1875.] The Atmospheres of the Planets. 471 
can perhaps its actual surface be seen. Supposing the law 
of variation in temperature to be analogous to that on 
Venus, the rate of decrease of density on Mercury would be 
as under, being slower on this planet than any other in the 
solar system ; and thus, were not the planet always in so 
close proximity to the sun, would render Mercury the most 
favourable member of our system to study atmospherical 
effects. 
At a height of rz miles the density would be halved, at 33 
miles reduced to one-tenth, at 62 miles to only one-hundredth; 
at go miles to one-thousandth, and at 200 miles become 
utterly insensible. As under the most favourable conditions 
one second of arc at the distance of Mercury from the earth 
isover 200 miles in length, it is evident that on Mercury, as all 
the other planets, the border of atmosphere is so shallow 
that on the limb it would be to all intents perfectly undis- 
tinguishable. From grey spots that have repeatedly been 
seen by observers during the transits of Mercury on the 
planet, it is possible that the atmosphere of Mercury may 
be of very considerable density, so as from the combined 
effects of refraction and reflection to produce these otherwise 
inexplicable markings. And the supposed great density of 
Mercury’s atmosphere receives some support from the rosy 
tinge of the planet noticed by Dela Rue, and the ill-defined 
border detected by Secchi. 
From the formule already given, byassuming other condi- 
tions to hold upon the planets besides those that have been 
adopted in the above, any possible atmospheric condition 
can be easily found, it only being necessary to suitably 
determine the several constants. It will, therefore, be pos- 
sible to test any supposed condition by assuming it to hold, 
and ascertaining whether it infringes any of the theoretical 
conditions; but it will appear that the two extreme limits 
of an atmosphere to a planet of the solar system will lie 
between the two conditions of a uniform temperature and 
uniform density, the one giving the maximum and the other 
the minimum depth. 
VOL. V. (N.S.) : 30 
