666 fT. C. CHAMBERLIN 
The considerations that grow out of altitude above the sur- 
face which reduces the parabolic velocity have been neglected 
thus far. These are not very important in a shallow atmos- 
phere as may be seen by reference to the tables previously 
given, but they might be consequential in an exceedingly 
extended atmosphere. While it would be hazardous to esti- 
mate the height of a superheated atmosphere embracing the 
whole present hydrosphere, it seems not improbable that its 
outer portion would be appreciably affected by the reduction 
of the parabolic velocity due to its high altitude. 
To this is to be added also the effect of the high rotation 
which the earth is assumed to have had. Inthe supposed dis- 
charge of the moon under either the nebular or fission hypothesis 
the attraction of the earth in the equatorial zone must have been 
nearly or quite neutralized by the centrifugal effect of rotation. 
This must have greatly promoted the expansion of the atmos- 
phere in that zone and correspondingly reduced the earth’s power 
to control its outer portion, indeed it is difficult to see how the 
moon could have separated from the earth without carrying away 
the atmosphere, unless indeed the separation took place while 
the material of both bodies was in a perfect gaseous condition 
and the atmospheric constituents were diffused throughout the 
entire gaseous mass. But even in this case the subsequent con- 
traction of the earth should apparently have accelerated its rota- 
tion to such an extent that the retention of the outer equatorial 
atmosphere would be put in jeopardy. 
There is still another consideration whose importance may 
possibly be decisive—the dissociation of water. Dr. Stoney 
has maintained that even under present conditions the earth is 
incompetent to retain hydrogen. This conclusion is in harmony 
with the fact that hydrogen does not permanently exist in 
the atmosphere, though this absence may be otherwise explained.* 
At 1000° C. all molecules of hydrogen would acquire the para- 
bolic velocity of the earth some hundreds of thousands of times 
‘In his last paper, referred to in a previous footnote, the weakness of the argu- 
ment from the absence of hydrogen owing to the ease with which it may combine 
