[October, 
600 Sunlight . 
According to the more advanced theories regarding 
comets, they are thought to consist of solid matter, and 
rigorously to conform to the laws of gravitation, and 
strictly to follow out their orbits. When a comet is far 
away from the sun it appears very much like ordinary 
stellar matter ; but as it gradually approaches the sun, and 
as its motion increases, it becomes more brightly illu- 
minated, and sends out short streaks of light sunwards. 
Shortly these streaks are forcibly driven back as by some 
repellent force, and gather about the head of the comet in 
a bright halo. On a nearer approach this halo appears to 
be partly thrown off, and one or more tails formed on the 
side away from the sun. When the comet sweeps round 
the sun its tail wheels round along with it, always keeping 
away from the sun, and as it recedes its tail — which is now 
in front — gradually dwindles away, until it appears as ordi- 
nary stellar matter again. 
As comets (at least those that we have most to do with 
at present) travel in infinitely long ellipses, and strike the 
plane of the ecliptic at a considerable angle, they may be 
considered, for all practical purposes, not to participate in 
the general circular movement of the planetary system ; and 
when crossing the plane of the ecliptic they will therefore 
be altogether stationary, and under the same influences as 
the sun himself. 
Now let us follow a comet in its passage from outer space 
to this particular point. When it first makes its appearance 
we see it through our telescopes as a faint, hazy, undefined 
body, which appearance very probably results from a defi- 
ciency of power in our instruments. As it approaches the 
sun, short streaks of light spring forth sunwards, or rather 
an eleCtric discharge passes from it towards the sun, owing 
to its having been formerly the rubber, and consequently 
endowed with a different kind of electricity. As it gets 
further within the influence that (according to this theory) 
forms a sun, a repulsive aCtion takes place between it and 
the sun, owing to their now being charged with the same 
kind of electricity ; in faCt, for the time being, it becomes a 
sun, and emits light in the direction of the rubber, or 
outer planetary matter, in the same manner as the sun 
himself. 
Throughout our statements we have been making use of 
these two powers, Motion and attraction ; but if these 
speculations be correct, attraction is but the product of the 
aCtion of motion upon mass, in the same manner as light 
and heat are the outcome of the aCtion of motion upon 
