326 Sroney—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 Orin hla 
A Telit ta Lik oa 
Therefore the joint potential of the Sun and Earth at that station is 
Lia 
NAN tc 8) 
This, then, is equal to »* / 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 travelling, 
it has already got, in common with the rest of the Earth, 30 km./sec. of this 
velocity ; and therefore, if fired off in that direction, the speed with which it 
would need to part from the Earth is 13°83 km. /sec. Now, as 11 km. /see. is 
enough to enable a molecule to leave our atmosphere, it can be but very seldom 
that a molecule quits it with a speed of 13°83 km./sec.; and, accordingly, nearly 
all the molecules that have left the Earth have remained in the Solar system, and 
are in fact now travelling as independent planets round the Sun. 
We have taken the special case of a molecule leaving the Earth’s atmosphere. 
A similar treatment applies to molecules leaving the atmospheres of other planets 
and satellites. In every case the velocity required to enable a molecule to quit the 
Solar system is markedly in excess of that which enables it to escape from its own 
atmosphere. Accordingly, almost all such wandering molecules are still denizens 
of the Solar system. 
= 960°5. 
Cuaprer XIV.—Former size of the Sun. 
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 if 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 
