Page:Elementary Text-book of Physics (Anthony, 1897).djvu/154

140 '''121. Waves.'—When a disturbance is set up at a point in the free surface of a liquid, it moves over the surface of the liquid as a wave'' or series of waves. Each wave consists of a crest or elevated portion and a hollow or depressed portion of approximately equal length, and the distance from a particle at the summit of one crest to a particle at the summit of the next succeeding crest, or the distance between particles in successive waves which are in the same condition of motion, is called a wave length. A line which is drawn along the crest of any one wave or through the particles in that wave which are in the same condition of motion, and which at every point is at right angles to the direction in which the wave is propagated, may be called the wave front.

The formation of waves is explained by inequalities of hydrostatic pressure arising in the liquid if by any cause one part of it be elevated above the rest. H. and W. AVeber examined the peculiarities of waves in water and the motions of the water particles in them by the aid of a long trough with glass sides; by immersing one end of a glass tube below the surface, raising a column of water in it a few centimetres high by suction, and allowing it to fall, they excited a series of waves which proceeded down the trough and could be examined through the sides. The motions of the particles in the wave were studied by scattering through the water small fragments of amber, which were so nearly of the same specific gravity as the water that they remained suspended without motion except during the passage of the wave, and took part in the motion excited by the wave as if they had been particles of water. It was found that the wave motion was a form of motion transferred from one portion to another of the water, and did not involve a displacement of the particles concerned in it,—at least when the successive waves had the same wave length. In that case—which is the typical one—the particles in the surface of the water described closed curves, which were elliptical or circular in form, the diameter of the circle being equal to the vertical distance between the crest and the hollow or the height of the wave. In the upper part of the circle the particle moved in the direction in which the wave was