Page:Popular Science Monthly Volume 39.djvu/668

650 establish beyond a doubt that the motion of a jet of water is never continuous, but is intermittent, periodic. Design vii, Fig. 5, which represents a jet from a hydrant or fire-engine at the moment of elevating, and design viii, representing a vertical jet from a narrow hole, are schematic but faithful reproductions of numerous observations. The photographs of Figs. 6 and 7, taken from nature at the overflow of a factory flume on Sunday, when no wheel or turbine was moving, are convincing.





This intermittent character of the stream is due to the same cause as the sound produced by the air in blowing through a keyhole; it is produced in the same way as the wavy tufts in smoke escaping from a chimney. Water, as well as air, is elastic, but in a less degree, and differences in its pressure are propagated in waves. Ample explanations on the subject may be found in the chapters on undulations in treatises on physics; but the theory of the liquid wave is complicated, and far from being exhausted. We limit ourselves to saying that water, like air, is greatly disposed, every time the velocity of the stream is changed, by shock against a foreign body or by an abrupt contraction or enlargement of the channel, to go into vibrations and to communicate them to the walls of the orifice whence it escapes or to any object against which