Sronry—Of Atmospheres upon Planets and Satellites. 327 
at the freezing temperature. Itis got by putting 7’=273 and p=1 into Clausius’s 
formula, page 310. We thus find w = 1:841 km./sec. This multiplied by 9:27 
(see page 314) gives us a velocity 2, which the molecules of hydrogen could, at 
this temperature, get up sufficiently frequently, for the purposes of escape. And 
if multiplied by 18 (see page 320), it furnishes a velocity 7, which hydrogen is 
unable to get up sufficiently frequently for effective escape. We thus find 
v, = 17 km. / sec. v, = 33°14 km./sec. 
We have next to find how large the Sun should be in order that one or other 
of these velocities should be that which is just sufficient for the escape of a 
molecule. For that, 7, and 7, being the corresponding radii, the potentials must 
amount to Pa 172 ™, (33:14 
ERRATA. 
Page 326, line 7, after “both these bodies” add ‘if stationary.” 
» 99 line 12, after ‘13°83 km. /sec.” add the following :— 
Accordingly this is a velocity which would suffice to set the 
molecule completely free, if the Earth were arrested in its 
orbit immediately after the molecule left it. But since, 
on the contrary, the Karth persists on its course, a slightly 
greater speed of projection is actually needed. 
»» 9» line 14, change “with a speed of” into “with a velocity somewhat 
exceeding.” 
In carrying on an inquiry such as that of the present Memoir, we should keep 
in mind that the encounters between molecules have not the same effect on their 
subsequent motions as mere collisions between elastic or partially elastic solids 
would have. Let us, for simplicity, picture to ourselves two molecules which 
approach one another along a straight line, and after an encounter, which is in 
fact a complex struggle, recede from one another along the same line. 
If they were solid particles with elasticity e, the equations of their motion 
would be 
MU, + MU = MV, + MV2 5 
Uy — Uy + e (V; — 2») = 0, 
where 2,2; are the velocities before, and w,% the velocities after, the collision, 
