Page:Popular Science Monthly Volume 25.djvu/77

Rh of running over smooth bodies, but that the power should be attributed to the molecular action between solid and liquid bodies; or, in other words, to capillary adhesion.

If we examine the under part of the pulvilli (Fig. 1) with a microscope, we shall see distinctly that it is furnished with numerous hairs, regularly distributed. These hairs terminate, at their lower end, in a



kind of bulb, the form of which varies, whence flows an oily liquid that dries slowly and does not harden for a long time. The minute drops left on the glass by the hairs may be taken away, even after two or three days have passed, without our having to moisten them, by simply rubbing a piece of fine paper over them. I have devised an apparatus for collecting these drops by cutting a hole in a piece of board over which I fix a glass slide. Turning the board over so that the glass shall be at the bottom, I have a little cell with a glass floor. With the aid of a piece of paper gummed to the wings, I introduce a fly into this cavity in such a manner that the pulvilli shall rest upon the floor. Then, putting the board under the microscope with the glass slide uppermost, we have the fly's feet under our eyes. The insect, struggling for liberty, places his pulvilli against the glass, and leaves after each effort, traces that may be observed very distinctly, for they are perfectly visible in a good light (Fig. 2).