The President's Address. 19 
be always the case; on the contrary, a gradual change is continually 
taking place during a cycle of 21,000 years. Taken by itself the 
balance of astronomical authority is not, I think, of opinion that 
this would greatly influence our climate. The effect, however, which 
the obliquity of the ecliptic would exercise depends greatly on the de- 
gree of eccentricity of the earth’s orbit. This is continually changing, 
and the more elliptical it is the greater the effect produced by the 
above-mentioned causes. At present the orbit is nearly circular, 
and consequently the difference of temperature between the two 
hemispheres is less than usual. Mr. Croll and Mr. Stone have 
caleulated the eccentricity for the last million of years, and have 
_~ shown that there are two periods especially, one namely from 850,000 
SS eee Se 
Pe ig es 
to 750,000 years ago, the other from 200,000 to 100,000 years ago, 
when the eccentricity of the orbit was far greater than usual, and 
when, therefore, the difference of temperature between the two 
hemispheres would also have been unusually great. From 100,000 
to 200,000 years ago, then, there was a period when our climate 
underwent violent oscillations, being for 10,500 years far colder than 
now, then for a similar period far hotter, then far colder again, and 
so on for several variations. These alternations of hot and cold 
periods beautifully explain the difficult problem of how to account 
for the existence of remains belonging to tropical and to Arctic ani- 
mals associated together in the same river gravels. It also throws light 
on the fact, first pointed out by my friend, M. Marlot, that there 
are in Switzerland geological indications of several periods of extreme 
cold with others of more genial climate, and Mr. Croll,in his “ Climate 
and Time,” has shown, from the evidence of two hundred and fifty 
borings in the Scotch glacial beds, that many of them show evidence 
of the existence of warm interglacial periods. The antiquity of this 
period therefore really must be solved by the mathematician and 
physicist rather than by the antiquary, and it affords us an excellent 
illustration of the manner in which the different branches of science 
depend upon one another, and of the fact that the more science ad- 
vances the more necessary it is that our higher education should be 
based on a wide foundation. 
The Bisuor proposed a vote of thanks to Sir John Lubbock for 
c 2 
