Sullivan — On Thenardite. 
3 
ceived, but also upon the amount retained. That is, although the actual 
amount of heat now received during 4296 hours of day in the 
southern hemisphere is as much as that receiyed in the northern hemi- 
sphere during 4464 hours of day, yet, as the radiation during 4464 hours 
of night in the southern hemisphere exceeds that which takes place 
in 4296 hours of night in the northern, the mean temperature of the 
former hemisphere must necessarily be lower than that of the latter. 
Owing to the joint action of the precession of the equinoxes and the 
revolution of the apsides, the seasons make a complete revolution in 
21,000 years, so that in each hemisphere summer occurs alternately in 
perihelion and aphelion, and each consequently endures in turn the 
longer winter in aphelion. 
The effects of these astronomical changes, though not recognizable in 
the present condition of things, being completely masked by the far greater 
effects due to physico- geographical causes, increase with the excen- 
tricity of the earth's orbit, which is always altering, so that with a very 
high excentricity considerable changes in climate may result from 
purely astronomical causes. At present the excentricity is small ; but 
there was a time when it was less, and periods when it was from three to 
four or five times greater. Thus the number of days by which winter oc- 
curring in aphelion exceeds summer in perihelion varied from 4-9 to as 
much as 36*4, or 28'3 days more than at present. Mr. -Stone, acting 
upon a suggestion made to Mr. Airey, the Astronomer-Royal, by Sir 
Charles Lyell, calculated by Le Yerrier's formula the excentricity of the 
earth's orbit at different periods. Mr. James Croll completed these cal- 
culations for the last 1,000,000 years. With the value for the excentricity 
thus obtained, and assuming the temperature of space to be -239° Eahr., 
the mean temperature of the hottest month in London to be 64° Fahr., 
and of the cold months 38° Fahr., Mr. James Carrick Moore calculated 
the number of days in which the winter occurring in aphelion should be 
longer than the summer in perihelion, and the mean temperatures of 
the coldest months during the former, and of the hottest during the 
latter, at the locality of London for a number of periods in the last 
1,000,000 years. 
Such calculations are necessarily only rough approximations ; and 
as they do not, and could never take into account the great disturbing 
influences of the relative positions of land and water, they merely indi- 
cate the probable extentto which these astronomical causes may influence 
climate, supposing the temperature of space assumed to be not very 
far from the truth. The table of results calculated for intervals of 
50,000 years given by Sir Charles LyelP are nevertheless of very great 
interest, and deserve the serious attention of geologists. Among other 
points of interest, they suggest a definite date for the Glacial Period, 
and give one of the elements upon which the temperatures which pre- 
vailed during that period depend. They even give, as I have stated, 
the approximative mean temperatures of the hottest and coldest months 
* Priuciples, 10th Ed., vol. i., p. 293. 
