MECHANICAL PARADOXES. 



at the A end the back cutting edge is beneath, 

 at the B end it is above. 



It can be readily seen that such a boomerang 

 is on the plan of the spinning toy shown in 

 the same figure. This toy has oblique vanes, 

 and, supposing it to be spun so that those 

 on the left advance and those on the right 

 recede, then the upper edges of the vanes, 

 C, D, E, F, are the leading edges, and cut the 

 air on such a slope that they tend to climb 

 up it and all rise together. The spinning is 

 done by means of a string wound round the 

 stem in a suitable holder, and if the spin be 

 strong enough and the toy large enough, it 

 will fly up to the ceiling or even as high as a 

 house. 



A boomerang with its vanes constructed 

 on the same plan, though it has only two of 

 them, and they are much heavier in proportion 

 to their area, will behave to some extent in 

 the same manner if spun in the same way. If 

 the boomerang in Fig. 15 be so spun that the 

 end A comes forward and the end B goes back, 

 since A slopes upward towards the front and 

 B slopes upward towards the back, each end 

 will tend to rise that is, the whole boomerang 

 will tend to rise, and with a force proportionate 

 to the vigour with which it is spun. 



Thus, recurring to Fig. 14, the rising 

 tendency of such a boomerang might counteract 

 the falling tendency due to gravity, and instead 



56 



