318 Stonsy— Of Atmospheres upon Planets and Satellites. 
round the Sun). Possibly observations could be made in the daytime which 
would determine between these. Meanwhile 
w= 2m./sec., if the rotation period is 88 days, 
u= 175 m. /sec., if the rotation period is 1 day. 
By using the above values for 7 and m / Min equations 13 and 14, we find 
v (the minimum velocity of escape, if Mercury were at rest) = 4643 m./sec., 
which is a little more than 43 km. / sec. Hence 
v =v—u= 4641 m. /sec., if the rotation period is 88 days, 
and 
= 4468 m. /sec., if the rotation period is 1 day. 
By employing these values in equations 16 and 17, we find that 
p (the density of the gas that will escape 
from Mercury, as freely as helium 
does from the Earth) —. . . = 10°25, on the 88-day hypothesis. 
and =11, on the 1-day hypothesis, 
and on the further supposition that the absolute temperature of the gas where it 
escapes is 207, that is, 66° C. below zero. 
If the highest temperature at the upper surface of Mercury’s atmosphere over 
his equator is higher than this, and it is probably much higher, the foregoing 
values for p will have to be increased in the ratio of 7/207, where 7 is the 
highest temperature reached. It must also be remembered that helium is so 
prompt in escaping from the Earth that it is probable that gases somewhat 
denser could escape; and, as a consequence, that the limiting density of the 
gases that can escape from Mercury has to be increased in the same proportion. 
The general conclusion then is— 
1. That water with a density of 9 certainly cannot exist upon Mercury. Its 
molecules would very promptly fly away. 
2. That it is in some degree probable that both nitrogen and oxygen, with 
densities of 14 and 16, would more gradually escape. 
It is, therefore, not likely that there are, in whatever atmosphere Mercury may 
be able to retain, any of the constituents of the Earth’s atmosphere except perhaps 
argon and carbon dioxide. 
