OF PLANETARY ATI\IOSPHERES. 
19 
The rate of effusion is proportional to the quantity of gas present, so that in cases 
where a gas does not exist in appreciable quantities in the atnios])here of the planet, 
the figures enable us to calculate what would happen siqiposing the planet to be 
endowed with an atmosphere of any given gas at a given temperature. 
We now obtain the results given in the following tables : — 
Number of Years in which the Efflux across the Critical Surface would equal the 
Removal of the Gas in a Superficial Layer 1 centim, thick. 
I. Earth’s Atmosphere. 
Hydrogen at 
absolute temp. 
Helium at 
absolute temp. 
Years. 
100“ 
200“ 
3‘54 X 10^'^ 
150 
300 
3-06 X 1010 
200 
400 
8-40 X 1010 = 84,000,000,000 
250 
500 
6-02 X 100 = 602,000 
300 
600 
2-22 X 102 ^ 222 
II. Atmosphere of Mars. 
Vapour of water at 
absolute temperatm-e. 
Years. 
200“ 
1-22 X 1033 
250 
3-37 X 1023 
300 
1-94 X 1013 
400 
2-40 X 103 = 2,400,000,000 
500 
4-28 X 10-1 = 42,800 
600 
1 -06 X 102 206 
The following table shows tiie corresponding absolute temperatures on the Earth 
and Mars at whicli the calculations for helium and water respectively would lead to 
approximately the same numerical results :— 
Earth and helium . . 
200“ 
300“ 
400“ 
500“ 
600“ 
Mars and water . . . 
187 
281 
376 
472 
571 
The removal of a layer of atmosphere 1 centim. thick from tlie surface of a })lanet 
would of course decrease the barometric pressures on that planet by an amount equal 
to the decrease for 1 centim. of altitude, and such a change would therefore be prac¬ 
tically quite imperceptible. A secular diminution in atmospheric pressure would not 
make itself noticeable to any practical extent till it corresponded to an altitude com¬ 
parable with, say, 100 metres. For such a change the iiumbers of years in the fore¬ 
going table would have to be multiplied by 10,000. 
i> 2 
