Page:Popular Science Monthly Volume 75.djvu/37

Rh disk is to cause precession, the axis of spin of the disk in Figs. 26 and 27 turns towards the vertical, bringing the right-hand side (with reference to the thrower) of the disk in Fig. 26 upwards and bringing the left-hand side of the disk in Fig. 27 upwards.

Fig. 28 represents a propeller-wheel boomerang. In the following discussion the propeller is supposed to be right-handed, that is to say, if it were set spinning in the direction of the curved arrow S in Fig. 28, it would blow air towards the reader like a desk fan. Fig. 28 represents the boomerang as it leaves the hands of the thrower, who is supposed to be standing at MM, V being the direction in which the boomerang is thrown, and the curved arrow S representing the direction in which the boomerang is set spinning. The upper vane of the

boomerang in Fig. 28 is traveling forwards at a greater velocity than the lower vane, because the forward velocity of the upper vane is the velocity of forward motion of the boomerang plus a forward velocity Sr which is due to the spinning motion of the boomerang, whereas the forward velocity of the lower vane is the forward velocity of the boomerang minus Sr. Fig. 29 is a top view of the boomerang as it leaves the hand of the thrower at M, V is the velocity of forward motion of the boomerang, and the arrow S represents the spin of the boomerang. The arrow F represents the force with which the air pushes sidewise against the upper vane because of propeller action. A force pushes sidewise in the same 'direction on the lower vane because of propeller action, but the sidewise force on the upper vane is the greater because of the greater velocity of the upper vane. The inequality of these forces constitutes a torque upon the boomerang, and this torque is represented by the arrow T in Fig. 29. The effect of