Page:Popular Science Monthly Volume 75.djvu/36

32 it with its side flat against the air. This may be shown by dropping a thin paper disk through the air or by blowing a blast of air against a disk which is pivoted about a diameter as an axis. Fig. 25 represents

a thin metal disk DD which is thrown in the direction of the arrow V and at the same time set spinning in the direction of the curved arrow S, the thrower standing at MM. Figs. 26 and 27 are top views

of the disk. Fig. 26 shows the disk starting to glance to the right (with reference to the thrower at M), and Fig. 27 shows the disk starting to glance to the left (with reference to the thrower at M). This glancing action of the disk causes the air to exert upon the

disk a torque about a vertical axis in Figs. 26 and 27, which torque is represented by the forces FF in Fig. 26 and by the forces F'F' in Fig. 27. This torque would turn the disk flatwise against the air if the disk were not spinning, but the effect of the torque on the spinning