Proceedings of the Cambridge Philosophical Society. 389 
of the telescope. It was shewn that the change would be such 
as might, under favourable circumstances, be capable of being 
estimated by observation ; and the precautions were mentioned 
which would be necessary for its accurate determination. 
April 22.— A paper was read “ On the Distribution of the 
Colouring Matter , and on certain peculiarities in the structure 
of the Brazilian Topaz f by David Brewster, LL. D. F.R.S. 
&c. — This paper was divided into the following heads : 
1. On the distribution of the Colouring Matter in Topaz. 
2. On the Tesselated Structure of the Brazilian Topaz, and 
the singular superposition of its External Laminae. 
3. On the Optical Structure and Properties of Brazilian 
Topaz. 
4. On substances found in the Brazilian Topaz. 
5. On the probable difference in the Chemical Composition of 
the Brazilian and other Topazes. 
Under the last head, the author observes that the late Rev. Wil- 
liam Gregor detected Lime and Potash in the Brazilian Topaz 
which sufficiently accounts for the difference between its Optica 
Structure, and that of the other Topazes. 
May 6 . — “ On the Rotation of Bodies by W. Whewell, 
M. A. Fellow of Trinity College. — Mr Whewell began by 
giving a short history of the problem of the motion of a body of 
any figure, about its centre of gravity. D’Alembert first solved 
this problem in 1749 ; and Euler, in 1758, put the formula in 
what is now the usual form. Mr Landen gave a different solu- 
tion in the Philosophical Transactions for 1785, and to the end 
of his life maintained the results of the continental mathemati- 
cians to be erroneous. Mr W.’s object was to put the error of 
Mr Landen in the clearest point of view, by reducing the ques- 
tion to the formulae for the motion of points. If a triangular 
pyramid, without inertia, having material points at three of its 
angles, revolve anyhow about its remaining angle or vertex, it 
may be considered as a solid body ; and, by investigating this 
case, the truth of Euler’s formula is fully established. It ap- 
pears also, that if the angles at the vertex of the pyramid be 
right angles, its motion may be made to coincide with that of 
any given solid body, by properly adjusting the magnitudes of 
the three material points. 
