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THE POPULAR SCIENCE MONTHLY 



this torque during a short interval of time is to produce an increment 

 of spin-momentum A#, and the addition of this increment of spin- 

 momentum to the previously existing spin-momentum 8 gives a re- 

 sultant 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. 



M 



top view 



Fig. 29. 



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 pro- 

 duced by propeller action (see Figs. 28 and 29) produces precessional 

 motion which tends to raise the forward edge a and lower the backward 

 edge i 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 



