522 Tr ansae tions . — Miscellaneous . 



(199 days) ; yet in both cases (supposing the obliquity to re- 

 main constant) the same inequality will exist in the total 

 amounts of heat received during summer and winter- in both 

 hemispheres — i.e., 63 parts in summer and 37 parts in winter. 

 Hence the hemisphere which was enjoying a long summer 

 would have its heat tempered by the length of time over 

 which it was distributed, and its short winter would have a 

 fair amount of heat each day ; but the other hemisphere would 

 have only the same amount of heat distributed over all the 

 days in its long winter, while its short summer would be in- 

 tensely hot. Such conditions, he considers, and shows 

 mathematically, would be very different from those now ex- 

 perienced on earth, and might well produce long periods of 

 genial or arctic climate on one hemisphere and then on the 

 other. The alternations in climate between one hemisphere 

 and the other would, he states, each have a period of 10,500 

 years from commencement to end, a complete cycle lasting 

 21,000 years, and these cycles would probably be recurrent 

 two or three times or more during each period of extreme 

 eccentricity. 



Dr. Croll gives the dates of these periods. The latest cul- 

 minated 200,000 years ago, and ended 80,000 years ago ; 

 another culminated 750,000, another 850,000, another 250,000 

 years ago ; and the next will occur 500,000 years hence. 



Now, if we accept the geological inferences from the known 

 facts of the last ice age in the Northern Hemisphere, — and they 

 are so convincing that we cannot refuse our assent, — it is clear 

 that the cause of that ice age was not the eccentricity of the 

 earth's orbit combined with a favourable position of the line 

 of equinoxes, whatever effect may have been produced by 

 such a conjunction in past ages. 



I will now pass on to put before you the view of Major- 

 General Drayson, an artillery officer who has devoted the 

 greater part of his life to the study of astronomy, and who has 

 made a discovery which, although it has not yet met with uni- 

 versal recognition, is steadily making its way into the position 

 of a fundamental astronomical fact. The discovery is this : that 

 our earth is not only revolving daily on an imaginary axis at 

 present inclined 23° 27' 22-3" to the plane of the ecliptic, or 

 its orbit round the sun ; but that it is also slowly revolving in 

 nearly the opposite direction on another imaginary axis, the 

 pole of which is 6° from that of the ecliptic, 29° 25' 47" from 

 the pole of daily rotation, and has a right ascension of 270°, 

 or 18 hours (Plate XLV., fig. 3). The discovery of this second 

 rotation of the earth is fraught with most important conse- 

 quences. It explains most simply the cause of the precession 

 of the equinoxes and the varying obliquity of the ecliptic, and 

 defines the rate of precession and the amount of the obliquity 



