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transparent gas upon the removal of the body it surrounds 
to its second position nearer to the sun. But the atmo- 
spheric condensation into cloud-like mist which follows the 
removal of our little planet from the influence of the solar 
rays would also result from the removal of those solar rays 
from that little planet, such for instance as would be caused 
by the interposition of one of the planets. Under these 
circumstances a precipitation of misty material would take 
place, a precipitation which would as before be dissipated at 
the termination of the eclipse. 
A comet, however, is not circumstanced as our hypothe- 
tical planet has been. It is not placed at some given 
distance from the sun and allowed to remain there until 
the maximum thermal effect has been produced, and then 
removed elsewhere. It is continually altering its distance 
from the sun, and, apart from any axial rotation it may 
have, is continually presenting a fresh aspect to the opera- 
tion of the solar heat. Vapourised materials issue from its 
heated surface in jets like steam, and rise towards the sun 
into the cooler atmosphere above, where they lose a portion 
of their heat, become partially condensed, and form a canopy 
of cloud, which, when viewed from the side by the inhabi- 
tants of another planet, presents the appearance of a 
crescent with horns turned from the sun of a hemisphere or 
a sphere of nebulous matter, according to the amount and 
aggregation of the misty particles. As the comet approaches 
its perihelion this misty canopy is dissipated as transparent 
gas into the upper and surrounding regions of its atmosphere 
by the ever increasing power of the sun, whilst fresh jets of 
steam arise from the heated surface of the central mass and 
replenish the stratum of clouds. It is not difficult to find 
