oj the Native Australians called the ^^Boomarang" 333 



spicuity, endeavoured to develop the causes by which the 

 body is made to ascend ; and before I explain those which 

 effect its return, I shall, with the aid of the annexed diagram, 

 enumerate them, and their effects, for the satisfaction of some 

 of your readers. The figure is a parallelipipedon. 



Let the body be projected from A in the direction A D with 

 a velocity which would carry it to D in the 

 same time that it would descend by gravity to 

 R. Also let A N represent in quantity and 

 direction a third force, acting on the body for 

 the same time, and equivalent to the force so 

 often mentioned already as acting on the plane 

 side. I need not say that A N is composed of 

 all the successive forces acting on the plane 

 side of the body, during its motion over dif- 

 fereht volumes of air in the same time that 

 the projectile force would carry it to D, and gravity to R. 

 The fourth and fifth forces spoken of (1.) are already ac- 

 counted for, and not estimable here (3.). Since these three 

 forces act on the body at once, it will be found at the end of 

 the given time at I (see Comp. and Res. of Forces,) the solid 

 angle of the parallelipipedon opposite to A ; and, because the 

 forces are not uniform, it will not have described the diagonal 

 A I, but a curved line, as A O I. It is evident that the ele- 

 vation of I, and consequently of the body, will depend on the 

 magnitude of the angles which A D and A N respectively 

 make with the horizon. By drawing the diagonal A P, which 

 is equivalent to the forces of projection and gravity, (see Me- 

 chanics, Comp. and Res. of Forces,) we may see that gravity 

 contributes to augment the resistance to the plane side; for 

 combined with the force of projection, it increases the action 

 of, and thereby causes greater reaction to, the plane side. 



1 1. I shall now mention what appear to me the causes of its 

 return. When arrived at its greatest elevation the force 

 of projection has ceased, and with it the forces generated by 

 it. It is evident, however, that although the projectile force 

 is spent, the gyration on its axis will not instantly cease, but 

 continue for a few seconds (First Law of Motion). The only 

 force now acting on the body is gravitation. You may not 

 be aware that the angle of inclination, i. e. the angle which 

 the plane side makes with the horizon, has increased very 

 much, so much indeed that if it was projected at 40° or 50° 

 it may have become 60° or 70°, the cause of which I may 

 have occasion, perhaps, to explain in a future communication. 

 Should it increase so far as to become a right angle, it is ob- 

 vious that the body would instantly descend by gravitation. 



