Page:Popular Science Monthly Volume 6.djvu/753

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water the splash or rebound is visible enough, as are also the waves which diverge from the point of contact; but the effect caused by the drop under the surface is not apparent, because, the water being all of the same color, there is nothing to show the interchange of place which may be going on. There is, however, a very considerable effect produced. If instead of a drop of rain we let fall a drop of colored water, or, better still, if we color the topmost layer of the water, this effect becomes apparent. We then see that each drop sends down one or more masses of colored water in the form of vortex rings. These rings descend with a gradually diminishing velocity and with increasing size to a distance of several inches, generally as much as eighteen, below the surface. Each drop sends in general more than one ring, but the first ring is much more definite, and descends much quicker than those which follow it. If the surface of the water be not colored, this first ring is hardly apparent, for it appears to contain

PSM V06 D753 Raindrop behavior when falling on water.jpg
Drops descending below the Surface.

very little of the water of the drop which causes it. The actual size of these rings depends on the size and speed of the drops. They steadily increase as they descend, and before they stop they have generally attained a diameter of from one to two inches, or even more. The diagram above shows the effect which may be produced in a glass vessel. It is not that the drop merely forces itself down under the surface, but, in descending, carries down with it a mass of water which, when the ring is one inch in diameter, would be an oblate spheroid, having a larger axis of two inches and a lesser of about one and a half inch. For it is well known that the vortex ring is merely the core of the mass of fluid which accompanies it, the shape of which