Trans. N. V. Ac. Set. 90 April 14, 



the ecliptic, and that once in any position it can never change* "by 

 any force known to science." To all of which those of us who are not 

 astronomers can only answer — What you say may all be true, but, 

 nevertheless, the axis has been tilted over to 23^° by means of some 

 force not known to science. And if we cannot tell how it occurred — 

 an inability which extends to a great many things besides this — our 

 business is to discover, if we can, when it occurred. It is not a ques- 

 tion of possibilities, but of chronology. The uniformity of biological 

 conditions in high and low latitudes indicates that the present obliquity 

 had not been attained in Archaean, or in Palaeozoic, or in Mesozoic 

 times. A similar uniformity prevailed in the Tertiary till toward its 

 close ; then comes a blank of unknown length, during which the Glacial 

 epoch came and went, and then, when the record again begins to be 

 legible, there are, for the first time in the world's history, indications 

 of zones of climate and of alternating seasons. This seems to fix the 

 date of the increase of axial obliquity as corresponding to the Glacial 

 epoch. 



It requires no argument to show that a perpendicular axis would 

 account for the otherwise inexplicable evenness of ancient climate. 

 Ocean currents might bend the isotherms, but at any place the tem- 

 perature, whatever it was, would be the same all the year. The hours 

 of light and darkness would be the same everywhere. 



This kind of uniformity is, however, compatible with great cold, 

 and does not touch the question of a warm polar climate. Many 

 theories have been proposed to account for the remarkable tempera- 

 ture. Six or seven are ably treated by Searles V. Wood, Jr., in the 

 Geological Magazine for September and October, 1876, and by Dr. 

 Croll in Climate and Time. Dr. Croll's theory differs from those. 

 To it there are what seem insuperable objections. I have discussed 

 this theory somewhat fully in Penn Monthly for June, July, and August, 

 1880, and shall not now repeat. 



Professor Whitney has lately put forth another explanation, in which 

 he attributes the preglacial warmth to the sun itself being hotter. All 

 conclusions in regard to the sun's former temperature must be hypo- 

 thetical ; but if it be a gaseous body, as suggested by Professor YouNG, 

 it has been growing hotter all the time it has been condensing. f To 

 this, as to all other theories heretofore advanced, lies the objection that 

 they ignore the uniformity which is such a remarkable feature of 

 geological climate. 



♦ There is a small secular change of obliquity going on now, but it oscillates about a mean 

 position. 



t A fact discovered by Mr. J. H. Lane. See American Jour. Science, July, i88o ; also. New- 

 comb's Astronomy, page 508. 



