Practical Mechanics for Boys

by James Slough Zerbe

This may be illustrated by three tubes of different internal diameters. The liquid rises up higher in the second than in the first, and still higher in the third than in the second. The smaller the tube the greater the height of the liquid.

This is called capillary attraction, the word capillary meaning a hair. The phenomena is best observed when seen in tubes which are as fine as hairs. The liquid has an affinity for the metal, and creeps up the inside, and the distance it will thus move depends on the size of the tube.

The Sap of Trees.—The sap of trees goes upwardly, not because the tree is alive, but due to this property in the contact of liquids with a solid. It is exactly on the same principle that if the end of a piece of blotting paper is immersed[p. 87] in water, the latter will creep up and spread over the entire surface of the sheet.

In like manner, oil moves upwardly in a wick, and will keep on doing so, until the lighted wick is extinguished, when the flow ceases. When it is again lighted the oil again flows, as before.

If it were not for this principle of capillary attraction, it would be difficult to form a bubble of air in a spirit level. You can readily see how the liquid at each end of the air bubble rounds it off, as though it tried to surround it.

Sound.—Sound is caused by vibration, and it would be impossible to convey it without an elastic medium of some kind.

Acoustics is a branch of physics which treats of sounds. It is distinguished from music which has reference to the particular kinds.

Sounds are distinguished from noises. The latter are discordant and abrupt vibrations, whereas the former are regular and continuous.

Sound Mediums.—- Gases, vapors, liquids and solids transmit vibrations, but liquids and solids propagate with greater velocity than gases.

Vibration.—A vibration is the moving to and fro of the molecules in a body, and the greater their movement the more intense is the sound. The intensity of the sound is affected by the density of the atmosphere, and the movement[p. 88] of the winds also changes its power of transmission.