124 
THE ICE AGES, PAST AND FUTURE. 
June, 1892. 
Ball, wlio lias contributed a further step in completing the 
explanation ; and this discovery of the astronomical cause of 
the Ice Age is a remarkable triumph of mathematical calcu¬ 
lation based upon very careful and accurate observations. 
The first named of the two disturbing causes that 
bring about the Ice Age, namely the action of the Sun 
and Moon, consists in a continued shifting of the direction 
of the 'polar axis of the earth. This axis is inclined 28° 
to the vertical, and at .the present time it points nearly 
to the pole star; but each year it points to a spot slightly 
more eastward, and continues this slow change in direc¬ 
tion until in a period of 10,500 years it will have made 
half a revolution, and will point to a spot as far removed 
from the zenith on the opposite side to the pole star; and 
the bright star Lyra will then take its place as the pole star 
of that future time ; while a further period of 10,500 years 
will bring it back to its present position. The former occur¬ 
rence of the above event may even have been within the 
period of human history, for there is some ground for thinking 
that Lyra was actually the pole star at the time of the build¬ 
ing of the oldest Egyptian pyramid, from some astronomical 
considerations connected with its structure and position. 
This gradual and regular change in the direction of the 
earth’s axis has long been known as the Precession of the 
Equinoxes., and is caused by the attraction of the sun and 
moon (principally the sun) upon the protuberant portion of 
the earth round the equator; the earth being not truly 
spherical, but orange-shaped and bulged at the equator to 
the extent of ¥ i_th part of its diameter. In consequence of 
the inclination of the axis of the earth, this protuberance is 
thrown out of the direct line of the pull exerted by the sun 
upon the earth, which causes a tendency to pull it back into 
that direct line ; and if there were nothing to counteract this 
action the ultimate result would be to bring the axis of the 
earth gradually more vertical until it became quite vertical, 
and without any inclination. But the rapid rotation of the 
earth upon its axis—a speed of 1,000 miles an hour at the 
equator—causes it to oppose a stubborn resistance to any 
change in its inclination, and the only result of the pull is 
to cause the direction of the axis to shift gradually sideways, 
preserving the same inclination, and gradually rolling com¬ 
pletely round, and returning to its original direction. This 
action is well known in the gyroscope, or in an ordinary 
spinning-top going to sleep. 
The present consequence to us of the inclination of the 
earth’s axis is the difference between winter and summer ; 
in summer the north pole is inclined towards the sun, and 
