Dr. CrolPs Hypotheses on Geological Climates. 233 



over very large extents of sea, they bring to the equatorial 

 regions the temperature which prevails over tropical seas. 

 There can be no question of an influence of aerial currents 

 from the middle latitudes on the temperature of the equator, 

 and the latitudes 10° N. to 5° S. at least. Two examples 

 show how the thermal influence of stronger winds than the 

 trades nearly disappears as soon as they have blown over an 

 extent of about 1000 miles of sea. The coldest region of the 

 tropics in winter is Southern China ; near Canton frosts 

 are not rare in winter ; the N.E. monsoon blows as a strong 

 steady wind towards the coast of Cochin China; and yet at 

 Saigon, but 12 J° south of the tropic, the mean temperature of 

 January is not below the normal of the parallel. We have : — 



Mean temperature 

 of January. 



Canton, 23° N 54*8 



Hong Kong (Victoria), 22° N . . 53*5 

 Saigon, 11° 1ST 77-5 



Thus, between Hong Kong and Saigon the difference per 

 degree of latitude is 1 0, 8 F., while the mean difference in 

 January between 10° and 20° N. is 0'77 F. per degree of lati- 

 tude. Thus we see, that when a cold wind from the middle 

 latitudes reaches the borders of the tropical zone, it is soon 

 warmed on passing over the broad expanse of the seas, and 

 already north of 10° N. the cooling influence is not felt. 

 The exceedingly small difference of temperature in the zone 

 between 10° N. and 10° S. is a proof that the cooling influence 

 of winds from middle latitudes is not felt there. 



Another cold wind reaches the tropical zone — the famous 

 Northers of the Gulf of Mexico. This is not a steady wind 

 like the N.E. monsoon of China, but a cold wind blowing at 

 times with the utmost violence. At the mouth of the Bio 

 Grande (26° N.) frosts happen every winter, and even tem- 

 peratures of 23° ha\e been observed during Northers ; the 

 latter are frequent and dangerous at Yera Cruz (19° N.) ; 

 but the temperature does not sink below 51°, and not below 

 59° on the exposed, relatively very cold plateau of Tarifa, on 

 the isthmus of Tehuantepec. Owing to their extreme vio- 

 lence, the Northers keep for a longer distance a much lower 

 temperature than other winds of the tropical zone. Not- 

 withstanding the Northers, the mean January temperature 

 of Yera Cruz (71°*7) is not below the mean for the latitude. 



On the extensive wooded plains of the Upper Amazons 

 weak winds and calms prevail for a great part of the year, ana 

 the S.E. winds of the drier months have so little the power 



