24 WILLIAM H. PICKERING 
planetesimal condensation or otherwise, does not concern us here. 
We merely assume that in these early days the Earth was in much 
the same condition that we find it at present, except that it was 
hotter. We also assume that it was slowly condensing from a more 
bulky form, rendering fission possible. 
These processes of fission and condensation we see going on all 
around us at the present time in the stellar universe, as indicated by 
the variable stars of short period and the spectroscopic binaries. 
It therefore requires no great stretch of the imagination to conceive 
that it may also have occurred on a smaller scale in the case of our 
Earth and Moon. 
It does not follow, however, that our combined planet was ever 
incandescent. Indeed, this seems to be unlikely. A cold nebula 
which is later to condense into a sun must almost necessarily be com- 
posed largely of solid matter. The electric disturbances by which 
we see it, illumine only the gaseous portions, but the metallic elements 
must be there nevertheless, all the time unseen. 
Assuming then a hot, solid, ellipsoidal Earth, with an interior 
more or less liquid, at least beneath the Equator, revolving on its 
‘axis once in about four or five hours, we have a picture of our as yet 
moonless planet as conceived by the astronomer. As it continued to 
cool, vast volumes of steam and other gases escaped from its interior, 
increasing its density and diminishing its volume. 
As its volume diminished, its speed of rotation increased, until 
by centrifugal force, as explained by Darwin, the Moon was born. 
If the crust was solid, and if the Moon escaped from it, it is almost 
certain that a scar of some sort would have been left, and it is of 
interest to see if we can find it. 
The specific gravity of the Earth as a whole is 5.6. That of 
the surface material ranges in general between 2.2 and 3.2, with an 
average of 2.7. Ihe specific gravity of the Moon is 3.4. his 
indicates clearly that the Moon is composed of material scraped off 
from the outer surface of the Earth, rather than of matter obtained 
from a considerable depth. At the same time, the specific gravity 
3.4 indicates that the layer of material removed had an appreciable 
thickness. 
As is well known, the land and water are very irregularly dis- 
