368 Proceedings of the Royal Society 
Prof. Clerk-Maxwell, while on the whole favourable to the theory, 
pointed out the necessity for a full dynamical investigation, whose 
results might be compared with observation. The author’s own con- 
viction has all along been that the difficulty is not so much dynami- 
cal as constructional : — i.e., it lies mainly in obtaining a proper 
conception of the problem to be treated in the case of any particular 
comet, and not in the way of obtaining at least an approximate solu- 
tion when once the problem is stated. The fact is that the 
hypothesis is so very general that almost anything could be explained 
by it. When two considerable masses of stone, moving approxi- 
mately in the same orbit, impinge on one another with given velocities, 
what is the amount of smashing — how many large fragments, how 
many small, how much mere dust, will be produced — and in what 
direction and with what relative velocity will each of these on the 
average be projected h What amount of glowing gas will be pro- 
duced % Again, if there be many millions of such masses, forming a 
group in which all describe approximately elliptic orbits in something 
like equal periods, but of various sizes and in any planes about their 
common centre of inertia, the group itself being subject to a sort of 
tidal disturbance by the sun, at what part of the group will the 
impacts mainly occur 1 Questions so entirely vague as these are not 
yet ready for the application of mathematical methods. 
The main difficulty felt by the critics above named seems to be 
with respect to the production of the tail of a comet. The hypo- 
thesis of course involves as an immediate consequence that extensive 
regions of space all round the nucleus of the comet (but specially 
extended in the plane of its orbit) are full of fragments large and 
small, driven out at different times from the main ranks which (on 
the whole) become gradually extended along an arc of the orbit. 
Eays or tails will thus be seen wherever a visual line can be drawn, 
along and near to which there is an assemblage of particles fitted to 
give back a maximum of solar light. And, if the particles be not 
very large, the mass in each cubic mile of space may be very small, 
while the whole has considerable brightness, and yet does not 
sensibly weaken the light of a star seen through it. 
The author stated that he had investigated the form assumed by a 
train of particles ejected at different times from the head of a comet in 
the plane of its orbit ; always with the same relative velocity (so small 
