Vol. 57.] THE CLIMA.TE OF THE PLBISTOCEXE EPOCH. 453 



at present. A study of Dr. Buchan's monthly charts shows, how- 

 ever, that as spring advances and the laud-tracts of the temperate 

 regions of the ^N'orthern Hemisphere are heated by the sun, 

 the continental anticyclones shrink in extent and move north- 

 ward, finally coalescing over the iS^orth Pole as represented in 

 the statistical chart for July (fig. 19, p. 454), and forming there 

 a small anticyclone with a maximum pressure of 29 "90 inches only, 

 that of the circumpolar regions as far south as lat. 50° jN". being but 

 little lower. At that season, moreover, the isotherms of the northern 

 regions coincide more nearly with the parallels of latitude than they 

 do in winter. 



jS'o permanently ice-clad area now exists (except in the southern 

 part of Greenland) south of the present July isotherm of 40° Pahr.^ 

 The American ice-sheet is believed to have extended, however, 

 considerably beyond the present isotherm of 70° Pahr., and in one 

 direction to have approached that of 80°. (See fig. 5, p. 415.) 



The existence in North America during summer at that stage of 

 the Glacial Period of a cold area of such magnitude so far south must 

 have been accompanied, as already urged, by a distribution of pressure 

 different from that which now prevails. Under such circumstances 

 the Polar anticyclone may have been stronger, and may have reached 

 farther south at that season than it now does. At the same time, 

 pressure over the ice-sheet must have been prevalently higher than 

 that over the adjoining oceans, though the baric gradient may not 

 have been so steep as during the winter. The barometer is not 

 statistically high in Greenland in summer at present, but there is a 

 distinct though gentle gradient of lower pressure from that country 

 towards the south (fig. 19, p. 454), the winds blowing in the same 

 direction as they would do if the pressure were greater all round. 

 While, therefore, during the maximum gla elation of North America 

 there might have been in winter a frequent recurrence of conditions 

 like those described on p. 437 (fig. 11), south-westerly winds may 

 also have been then frequent during the summer on the eastern side 

 of the North Atlantic, If so, the climate of the British Isles 

 would have been warm at that season, and no permanent accumu- 

 lation of ice could have taken place there. On the other hand, north- 

 easterly winds would have been prevalent during the summer in 

 North America, instead of mild or warm winds as at present.^ 



We have little knowledge as to the conditions obtaining in Central 

 Asia at the different stages of the Glacial Period. There is evidence, 

 however, that at some period the glaciers of the Himalayas and of 

 other mountain-regions on that continent attained a greater exten- 



1 The July isotherm of 45° Fahr. includes the whole of Greenland. 



^ In connection with the suggested anticyclonic condition of the ice-sheet 

 during the summer, it is necessary to remember, as Mr. H. N. Dickson has kindly 

 reminded me, that the air descending upon the anticyclone would have been 

 warmed by compression, and it would therefore have tended to melt the ice. 

 On the otlier hand, the melting of the ice would have lowered the temperature, 

 as would radiation during the night. Moreover, some of the warmed descend- 

 ing air might have flowed outward at a higher level, without reaching the 

 earth. See Ekholm, Quart. Journ. Koy. Met. Soc. vol. xxvii (1901) p. 2.X 



