444: MR. r. W. HAEMEE ON THE [^^o* IQOT* 



admission that the isotherm of 50° Fahr. or thereabouts ^ reached 

 at that period in summer coincidently to lat. 37° 35' N. in North 

 America, the supposed southern limit of the American ice-sheet, 

 that is, to within 14° of the Tropic of Cancer, where the sun's rays 

 are then vertical, and to about lat. 50° N. in Europe. 



The meteorological equilibrium of such a state of things, if even it 

 could have been for a time established, would have been, I think, of 

 a most unstable character. Such a theory requires, however, not 

 that these conditions might have occurred occasionally, but that they 

 should have been persistent for periods extending over thousands of 

 years, which seems to me to be highly improbable.^ At present it is 

 in winter, when the isotherms are crowded together in North America, 

 that cyclonic storms arise there most frequently. In summer, when 

 the difference in temperature between adjoining regions is not so 

 great, they are less numerous. The marked contrast between the 

 summer climate of the southern part of the American ice-sheet 

 during its greatest extension, and that of the region immediately 

 to the south of it, might have caused (to an even greater extent than 

 now) atmospheric disturbance at that season, and cyclonic storms, 

 with southerly winds to the east of their centres, would have 

 then occurred more frequently in the eastern part of the North 

 Atlantic.^ 



It may, perhaps, be suggested that by the pressure of an anti- 

 cyclone, extending from the Polar regions over North America, 

 Greenland, and Northern Europe at the same time, the Atlantic 

 cyclone might have been driven so far to the south, that the 

 influence of the south-westerly winds thereby caused, even in 

 summer, although affecting Spain and France, may not have 

 extended to the British Isles. We have seen, however (fig. 11, 

 p. 437), that when the North Atlantic cyclone reaches as far south 

 as lat. 35° N., no such effect is produced, the contrary being rather 

 the case.^ 



The necessary existence in the southern part of the North Atlantic 

 at that time of a high-pressure system, complementary to the Equa- 

 torial trough of low pressure, raises the question whether the 

 Icelandic cyclone could have been forced indefinitely southward. 

 The farther it was driven in that direction, moreover, the warmer 

 would have been the winds on the eastern side of its centre. The 



^ The southern coast-line of Greenland now coincides with the isotherm of 

 45° Eahr. An average summer-temperature in Labrador of 50° -Fahr., on the 

 contrary, seems sufficiently high to prevent the permanent accumulation of 

 snow. 



^ The farther southward the isotherms of low temperature reached, the 

 greater, I think, would have been the instability of the atmospheric equilibrium. 



^ Mr. Borchgrevink states that sudden storms of great violence are prevalent 

 near the edge of the Antarctic ice-sheet ; see ' First on the Antarctic Continent ' 

 1901, passim. 



* It is the existence of an ice-sheet under such conditions, and especially 

 during the summer, in Great Britain (leaving Scandinavia out of the question), 

 that is the difficulty. No great ice- sheet could have accumulated in a region 

 under the prevalent influence of warm winds. 



