Fred was quite satisfied with all his father had said and started to get ready. Jessie began to question him about several things she did not fully understand in her father's talk the night previous. Fred explained matters, made them quite clear to her, and then asked her to get her memorandum book and write down the following, which he said, she would often find useful: "There are six mechanical powers, two of which father has not told us about, but will no doubt do so, before long. These are called, the Lever, Pulley, Wheel and Axle, Inclined Plane, Wedge, and Screw. The Screw and the Wheel and Axle, you have yet to hear about. Now, study carefully the following rules:
"The Lever.—Rule: The power required is to the weight as the distance of the weight from the fulcrum is to the distance of the power from the fulcrum.
"The Pulley.—A fixed pulley gives no increase of power. With a single movable pulley the power required will equal half the weight, and will move through twice the distance. Increasing the number of pulleys, diminishes the power required. Rule: The power is equal to the weight, divided by the number of folds of rope passing between the pulleys.
"The Wheel and Axle.—Rule: The power is to the weight as the radius of the axle is to the length of the crank or radius of the wheel.
"The Inclined Plane.—Rule: The power is to the weight as the height of the plane is to the length.
"Wedge.—Rule: Half the thickness of the head of the wedge is to the length of one of its sides as the power which acts against its head is to the effect produced on its side.
"The Screw.—Rule: As the distance between the threads is to the circumference of the circle described by the power, so is the power required to the weight."
Fred told George also to copy the foregoing in his memorandum book, so that he would be able to work out any problems for himself.