8i6 THE POPULAR SCIENCE MONTHLY. 



With Dr. Croll's connection with this subject a new era may be 

 said to have been inaugurated. Previously it had been deemed suffi- 

 cient to point out certain agencies which seemed to be adequate to 

 produce the observed effect, without making any effort to show mathe- 

 matically that they ^oere adequate ; but this physicist contended from 

 the first that cause and effect should be determined in absolute mea- 

 sure, just as in the other branches of physical science. His failure to 

 observe this excellent rule in one case is to be attributed to the same 

 paucity of trustworthy observations which is the occasion of the ob- 

 scurity enveloping the whole subject. 



In Chapter IV. the physical agencies leading to changes of climate 

 are discussed, and an explanation of the present low temperature of 

 the southern hemisphere is offered in Chapter V. As has already 

 been intimated, that hemisphere, which has its winters in aphelion^ 

 has a longer winter and a shorter summer than the mean. Now, it is 

 perfectly obvious that this variation in the length of the seasons 

 increases with any increase in the eccentricity of the terrestrial orbit, 

 and similarly diminishes with any diminution of eccentricity ; for the 

 eccentricity of the planetary orbits may vary within pretty wide 

 limits, which have been determined by La Grange, Leverrier, and, 

 more recently as well as more satisfactorily, by Mr. J. N. Stockwell, 

 of Cleveland. The present eccentricity (0-0168) is such that there is 

 a difference of about eight days in the length of summer and winter 

 in either hemisphere, when the solstices coincide with the apsides ; 

 and, the winter solstice of the southern hemisphere being now not far 

 from aphelion, that hemisphere has the long winter and short summer. 

 It is admitted, however (even too readily, it would seem), that the 

 present degree of eccentricity is too insignificant to exercise much 

 influence on the climate of the globe ; but 210,000 years ago, when, 

 according to Dr. Croll's elaborate computations, the eccentricity was 

 0'0575, the excess of winter over summer, due to this cause, amounted 

 to 26*7 days ; and 850,000 years ago, the eccentricity then being 

 0-0747, it amounted to 34-7 days ; and it is argued that so gi'eat a 

 difference in the relative length of the seasons would indirectly, through 

 the intervention of a number of physical agencies, materially affect the 

 earth's climate. These agencies are shown to be such as would be 

 brought into operation by an increase in the length of winter, even if 

 its severity was not increased ; and they are mainly dependent on the 

 increased proportion of moisture precipitated as snow instead of rain. 

 Of course, this snow would remain until melted by approaching sum- 

 mer, just as it does in every region where much snow falls. 



" There are three separate ways whereby accumulated masses of 

 snow and ice tend to lower the summer temperature " : 1. By means 

 of direct radiation [and by direct contact] ; 2. By direct reflection 

 back into space* of the solar rays ; and, 3. By chilling the air and con- 

 densing the vapor into thick fogs which intercept the solar rays. This 



