186 ADDRESS. [LECT. 



In consequence, however, of the obliquity of the ecliptic, 

 this only happens twice in the year, namely, on the 20th 

 of March and 23rd of September, which days divide the 

 year into two halves, the day being longer than the night 

 in the spring and summer, and shorter, on the other 

 hand, in autumn and winter. Under existing circum- 

 stances then, we have in the northern hemisphere seven 

 days more of summer than of winter, while in the southern 

 hemisphere they have, on the other hand, seven days 

 more of winter than of summer. This, however, has not 

 been, nor will it 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 astro- 

 nomical 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 degree of eccentricity of the earth's orbit. 

 This is continually changing, and the more elliptical it is, 

 the greater is the effect produced by the above mentioned 

 causes. At present the, orbit is nearly circular, and con- 

 sequently the difference of temperature between the two 

 hemispheres is less than usual. 



Mr. Croll and Mr. Stone have calculated the eccentri- 

 city for the last million of years and have shown that 

 there are two periods especially, one namely from 850,000 

 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 must 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 



