10 Professor Booth on the Rotation 



that -r-^ and -^ and -, — ouijht to be 0, according to M. 

 dx ax ax " ° 



Mossotti's views ? If so, I am assuredly at issue with your 



Correspondent. But most probably something else is meant 



which does not strike one who thinks differently from the 



writer. 



That I may not occupy much of your space in a matter 



not universally interesting, I will conclude with stating, that 



the real difficulty in the way of a molecular theory is our 



want of power to sum expressions in Finite Differences. To 



convert them into definite integrals is to omit that part of their 



value which is the most important. Should any of your 



readers desire to examine the subject, I would recommend 



them to consult, in addition to M. Mossotti's papers in the 



Scientific Memoirs, and my memoir in the Transactions of 



the Cambridge Philosophical Society, some of the following, 



which are amongst the most valuable contributions to this 



branch of theoretical physics : — Laplace, Mecanique C^leste^ 



liv. 12. ch. 2., particularly art. 3; Poisson, Journal de V Ecole 



Polytech,^ cap. 20; and Mem. de VListitut, vol. viii. and ix.; 



Cauchy, Exercices, vol. iii., particularly p. 226; Avogadro 



Fisica di Corpi ponderabiliy vol. i. p. 159. 



I have the honour to be, 



Dear Sir, yours very truly, 



P. Kelland. 



IV. On the Rotation of a rigid Body round ajixed Point. By 

 James Booth, jEsg'., M.A., Principal qf^ and Professor of 

 Mathematics in Bristol College. 



[Continued from vol. xix. p. 441.] 

 XX. npO determine the position of the body in terms of 

 the time. 

 It is clear that the indefinitely small portion o s, of the line 

 m parallel to the tangent at Q* to the section of the ellipsoid, 

 made by the plane of u and ?«, may be considered as the dif- 

 ferential of the arc of the spherical conic, traced out by the 

 vertex of m during the motion : calling the differential of the 



ds 

 curve -Tj, and recollecting that o i) : m : : G : K ; see (Xl.)t 



and putting for o v its value -rrj we find -j- = -^^ ; then 



• See fig. in art. XI. 



■f The property of the Ellipsoid given in Number XL, first appeared in 

 an essay by Prof. MacCullagh, on the Wave Theory of Light, published in 

 in vol. xvi. of the Transactions of the Royal Irish Academy. 



