1875.] Variation in the Obliquity of the Ecliptic. 291 
change in direction of the axis, making it describe a small 
circle of the sphere round the pole of the ecliptic. 
Let us grant that this theory is perfectly sound, supposing 
the preliminary data be correct, and we have yet not touched 
the main question at issue; but let us first examine the 
preliminary data. 
How do we know that the earth is homogeneous ? 
Let us suppose that every part of the earth’s surface in 
the southern hemisphere is at exactly the same distance 
from the true centre of the earth,—that the surface in a 
corresponding latitude in the northern hemisphere is from 
the true centre,—and it is quite possible that the mass of 
the two hemispheres is different, and hence the assumption 
that the earth is homogeneous may be a false one. 
Let us, however, come to facts. Let us examine the 
earth as it is, and deal with what we know. Water finds 
its »wn level, and the surface of the ocean in say 50 N. 
latitude must be at the same distance from the earth’s 
centre as is the surface of the ocean in 5c° S. latitude, and 
so for every degree of latitude north and south. When we 
examine the globe we find north of the equator the enormous 
and elevated mass of land comprising Asia, also the whole 
of Europe, also fully two-thirds of Africa, and the whole of 
North and part of South America. 
These vast continents of land in the northern hemisphere 
have as a set off, in the southern hemisphere, Australia, a 
few islands, one-third of Africa, and part of South America. 
At a rough estimate we may state there is four times as 
much land, above the sea, north, as there is south of the 
Equator. 
Again, the great depths of the ocean in the Southern 
Pacific, in the Southern Atlantic, and Indian Ocean, admit 
of vast quantities of water being now in the southern hemi- 
sphere, whereas in the northern there are massive granite 
mountains, raised 3 and 4 miles above the sea-level, and 
having in place of water only, 5 miles below the surface, 
solid granite. 
If, then, the materials of the earth be homogeneous, it 
follows that the preponderance of the land in the northern 
hemisphere must cause the centre of gravity of the earth to 
be situated considerably north of the Equator. Every com- 
petent mechanician knows that the second rotation of a 
rotating sphere is, as regards its amount and direction, 
mainly dependent on the situation of the centre of gravity. 
If the centre of gravity coincide with the centre of the 
sphere, regularity of movement may be expected; if the 
