266 
The Evolution of the Solar System. 
[May, 
ceases, and it becomes continuous in condition with the 
matter of space. At this undefined point we reach the true 
atmospheric limit, or the outermost point of diredl alle- 
giance of outer matter to the earth. This limit perhaps 
constantly varies. At every approach of a planet the gravi- 
tative influence of the earth is partly neutralised, the density 
of the atmosphere decreased, and its fridtional power re- 
duced. The influence of the sun and moon must produce 
still more marked results ; and the atmospheric perturba- 
tions and storms are, no doubt, very largely due to these 
exterior influences upon its density. 
If the earth were at rest in space the result of the above 
action would be to give the atmosphere a shape markedly 
different from that of the solid earth. For as we pass from 
the Equator towards the Poles the rotatory vigour decreases, 
and becomes null at the immediate poles. Consequently 
atmospheric fridtion might be more effective in producing 
the decreased centrifugal motion required, and the true at- 
mosphere would assume the shape of an elongated spindle, 
its axis extending in the line of the poles. 
But the earth is not at rest in space. It is moving through 
space at a very rapid speed. Thus the fridtion of its atmo- 
sphere is not alone concerned in producing rotation in sur- 
rounding matter, but also in carrying this matter forward 
with the earth in its flight through space, as a permanent 
envelope. Here, too, there is a resistance of outer matter 
to this influence, and a disposition to lag. Probably to a 
certain distance outward the combined gravitative and fric- 
tional vigour of the earth carries this atmospheric matter 
forward at its full speed. Beyond this distance these influ- 
ences so weaken that the matter of space lags behind in the 
race. This lagging increases until, at a certain limit of 
distance, the atmospheric matter quite ceases to accompany 
the earth in its orbital flight. Here is another atmospheric 
limit, shading off and imperceptibly disappearing like the 
former. Its shape, however, should be spherical, since its 
producing influence would be equal in all directions from the 
centre. 
As remarked above, the effedt of absorption of the existing 
atmosphere into the earth’s crust would necessarily be to 
increase the earth’s gravity, and thus to attract new matter 
into this denuded space. But another consideration arises 
here. Our atmosphere, in its lower portions at least, is not 
composed of disintegrated matter, but of chemical atoms 
and molecules, matter in a state of partial condensation. 
It is not at all probable that the matter of space exists in 
