1900. ] LOWELL—MARS ON GLACIAL EPOCHS. 661 
The minus sign denotes that x diminishes as D increases. 
If we assume a — 1 we have 
aD = -— Lax 
D 
whence 
aD Vo Der 
D 
or 
10) (HS EE 
For another planet we have in the same way 
D=e-s 
Curves showing variation in density of atmospheres7on the Earth and on Mars. 
Thus the height ‘necessary to bring about the same relative 
amount of density upon two planets varies inversely as their respec- 
tive surface gravity. 
