145 
should have been exhibited in an exceptional degree by the 
comet of which we speak. And all accounts agree that it 
presented '^a stupendous spectacle.” Its tail stretched to a 
hitherto unheard of distance, and so brilliant an object was 
this body that it could be seen by daylight and in the im- 
mediate neighbourhood of the sun. 
It is however only on the gratuitous assumption that 
comets are composed of the most refractory materials in the 
universe that any close proximity to the sun is needed as 
an argument in favour of their having atmospheres. Ad- 
mitting the community of matter and considering the rigor- 
ous cold they must experience in aphelion, it is but reason- 
able to suppose that as a rule the production of their 
atmospheres by the evaporation of their nucleii is com- 
menced at distances from the sun even greater than the 
distance of the earth, and thare is plenty of evidence to cor- 
roborate this view. The great comet of 1 81 1 presented 
irrefragable evidences of the existence of a transparent 
atmosphere certainly several thousands of miles in extent,^ 
and yet its nearest distance from the sun exceeded the dis- 
tance of the earth ! whilst during the last return to peri- 
helion of Halley’s comet in 1835 Sir John Herschel observed 
appearances which satisfied him that the nucleus of the 
comet was “ powerfully excited and dilated into a vapourous 
state by the action of the sun’s rays, escaping in streams 
and jets at those points of its surface which oppose the 
least resistance, and in all probability throwing that surface 
or the nucleus itself into irregular motions by its reaction 
in the act of so escaping.”'!* Yet the perihelion distance of 
this comet was as much as 0‘58 of the mean disttxnce of the 
earth. 
The close approximation to identity observed in the orbital 
* See observations of Dr. William Herschel in Phil. Trans., voL 102, 
pp. 134 to 136. 
t Herschel’ s Outlines,” p. 382, 
