THE 110YAL ARTILLERY INSTITUTION. 
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axis round which rotation is given is a principal axis, and thus there is 
no tendency to change its position in the top. If, when the point comes 
down on the ground, the axis is vertical, the top will at once spin 
steadily on the ground. Usually, however, the axis is inclined to the 
horizontal, and the weight of the top, acting through its centre of 
gravity, tends to produce a rotation round an axis perpendicular to the 
axis of the top. The positioil of the axis thus changes; and at first 
the point describes large circles, rolling on the ground, the axis always 
being inclined inwards. When the point has become fixed in one 
place, the top describes complete gyrations. A peculiar motion is then 
observed ; for the gyrations become smaller and smaller, until the top 
spins on a vertical axis, or “ sleeps,” as it is called. This is due to the 
friction of the point on the ground, which a little consideration will 
show ought to produce this effect. As, however, the rotation becomes 
slower, the top begins again to gyrate, and, in spite of the friction on 
the ground, the gyrations become larger and larger, until the point 
begins to describe small circles on the ground, and the top wabbles. 
This last sort of motion very well represents the motion of a free rigid 
body rotating under the influence of impressed forces of comparable 
magnitude; and, as we shall see, has to be considered in the case of 
rifled projectiles as they move through the air. 
The motion of the earth, which produces what is called the precession 
of the equinoxes, is another illustration of the same sort of motion. The 
shape of the earth is that of an oblate spheroid, and may be considered 
as a sphere enclosed in a hollow shell, which is thickest at the equator 
and tapers off towards the poles. The attraction of the sun on the 
sphere has no tendency to produce rotation; but, on the shell, the at¬ 
traction of the sun is greater on the half nearer than on the half more 
remote. The resultant action is a tendency to draw the equator into 
the plane of the ecliptic, and produce a rotation round an axis perpen¬ 
dicular to that round which the earth rotates. The consequence is that 
the earth describes a huge gyration, once in about 25,000 years, similar 
to the top in its final stages, but in the opposite direction. For it will 
be observed, that whereas the weight of the top tends to increase the 
angle at the apex of the cone of gyration, the attraction of the sun on 
the earth tends to diminish it; and we should expect to find, also, if 
observations could be made for the gigantic period that would be neces¬ 
sary, that the obliquity of the ecliptic is gradually becoming less and 
less. To this cause the glacial periods which seem to have covered the 
whole surface of the globe, may possibly be due; for if the obliquity be 
becoming less, it must at one time have been greater than it now is, 
and if the obliquity at one time has been very great, it would be suffi¬ 
cient to account for extreme heat and cold, every year, all over the 
globe. There is another action which, like the friction of the peg of 
the top, tends to retard the rotation of the earth—viz., the friction of 
the tides, which Meyer, in an elaborate calculation, has endeavoured 
to show is at present compensated for by the contraction of the bulk 
of the globe. The ultimate result must be, if sufficient time be 
allowed, that the earth will present only one side to the sun—a condition 
which we find in the case of the moon with regard to the earth, and 
