MORAINES OF RECESSION 429 
necessary to see ‘clearly how the position or place of the ice- 
front is related to this kind of climatic change. The change of 
climate from this cause does not go on with the rotation of the 
earth’s pole pari passu. For the pole, under the simplified con- 
ditions here postulated, moves in a circle and changes its posi- 
tion at a uniform rate. The effect of this movement on climate 
is to produce a periodic or oscillatory change to and fro between 
two extremes or climaxes, and these changes go on inan endless 
alternating series, from cold to warm, from warm to cold, from 
cold to warm, and so on. In glacial times a climatic variation 
of this sort, even if it were slight in amount, must have had its 
effect on the ice-sheet. As climate grew more severe the ice- 
sheet would advance its front all along and spread over a larger 
area, and as climate moderated the ice-front would draw back 
and the area of the ice-sheet would be reduced. Thus it may 
be demonstrated that the effect of a precessional oscillation of 
climate upon the ice-sheet would be to cause it to alternately 
increase and decrease its area by a series of expansions and con- 
tractions, and this process would necessarily be accompanied by 
a corresponding series of alternate advances and retreats of the 
ice-front. We are thus enabled to infer the character or manner 
of the oscillations of the ice-front, supposing them to be due to 
precession. They would obviously follow the manner of what 
is called simple harmonic motion. If a wheel be made to rotate 
on a fixed vertical axis a point or peg on its rim describes a cir- 
cle when viewed from above. But if we look at the wheel edge- 
wise, or from the side, the peg appears to move to and fro along 
a straight line, more slowly near the ends, fastest in the middle, 
and coming to rest for an instant at each extremity. The man- 
ner of the apparent motion is like the swinging of a pendulum 
viewed from below... This is the manner in which the forward 
and backward movements of the ice-front would take place if 
produced by precession. As the cold increased after a warm 
climax, the ice-front would advance, at first slowly, but at increas- 
ing rate, until the middle point of the oscillation was reached ; 
then more and more slowly until it came to rest at its cold or 
