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 space 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 scientific 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 Brahe 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 



