Page:Popular Science Monthly Volume 6.djvu/469

Rh $4⁄10$ of a metre in circumference; one-half of its circumference is covered with a metallic band, the other half is formed of insulating material. Metallic springs press against the periphery of the pulley, so that, while they touch the metallic band, a current from a battery depresses the lever which traces on the revolving cylinder. Hence the lever is depressed, while $2⁄10$ of a metre of the cord pass, and it is elevated during the next $2⁄10$ of a metre of passage of the cord. Thus are registered, in an indented line, the velocities of translation of the artificial bird. Evidently the greater the velocity of the bird the greater will be the number of indentations inscribed in a second on the uniformly-revolving cylinder.



2. Measurement of the Duration of the Depression of the Wings.—A second electrical recorder, similar to that which registers the turns of the pulley, serves to determine the duration of the depression of the wings. For this purpose it is necessary that, at the beginning of the depression, the current of a battery should be broken, and this action is registered on the revolving cylinder by an indentation in the trace of the writing-lever; also, at the end of the depression of the wings, the current must be closed again, and this instant is likewise registered on the cylinder.