Stoney—Of Atmospheres upon Planets and Satellites. 319 
Cuarter VIII.—Of Venus. 
The state of Venus’s atmosphere need not detain us long. The potential of 
gravitation is so nearly the same on this planet as on the Earth that its atmosphere 
almost certainly retains and dismisses the same gases as does the atmosphere of 
the Earth. The only element of uncertainty arises from its period of rotation 
being imperfectly known, but the nearly globular form of the planet assures us 
that its rotation cannot be swift enough seriously to affect the problem. 
The similarity of the two atmospheres is confirmed by the appearance of the 
planet. Venus is presumably a much younger planet than the Earth, and its 
temperature is consequently what the Earth’s was many ages ago, when through 
excessive evaporation water was the largest constituent of our atmosphere, and 
when clouds were present everywhere and without intermission. 
The conditions upon Venus are so nearly akin to those on the Earth that we 
cannot be mistaken in regarding the vapour which forms the abundant cloud we 
see on that planet as none other than the vapour of water. If we may assume 
this, we can advance a step farther than the statements made in Chapter LV. 
The detailed computations in the case of Venus give 
8-40 2 
r= 87-848 6378 = 6053 kilometres, 
m 
and as such observations as are practicable seem to indicate that on that planet 
P = 83779 seconds, 
we find that 
v = 10000 m./sec., 
u = 454 m./sec.; 
whence we infer that 
v’ =v—u= 9546 m. /sec. 
is the least speed which will carry a projectile away from Venus. 
Now, in water, p = 9. Whence, in accordance with Clausius’s formula, p. 310, 
the velocity of mean square in water, at the temperature of — 66° C., is 
Now 2’ is almost exactly 18 times this value of w; so that the circumstance that 
Venus is able to retain its hold upon water means that the molecules of a gas do 
