Page:Popular Science Monthly Volume 39.djvu/673

Rh and vertical in its origin, becomes necessarily the seat of horizontal oscillations—independently of the vertical vibrations which were first considered. Not a single molecule of which the



mass is composed follows a straight or parabolical line in its fall. All, without exception, describe a sinuous or zigzag course. Such is the general phenomenon. Let us follow out some of its details. For the jet to be rigorously cylindrical in its origin is not a sufficient condition of its preserving that form. When it issues horizontally in full gush from a fountain, the parabolic trajectories, upper and lower, lose their parallelism and cause by approaching one another a flattening of the jet. From this fact arise horizontal undulations, only slightly marked, it is true, but which suffice, if the light is favorable, for producing the illusion of swellings and contractions (Fig. 12, xxi). Even When a liquid Vein issues vertically from the bottom of a vessel by a circular hole, it assumes a helicoidal form in consequence of a motion of rotation which develops gradually within the vessel. This is independent