33 
Ordinary Meeting, November 28th, 1871. 
J. P. Joule, D.C.L., LL.D., F.R.S., Vice-President, in the 
Chair. 
Mr. Richard Samuel Dale, B.A., was elected an Ordinary 
Member of the Society. 
“ Encke’s Comet, and the Supposed Resisting Medium,” 
by Professor W. Stanley Jevons, M.A. 
The observed regular diminution of period of Encke’s 
comet is still, I believe, an unexplained phenomenon for 
which it is necessary to invent a special hypothesis, a Deus 
ex machina, in the shape of an imaginary resisting medium. 
I cannot be sure that the suggestion I am about to make 
has not already been made, but I have never happened to 
meet with it ; and therefore I venture to point out how it 
seems likely that the retardation of the comet may be recon- 
ciled with known physical laws. 
It is asserted by Mr. R. A. Proctor, Professor Osborne 
Reynolds, and possibly others, that comets owe many of 
- their peculiar phenomena to electric action. I need not 
enter upon any conjectures as to the exact nature of the 
electric disturbance, and I do not adopt any one theory of 
cometary constitution more than another. I merely point 
out that if the approach of a comet to the sun causes the 
development of electricity arising from the comet’s motion, 
a certain resistance is at once accounted for. Wherever 
there is an electric current some heat will be produced and 
sooner or later radiated into space, so that the comet in each 
revolution will lose a small portion of its total energy. In 
the experiments of Arago, Joule, and Foucault the conver- 
sion of mechanical energy into heat by the motion of a 
Peooeedings — Lit. & Phil. Soc. — Yol. XI. — No. 4. — Session 1S71-2 
