Woeikof — GrolVs Hypotheses of Geological Climates. 171 



We have — 



Mean temperature of January. 



Canton, 23° N 54-8 



Hongkong- (Victoria) 22° N ■ 59'5 



Saigon, 11° N. . 77-5 



Thus, between Hongkong and Saigon, the difference per de- 

 gree of latitude is 1*8° F. while the mean difference in Janu- 

 ary between 10° and 20° N. is 0*77° F. per degree of latitude. 

 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 on the zone between 10° N.- 

 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 Norther of the Gulf of Mexico. 

 This is not a steady wind like the northeast monsoon of China; 

 but a cold wind blowing at times with the utmost violence. 

 At the mouth of the Rio Grande (26° 1ST.) frosts happen every 

 winter, and even temperatures of 23° have been observed dur- 

 ing Won hers. The latter are frequent and dangerous at Vera 

 Cruz (19° N.) but the temperature does not sink below 59°-51° 

 F. and below the exposed, relatively very cold plateau of 

 Tarifa, on the isthmus of Tehuantepec. Owing to their ex- 

 treme violence, 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 plain of the upper Amazons, weak winds 

 and calms prevail a great part of the year ; and the S. E. 

 winds of the drier months have so little power of cooling the 

 air that at Iquitos the absolute minimum during a year was 

 66*8 F. It is clear that in continental regions also, cold winds 

 from the middle latitudes do not reach the equatorial zone. It 

 may be fairly asked, where and how did Dr. Croll get his opin- 

 ion about the cooling influence of winds from middle latitudes 

 on the temperature of the equatorial regions ? Would it not. 

 have been better to inquire about some of tne best known 

 facts of climatology, before speculating "& perte de vue " and 

 bluntly stating that the mean temperature of the equator 

 would be 55° above what it is now, if it was not for the heat- 

 abstracting action of ocean-currents. In fact, on the Upper 

 Amazons there is no such heat-abstraction, and aerial currents 

 certainly cannot have a cooling, influence of even a degree F., 

 and yet the mean temperature reduced to sea-level is not any- 

 thing like 135° F., but below 80°. Besides the immense influ- 



