674 GEOLOGY. 



Total thermal efficiency of the atmosphere. — It seems worth while in this 

 connection to try to estimate the total agency of the atmosphere in preserving 

 the temperature of the globe. The earth and moon are at practically the same 

 mean distance from the sun, and would doubtless have practically the same tem- 

 perature if the earth had no atmosphere or ocean. (The ocean is theoretically 

 a derivative from the atmosphere.) If the present mean temperature of the earth's 

 surface be taken at 15° C, the absolute temperature is —288° C. As the result 

 of a recent elaborate study, Very estimates the temperature of the moon at 

 its midday to be 97° C. 1 Several other astronomers have thought that the moon's 

 midday temperature did not rise much above the freezing-point. The range 

 of competent opinions may then be limited by the boiling- and freezing-points, 

 373° and 273° absolute. It is agreed by all that the lowest night temperature 

 cannot be far from absolute zero. Allowing this, however, to be 55° absolute, 

 the average temperature of the moon on the higher estimate is 214° and, on 

 the lower, 164° absolute. The difference between these, respectively, and the 

 mean temperature of the earth, 288°, is 74° and 124°, of which the mean is 99°. 

 While there is a rather large element of uncertainty in the estimates of the moon's 

 temperature, their mean gives countenance to the belief that the efficiency of 

 the earth's atmosphere, as a thermal blanket, may be in the neighborhood of 

 100° C. 



If the mean temperature of the earth in the Carboniferous period be placed 

 at 25° C, it will doubtless be about high enough, for that is a tropical tempera- 

 ture, and if the mean temperature of the glacial stages of the Permian be placed 

 at 10° C, this will perhaps be low enough. The range between these, 15° C, 

 would, on the basis above deduced, make the change from the one to the other 

 only 15% of the total efficiency of the atmosphere. When allowance is made 

 for the effects of increased vertical circulation, it does not seem that it is taxing 

 the assigned changes in the water- vapor and carbon dioxide too severely to make 

 them responsible for the remaining portion of the 15% of the whole thermal 

 competency of the atmosphere. 



IV. The Localization of the Glaciation. 



The remarkable distribution of the Permian glaciation is the most puzzling 

 feature of the whole problem. The localization of the recent Pleistocene gla- 

 ciation was also peculiar, as will be seen, but still it was measurably circum- 

 polar, as we should expect any general glaciation to be. Among the explana- 

 tions offered for this later puzzle is one which connects it with the areas of per- 

 manent low atmospheric pressure in the North Atlantic and North Pacific regions, 

 in which the present glaciations of Greenland and Alaska are located, and to 

 which they apparently owe their exceptional development. 



Tentative conception of the " fixed lows " and their relations to glaciation. — 

 The cause of the prevailing low pressure and permanent cyclonic character of 



1 Probable range of the temperature of the moon, Astrophys. Jour., VIII, 1898, 

 p. 265. See also the temperature of the moon, S. P. Langley, assisted by F. W. Very, 

 Nat. Acad. Sci., I. 



