Page:Popular Science Monthly Volume 75.djvu/38

34 this torque during a short interval of time is to produce an increment of spin-momentum AS, and the addition of this increment of spin momentum to the previously existing spin-momentum S gives a resultant spin-momentum S'. That is to say, the effect of the torque T is to cause the axis of spin to sweep around in the direction of the curved dotted arrow.

Fig. 30 shows a view as seen from above of an actual flight of the boomerang. The boomerang leaves the thrower at M with its plane approximately vertical, and the effect of the torque T of Fig. 29 is to cause the axis of spin of the boomerang to sweep about a vertical axis as represented by the curved dotted arrow in Fig. 29, thus tending to make the boomerang glance around a circular horizontal path. At the same time the boomerang acts more or less like a disk as represented in Figs. 25 and 27, and this action slowly brings the axis of spin of the boomerang into a vertical position (plane of boomerang horizontal). As the plane of the boomerang comes into an approximately horizontal position toward the end of its circular flight, the torque which is produced by propeller action (see Figs. 28 and 29) produces precessional motion which tends to raise the forward edge a and lower the backward edge b of the boomerang, thus causing the boomerang to tend to glance upwards. This tendency of the boomerang to glance upwards is helped by the propeller action of the boomerang, that is to say, the