they call a _lever_, and all the sticks that you have been using to-day are only levers of a different construction. By these short trials, you may conceive the prodigious advantage which they are of to men; for thus can one man move a weight which half-a-dozen could not be able to do with their hands alone; thus may a little boy, like you, do more than the strongest man could effect who did not know these secrets. As to that instrument by which you were so surprised that Harry could cleave such a vast body of wood, it is called a wedge, and is almost equally useful with the lever. The whole force of it consists in its being gradually narrower and narrower, till at last it ends in a thin edge, capable of penetrating the smallest chink. By this we are enabled to overthrow the largest oaks, to cleave their roots, almost as hard as iron itself, and even to split the solid rocks." "All this," said Tommy, "is wonderful indeed; and I need not ask the use of them, because I see it plainly in the experiments I have made to-day."