326 Sronsy—Of Atmospheres upon Planets and Satellites. 
We have already found, on p. 313, the potential of the Earth at the boundary 
of our atmosphere to be 
; M On 12] 
iy EEE Wo i pammuaal 
Therefore the joint potential of the Sun and Earth at that station is 
ape 
r R+h 
This, then, is equal to v?/ 2, when v is the least velocity which would enable a 
missile to escape from both these bodies. Therefore 
v= /(2 x 960°5) = 43°83 km. / sec. 
If the missile be shot off in the direction towards which the Earth is travelline 
= 960°5. 
The Sun is contracting, and therefore in past time was larger than it now is. 
The question then arises, how much larger may it have been while it was still 
globular ? We can place a limit on its possible size ¢f we assume that it was then, 
as now, able to prevent the escape of free hydrogen, and if we assign a temperature 
below which its outer boundary did not fall. 
In order to arrive at definite results, let us suppose this temperature to be 0° C. 
Here we might take into consideration the probability that, at a sufficiently remote 
period, the planets formed part of the Sun. But it is needless to do this, as the 
addition to be then made to its present mass would be only about 1/750th part, 
which is too slight an increase sensibly to affect our present computation. 
We have first to ascertain what the ‘velocity of mean square” of hydrogen is 
