THE RETURN OF HALLEY’S COMET IN 1910. XE 
visible for forty-six days. It was seen in all parts of the 
known world, but, as in most of the ancient returns, the Chinese 
records are much the most precise; indeed without their aid. it 
would have been quite impossible tu carry back this long chain 
ot identifications. Let us give them the credit they deserve for 
their patient, long sustained vigils, which have added so greatly 
to our knowledge of the history of this comet. 
We have now an array of some twenty-nine observed returns, 
many of them recorded as objects of great splendour. The first 
reflection suggested by them is the close touch that we are 
brought into with far-distant centuries, in being able to con- 
template the very same body that has so often filled the world 
with wonder and admiration; but besides the sentimental 
aspect, there are, I think, some deductions of value with regard 
to the constitution of this and other comets. Dr. Johnston 
Stoney some years ago developed the theory of planetary 
atmospheres from the standpoint of the kinetic theory of gases ; 
the gaseous molecules are moving with speeds of miles per 
second, hydrogen having the greatest speed, and the speeds of 
the others diminishing as their density increases. Now each 
planet has a certain speed which suffices to carry objects away 
from its surface. In the case of the sun it is 385 miles per 
second, for Jupiter 37, for the other giant planets upwards of 13, 
for the Earth 7, Venus 6, Mars and Mercury 3, the Moon 1} 
miles per second. 
An explanation is found of the fact that hydrogen is found in 
the sun and giant planets, but not in the smaller ones, its mole- 
cular speed being too high. The earth can retain the denser 
gases, but the moon cannot, and her airless condition is thus 
explained. Now there is no doubt, from what we know of the 
mass of comets, that their critical speed is much lower even 
than that of the moon; hence it is clearly impossible that they 
could permanently retain a gaseous envelope ; that which we 
see surrounding them is not, therefore, of the nature of a 
permanent atmosphere, but is perpetually escaping from the 
head of the comet, and perpetually being renewed. The tail that 
we may see in Halley’s comet to-day is a different one from what 
was seen a monthago. At every return for two thousand years it 
has been seen to eject a series of huge tails, which streamed away 
into space, and could not be recovered by it. Now there must 
be some storehouse to contain all this gas, and the storehouse 
must be of a much denser nature than the gas, since it moves 
as though under gravitation alone, while the tail does not. 
And, seeing that we know that a close connection exists between 
