Page:Popular Science Monthly Volume 25.djvu/270

260 needle, and having attached to its four arms four pieces of white card, perforated with holes which are depressed conically on one side and raised at the other, so as to present a surface something like that of a nutmeg-grater. Each card has twenty-five holes thus pierced, and the



whole apparatus a hundred holes. The rotations are more rapid if the cards are set on obliquely in the fashion shown at E in the figures, with the burred sides outward. The rotations are produced when the "mill" is set in front of the resonant box.

Dvorák's fourth apparatus is called by him an "acoustic anemometer," and is represented in Fig. 5. It consists of a little "mill" of simple construction, h i k, the vanes of which are made of small pieces of paper or card slightly curved, and a sounding-box, c d f g, placed a little way from it, while between them is held an ordinary Helmholtz's



resonator having its wide opening, b, turned toward the box, and its narrow opening, a, toward the mill. The stem of the tuning-fork is inserted in the socket F of the sounding-box. The internal increase of pressure induced by the vibrations of the tuning-fork through the sounding-box in the resonator at a has the effect of driving a jet of air gently against the sails of the mill, which consequently rotates. The two-aperture resonator of this apparatus may be replaced by a