RECENT ADVANCES IN GEOLOGY. 469 
I am not a believer in the doctrine of multiplied shocks. 
I would not, in the explanation of natural phenomena, resort 
to blind catastrophes. But is there not behind all, and over 
all, and pervading all, a great governing principle to whose 
operation we can refer these changes? Does it not exist in 
the celestial mechanism itself? To the solution of this prob- 
lem the attention of several physicists has been directed. 
The speculations of the French savant, Adhemar, are not 
altogether to be overlooked, based as they are on the preces- 
sion of the equinoxes and the movement of the apsides; a 
movement which, I believe, was unknown to the elder 
astronomers. If we compare the movement of the earth 
with the stars, it requires the lapse of 25,000 years to bring 
the equinox to correspond with the same point in space it 
now occupies; but the orbit itself being movable, this 
period is reduced to about 21,000 years. This is called the 
Great Year, being the measure of time before the winter 
solstice will again exactly coincide with the perthelion, and 
the summer solstice with the aphelion, and before the sea- 
sons will again harmonize with the same points of the terres- 
trial orbit. : 
The earth, at this time, approaches nearest the sun in the 
northern hemisphere during autumn and winter, and it is 
only when it recedes the farthest from the source of heat 
that the northern hemisphere receives the full effect of its 
vivifying warmth. As the earth between the vernal and 
autumnal equinox traverses a longer circuit than during the 
other half of the year, and also experiences an accelerated 
movement as it draws near the sun, the result is, that the 
northern summer is longer than the southern by about eight 
days ; but after the lapse of ten thousand five hundred years 
these conditions will be reversed. It was in the year 1248, 
. according to Adhemar, that the Great Northern Summer 
culminated, since which time it has continued to decrease, 
and that decrease will go on until the year 11,748, when it 
will have attained its maximum. 
