1887 .] Professor Tait on the Effects of Explosives. 
Ill 
a flash is always photographed as a line of finite breadth, even 
when the focal length is short and the focal adjustment perfect. 
This cannot he ascribed to irradiation. The air seems, in fact, to 
be driven outwards from the track of the discharge with such 
speed as to render the immediately surrounding air instantaneously 
self-luminous by compression. 
Such considerations show at once how to explain the difference 
between the effects of dynamite and those of gunpowder. The 
latter is prepared expressly for the purpose of developing its energy 
gradually. Thus while the flash of gunpowder fired in the open 
is due mainly to combustion of scattered particles, — that produced 
by dynamite is mainly due to impulsive compression of the sur- 
rounding air, energy being conveyed to it much faster than it 
can escape in the form of sound. 
4. Report on Possil Pishes collected in Eskdale and Lid- 
desdale. Part I. Ganoidei — Supplement. By Dr 
Traquair. 
5. On the Equilibrium of a Gas under its own Gravitation 
only. By Sir W. Thomson.* 
This problem, for the case of uniform temperature, was first, I 
believe, proposed by Tait in the following highly interesting 
question, set in the Ferguson Scholarship Examination (Glasgow, 
October 2, 1885): — “Assuming Boyle’s Law for all pressures, 
form the equation for the equilibrium-density at any distance from 
* Note of February 22, 1887. — Having yesterday sent a finally revised 
proof of this paper for press, I have to-day received a letter from Prof. 
Newcomb, calling my attention to a most important paper by Mr J. Homer 
Lane, “On the Theoretical Temperature of the Sun,” published in the 
American Journal of Science for July 1870, p. 57, in which precisely the 
same problem as that of my article is very powerfully dealt with, mathemati- 
cally and practically. It is impossible now, before going to press, for me to 
do more than refer to Mr Lane’s paper ; but I hope to profit by it very much 
in the continuation of my present work which I intended, and still intend, 
to make. — W. T. 
