glare wbicli the earth would feel were it illuminated by 
47,000 suns instead of one. A far inferior degree of heat 
has been found to melt the most refractory rocks and dis- 
sipate materials of prodigious fixity, and there can be no 
doubt that this comet acquired “a temperature high enough 
to convert even carbon into vapour.”^' 
The a priori evidence is complete. There can be no 
doubt the comet of 1843 possessed an atmosphere, and the 
only question now is one of its extent. I believe the actual 
depth of the terrestrial atmosphere is still unknown, though 
it is estimated as extending to a distance from the surface 
of more than seventy miles.*f* Were the mass of the con- 
densed portion of the earth by any means diminished so as 
to lessen the coercive power exercised thereby over its gase- 
ous parts, the volume of the latter would, according to 
Herschel, be increased in a proportion exceeding that in 
which the central mass had been diminished. The mass of 
comets is so small as to have furnished, so far as I can find, 
no data for its determination in any individual case. What- 
ever mass we may suppose the comet of 1843 to be possessed 
of consistent with this fact, it is clear its atmospheric limits 
must have been prodigiously remote from the solid matter 
of the nucleus. Indeed if we consider the recession result- 
ing from increased distance from the central gravitating 
mass, no volume of atmosphere containable within the known 
extension of our system would seem too great to be attri- 
buted thereto.! The deduction from this is that the pheno- 
mena resulting to a body of extended atmosphere and sub- 
jected to violent thermal change are phenomena which 
^ Eoscoe’s Spectrum Analysis,” p. 352. 
t Herschel’s ^‘Outlines,” tenth edit., p. 711. 
X “ Newton, with the view of illustrating this point, calculated that if 
a globe of common atmospheric air one inch in diameter was expanded 
so as to have an equal degree of rarity with the air situated at an eleva- 
tion above the earth’s surface equal to the earth’s semidiameter, it would 
fill the whole planetary regions as far as Saturn.” — Grant’s “ History of 
Astronomy,” p. 298. 
