180 Mr. A. Gages on Pseudomorphic Tremolite 



u=a9: so that this first centre is that of real reflexion, where, 

 . after the shock, the body moves in a direction opposite to its 

 actual motion u. On the contrary, the second root 



«■=— (X + «), 

 which is negative, gives for u' the value 



{\ + a)9 



u" = 



2 ' 



which is positive, or of the same sign as m ; so that the other 

 centre of maximum reflexion is in reality a centre oi progi-ession, 

 that is to say, a point by striking against which in the rear of 

 its translatory motion, the body is precipitated forward with the 

 greatest possible velocity. 



35. When ^=0, that is to say, when the body has merely a 

 translatory motion through space, the above equation gives ,r = 0; 

 whence we see that the centre of maximum reflexion then coin- 

 cides with the centre of gravity of the bodyj but x=0 renders 

 the reflexion m' zero, so that in this case the movement of the 

 body is des^i'oyed, as it evidently should be. 



36. "\Mien w = 0, that is to say, when the body merely rotates 

 on itself, we find 



4-Tf 1 ' -K^ 



.r = + K and u' = + -jr-. 



Thus for a body turning on itself, the centre of maximum re- 

 flexion is at the distance Kfrom the axis of rotation : hence it is 

 at this distance that the obstacle must be presented in order that 

 the centre of gravity of the body, at present at rest, may be sud- 

 denly animated with the greatest possible velocity; this velocity — 



expressed by -^ will be the half of that which the striking 



point of the body possesses. 



[To be continued.] 



XXIII. On Pseudomorphic Tremolite encrusted with Carbonate of 

 Lime and Magnesia, being apparently the inineral described by 

 Dufrenoy under the name of Miascite. By Alphonse Gages, 

 Esq., Curator of the Museum of Irish Industry^. 



IN the supplementary part of his Traitede Mineralogie (vol. iii. 

 p. 770, edit. 1837), M. Dufrenoy has described, under the 

 name of Miascite (Miaskite), two very distinct substances. The 

 first is a grayish felspathic rock, composed chiefly of felspar, 

 uniaxal mica, and elaeolite ; it was first described by G. Rose in 

 the account of the journey of Humboldt, Ehrenberg and G. Kose, 

 * Communicated by th« Author. 



