478 Transactions.— Miscellaneous. 
As, therefore, it is probable that comets enter the sun’s domain with a 
. great variety of momenta; and as, in the case of any particular comet, the 
probabilities are many millions to one against its momentum being in 
the direction of the little point represented by the sun and the planetary 
orbits, it is extremely probable that the great majority of visitors from 
interstellar spaee never come within human ken. 
Let us suppose that, for every comet seen, nine others make their peri- 
helion passage unobserved. Of 200 comets, the elements of whose orbits 
have been ascertained, twenty per cent. belong to the solar system, and 
eighty per cent. were visitors for the first and last time. If, therefore, 
astronomers record on an average one comet per annum, and nine others 
pass unrecorded, and if of these ten, two belong to the solar system, and 
eight are strangers, how great must be the wealth of cometic matter in the 
universe! For as these strangers are supposed to be some 5,000,000 years 
in the sun’s dominions before making their perihelion sweep, it is evident 
that at any moment the sun must have, under his control, a supply of 
foreign comets for 5,000,000 years, at the rate of eight per annum! That 
is 40,000,000; besides vast multitudes of comparatively domestic comets, 
to whom he saith ** Go,” and they go; ** Come," and they come. 
If, then, each sun of the midnight sky, and of the astronomer's “ optic 
tube,” can boast such a following of comets, it shall come very near 
to be thought that the objects so long beheld with terror, on account of 
their rarity, are, indeed, the most numerous family of bodies in the 
universe. 
Comets are among the very few celestial objects that are waiting to be 
explained, and although the question of their constitution, like the secret of 
the Pole, is of no practical importance, it is yet of absorbing interest. The 
solution, however, of many of the questions that may perplex us to-night 
may already be in the hands of those fortunate scientifie men whose posi- 
tion has enabled them to analyze our present brilliant visitor with first-class 
spectroscopes. 
For ages comets were regarded as vapours, and exhalations, more or 
less pestilential, floating in the atmosphere. Tycho Brahé was the first to 
rise to the conception that comets were beyond the moon. 
Kepler’s theory was wonderfully acute, considering the information at 
his disposal. He considered comets to be wholly or principally gaseous, 
_ and that the tail consisted of gaseous material, highly rarefied by the sun’s 
heat, and then carried away by the repulsive force of the sun's rays. 
Others supposed the tail to be a column of vapour lighter than the medium 
in which the comet moved, and therefore raised, as smoke is raised in the 
column of heated air from a chimney. Newton supposed comets to have 
