428 FRANK BURSLEY TAYLOR 
and the movement of the perihelion of the earth’s orbit. Mr. 
G. K. Gilbert has used this period of climatic oscillation in con- 
nection with another matter. His statement relating to the 
length and variability of the precessional period is concise and 
convenient. ‘The precessional period is about 26,000 years, but 
the position of the perihelion also moves—for the most part in 
a direction opposite to that of the equinoxes—and the resultant 
of the two motions has an average period of about 21,000 years. 
It is not absolutely regular, but ranges ordinarily within 10 per 
cent. of its mean value, and exceptionally to 50 per cent. above 
and below.’* The period at 21,000 years seems too long, but if 
we take it at its minimum of 10,500 years, it may not be.? 
THE THEORETICAL EFFECT OF PRECESSIONAL OSCILLATIONS OF 
CLIMATE UPON THE ICE-SHEET. 
Let us see in what manner the astronomical forces would work, 
supposing the oscillation of the ice-front to be due to precession. 
Precession is produced by the rotation of the axis or pole of the 
earth around the pole of the ecliptic. The figure thus described 
on the celestial sphere is not in reality a true circle, but for the 
purposes of this paper it may be assumed that it is, without in 
any way impairing the general truth or validity of the conclu- 
sions reached. The general idea of the influence of precession 
upon terrestrial climate has been so often discussed that it is 
hardly necessary to dwell at length upon it here. But it is 
of causes of advance orretreat proceed in waves downward from the névé to the end of 
the ice-tongue. From this and other causes of complication he finds it hard to make 
out clearly any regular period of variation. Nevertheless, it is probable that a thirty- 
five-year period exists. The progress of investigation along this line is well summar- 
ized by Professor H. F. Reid, “ Variations of Glaciers,” Jour. GEOL., Vol. III, No. 3, 
1895, p. 278 et seq. 
*“Sedimentary Measurement of Cretaceous Time,” by G. K. GILBERT. Jour. 
GEOL., Vol. III, No. 2, 1895, pp. 121-127. 
* There is a possibility that the period of precession during the glacial epoch was 
considerably shorter than the minimum of the modern calculation. Perhaps the 
chance of this seems very remote, but there are small changes going on, now appar- 
ently secular, which in so great a lapse of time may prove to be periodic and may 
come to be of prime importance in their effects on terrestrial climate. 
