G. W. Buhnan — Revised Theory of Glaciation. 265 



pass through the exceedingly attenuated upper strata of the at- 

 mosphere. Again, if cooling down to — 300° within six months 

 were possible, we should expect to find some approach to this in. 

 the lowest recorded temperatures of the Arctic regions during the 

 absence of the sun. It is true that in these regions the temperature 

 is modified by warm winds and waters. Yet on still, clear nights, 

 in places removed from the influence of ocean currents, these modi- 

 fying causes will be reduced to a minimum. I have not myself 

 come across any record of temperature so low as — 80°, and even if 

 we suppose it sometimes sinks lower, we have still no indication 

 that — 100° is too high for our calculation. And we have to 

 remember, as a set off against tbe influence of winds and currents, 

 and probable lower temperatures than any yet recorded, that the 

 intense cold of the Arctic regions may be due to other causes besides 

 radiation. The evaporation which taiies place from the surface of 

 ice and snow itself reduces the temperature already, it may be, far 

 below freezing. Snow, again, falling on the open sea melts even in 

 water already below freezing, and thereby adds to the cold. 



And there is another consideration which seems to show that 

 — 100° or even higher is a more suitable point to reckon from than 

 — 300°. According to Sir R. Ball the present difference in the mean 

 daily average of heat between summer and winter is 1'24 — 0-75, or 

 0-49. If --^Q of a unit means 30° difference in temperature, then 

 49 would be equivalent to 30° x 4-9 = 14.7°. And our present July 

 temperature is 60° ! The average difference between summer and 

 winter is about 20°. This would seem to indicate that —50° is the 

 most correct starting-point. 



Taking — 200° or — 100° as starting-points, then, according to 

 Sir R. Ball's calculation, the tenth of a heat unit will mean 20° or 

 10° instead of 30°, and the reduction in the average winter tempera- 

 ture would be i-Vx20°=14°, or t:Vx10°=7°. This would give 

 28-3°, or 35'3°, as the average winter temperature during the winter 

 of 199 days. 



With these figures, then, the Revised Astronomical Theory seems 

 insufficient to account for glaciation. On the other hand, they 

 permit to a warmer summer during a genial period. 



It appears, then, that a suitable temperature as a basis for calcula- 

 tion is the imperative need of the Theory, and that the use of — 300° 

 has not been justified. 



In his statements concerning the effects of varying eccentricity 

 and the precession of the equinoxes on the length of the seasons. 

 Sir R. Ball is not quite so clear as might be desired on a point of 

 such primary importance to his Theory. Thus, on p. 95, we find 

 the statement that, " loith the present eccentricity of the earth's orbit, 

 the great possible difference between summer and winter would 

 amount to 33 days. I do not mean that the actual disparity 

 between summer and winter at the present moment is so much as 

 this ; it only, in fact, amounts to seven days, because at present the 

 line of equinoxes does not happen to be adjusted in the manner 

 described." 



