196 



Fisher— Ages of the " Trail" and " Warp." 



of the S. Polar ice is due to the phase of precession in which we now 

 are, then it may be readily admitted that the present position of the 

 equator of warmth to the north of the " line," and the extension of 

 the S.E. Trades in the same direction, is due to that cause; and, 

 consequently, the change of seasons involved in the transference of 

 our winter to aphelion (and of the southern winter to perihelion) 

 would so far warm the southern hemisphere, as to weaken the S.E. 

 trades, and cause what is now the Gulf Stream to flow southward 

 instead of northward. 



In all these questions, and many similar ones, the real difficulty 

 is not to see in what direction the effect would be produced, but to 

 what extent. And this difficulty is enhanced in the present case 

 because we do not know with certainty what the absolute heating 

 power of the sun may be, the temperature — 239° F., attributed to 

 space being by no means certainly correct. 



If, however, Mr. Dawkins and myself are right in attributing this 

 recent deposit to a Glacial origin, it is a proof that some refrigerating 

 cause has been in operation during comparatively recent geological 

 times, when this country had, or rather, as I believe, finally received, 

 its present contour. Now, since one cause that we know of has been 

 in operation within such a recent period, viz., that of precession, it 

 seems extremely reasonable to attribute the observed effect to it. 



In considering the effect of precession, we must not forget that 

 its efficiency for producing climatal effects depends upon the eccen- 

 tricity of the earth's orbit. Let us, then, observe what values this 

 element had at those periods nearly preceding our own era, at which 

 the seasons were reversed. 



The northern winter solstice occurred in perihelion a.d. 1248. 

 It therefore occurred in aphelion 10468 years previously to that 

 epoch, that is, 11020 years before the year 1800; and at intervals 

 of 20937 years each, preceding that period. Hence we have the 

 following table : — 





Number of 



Eccentricity 



Winter tempera- 



Ditto of Scotland, 







years before 1800, 



of the earth's orbit 



ture of Snowdon 



the Gulf Stream 



Excess of 





■when the 



nearly,+ if itwas 



■without the 



being diminished 



•winter over 





Northern -winter 



greater, —if it 



Gulf Stream on 



in proportion 



summer in 





occurred 



■was less than the 



Hopkins' first 



to the eccentricity 



days. 





in Aphelion. 



value given. 



hypothesis. 1 



(Mr. Croll).2 







11020 

 31957 

 52894 



•0187 + 

 •0151— 

 •0131 + 



5°.05 



Not given. 



86 





73831 



•0316 + 



— r.oi 



5°1 



14-8 



A 



94768 



•0452 + 



-7M7 



— 3°2 



21-0 



115705 



• 0460— 



— 7°.52 



—3" 



21-3 



The fifth and sixth columns of Sir Charles Lyell's table, which give 

 the mean hottest and coldest months in the latitude of London when 

 winter is in aphelion, are calculated from the present means. It seems 

 to me, however, that, for cold epochs, it will be safer to calculate from 

 the mean as it would have been had the Gulf Stream not existed. 



* Geol. Jour., Vol. viii. p. 68. 



2 Phil. Mag. for Fet., 1867, p. 3. 



