1875.] Variation in the Obliquity of the Ecliptic. 293 
the point 6° from the pole of the ecliptic. As the earth 
rotates about 366 times in one year, it follows that during 
one revolution of the equinoxes there would be at least 
9,150,000 rotations of the earth on its axis. The fact that 
the centre of gravity of the earth is not situated in the plane 
of the Equator would produce a different movement of the 
earth to that which would occur if the centre of gravity 
were located in this plane ; and although during one rotation 
the differences would be very minute, yet when this difference 
is multiplied by 9,000,000 the variation would be con- 
siderable. 
We have, however, other important matters to consider 
in connection with this problem. We have supposed that 
the earth is a homogeneous spheroid, but we do not know 
this, nor can any person know it,—we can only consider it 
probable ; but we can now state what we do know. We do 
know that in former periods of the earth’s history there was 
a very different distribution of land and sea to that which 
now prevails, and the results of such changes we can 
define. 
Let us take for granted that the earth is homogeneous, 
and that at present the centre of gravity is not coincident 
with the Equator, nor is it passed through by the earth’s 
axis. Now the fact of an elevation of land—say in the 
southern hemisphere—by the action of internal forces, or 
the depression of land from any cause, would not make this 
land weigh more or less, and therefore the relative weight of 
the land in the two hemispheres would not be altered by any 
amount of elevation or depression, and no change in the 
position of the centre of gravity of the earth would be pro- 
duced by any amount of elevation or depression of the land, 
supposing the earth consisted of land only. Immediately, how- 
ever, there is an elevation of land, there is a transferral of 
the water of the ocean over the whole earth. The sea 
which before covered this land is spread over the whole 
globe; and when such masses of land as the greater part of 
Europe and America rose from beneath the sea, millions of 
tons of water were transferred to other parts of the earth, 
and the position of the centre of gravity of the earth was 
consequently altered thereby. The equilibrium (as we may 
term it) of the earth would not be altered by the actual fact 
of the elevation or depression of land in either hemisphere, 
but it would be altered by the resulting transferral of vast 
masses of water from one hemisphere or one locality, which 
masses would then be spread over the whole surface of the 
globe. 
