Suppose the hour at which he intends to rise be 8, and that he has placed the hand at 5.
Then, adding 12 to 5, you bid him call the hour at which the index stands, the number on which he thought; and by reckoning back from this number to 17, it will bring him to 8, the hour required.
The following Experiment shews the Power of Attraction.
If we take two pieces of lead, as two musket or pistol balls, and with a knife smooth two plane surfaces, and press them together, they will firmly adhere.
Two plates of metal made very smooth, when rubbed with oil and put together, will so firmly adhere, that it will require a great force to separate them.
If two pieces of wood, or of glass, be wetted with water, and placed together, the one may be lifted up by means of the other. Boys often have a piece of leather on the end of a string, which they wet and put on a stone, and thereby lift it up.
If we take a small tube of glass with a narrow bore, and put it in water, the fluid will rise higher within the tube than in the vessel. The narrower the tube is, the higher the water rises. This is called Capillary Attraction. If we put two pieces of glass together, and place the lower edge in water, it will rise between them, as it does in the capillary tubes. This experiment may be made more pleasing, by putting a shilling or a piece of paper between the two pieces of glass at one end. The water will then rise in a curve line, called an hyperbola, higher and higher as it recedes from the shilling or piece of paper, and the pieces of glass get nearer to each other.
Place a balance equally poised, so that one scale may be made to touch water in a vessel; considerable weight must be put in the other scale, to make it rise up. Put three or four bits of cork to float in a basin of water; they will gradually draw nearer to each other, and the more rapidly as the distance diminishes.
Experiments to shew the Power of Repulsion.
Dip a ball in oil and put it in water; a ditch will be formed all round it. Pour water on oiled paper, and it will run off.