35 
the same effect, it is quite unphilosophical to resort to a 
special hypothesis. Encke’s comet ought not to be quoted 
as evidence of the existence of such a medium until electric 
disturbance is shown by calculation to be insufficient to 
account for the observed diminution of period. 
“ On Cometary Phenomena,” by Professor Osborne Key- 
NOLDS, M.A. 
In all comets which have been observed through powerful 
telescopes there is an action going on which appears to be 
the result of evaporation. J ets of something like vapour 
are seen to issue from what is supposed to be a solid nucleus 
on that side which is towards the sun. 
No such signs of evaporation are observed on the planets, 
nor is there any phenomenon, that we are aware of, which 
can be compared with this taking place on our earth. At 
first sight it seems strange that the sun should act to more 
effect on such small bodies as comets than it does on the 
larger bodies, even when the latter are nearer to it than the 
former. When, however, we come to look closer, I think 
good reason may be given for this ; and I think that the 
difference of evaporation on the earth and on a comet may 
probably be the cause of electrical phenomena existing on 
the latter which certainly do not exist on the earth, and 
that the relation between the motion of the comet and the 
evaporation which might be expected to take place is 
precisely that which is observed between the motion and 
those appearances which I would explain on an electrical 
hypothesis. 
The first thing to be done is to take notice of the following 
facts : — 
1. Comets move in very eccentric orbits, whereas the 
planets move in orbits nearly circular. 
2. Comets are supposed to be much smaller than the 
planets. 
