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 of the winds also changes its power of transmission.
Sound is also made more intense if a sonorous body is near its source. This is taken advantage of in musical instruments, where a sounding-board is used, as in the case of the piano, and in the violin, which has a thin shell as a body for holding the strings.
Another curious thing is shown in the speaking tube, where the sound waves are confined, so that they are carried along in one line, and as they are not interfered with will transmit the vibrations to great distances.
VELOCITY OF SOUND.--The temperature of the air has also an effect on the rate of transmission, but for general purposes a temperature of 62 degrees has been taken as the standard. The movement is shown to be about 50 miles in 4 minutes, or at the rate of 1,120 feet per second.
In water, however, the speed is four times greater; and in iron nearly fifteen times greater. Soft earth is a poor conductor, while rock and solid earth convey very readily. Placing the ear on a railway track will give the vibrations of a moving train miles before it can be heard through the air.
SOUND REFLECTIONS.--Sound waves move outwardly from the object in the form of wave-like rings, but those concentric rings, as they are called, may be interrupted at various points by obstacles. When that is the case the sound is buffeted back, producing what is called echoes.