Perry.] 78 [February 28, 
may be drawn of no small moment, it is thought, to a right under- 
standing of much pertaining to the Glacial period. 
I am, of course, well aware that Mr. Croll, a distinguished Ene- 
lish physicist, has enthusiastically adopted one and another of these 
and of kindred explanations in succession, and that, after holding 
his.ground in favor of them for a while, he has with equal warmth 
rejected each of the suggested causes as insufficient to account for 
the facts requiring an explanation. And his rejection of any or of 
every one of them in succession as inadequate, when looked at in an 
isolated way, was no doubt in a certain sense justifiable ; ‘indeed, he 
was perhaps right in each particular instance; for probably no one 
of them by itself is able to produce the effects in question. But, while 
none of them is singly sufficient, the conclusion may be, and clearly 
must be, very different in respect to them all combined, and regarded 
somewhat in the way suggested. Would we arrive at the truth it 
should be our aim not to drop from the calculus any cause, however 
insufficient by itself, that can have the least bearing upon it, but to 
bring in all the elements so far as possible, and combine them in 
such a way as to approximate toward just the co-operative combina- 
tions which appear in nature. 
It should be remembered that, owing to the many different ele- 
ments which enter into the calculus, thése periods to which reference 
has been made cannot usually consist of a definite number of centu- 
ries or millennia, but must vary from ave to age. So, to the astro- 
nomic facts which have been briefly noticed, the exact relative 
‘mportance of which is not yet fully and clearly made out, there 
should be added, in order to the better understanding of the Aton- 
jan winter, two conjectures. These are of an astronomic kind, have 
been heretofore hinted at, and tend to show that the great climatic 
periods of the ages are liable to various mutations. First may be © 
mentioned the supposition that the sun is a variable heavenly body, 
and that thus after a time of new fuelling, and consequently of more 
than ordinarily intense ignition, there would naturally come a period 
during which less heat would be given off. Should such a state of 
the sun concur with the other conditions favorable to the greatest de- 
gree of cold, on the coincidence of the three astronomic elements al- 
ready mentioned, the winter, as must be evident, would be unusually 
severe, and perhaps very long-continued—probably far more pro- 
tracted than under ordinary circumstances. The second supposition 
implies that some portions of the celestial spaces have far less heat 
