170 Woeikof—GrolVs Hypotheses of Geological Climates. 



unexpectedly, expresses an opinion as to the power of air-cur- 

 rents in cooling the temperature of the ground and air at the 

 equator, which ought not to remain unchallenged. " No knowl- 

 edge whatever as to the intensity of the sun's heat can be ob- 

 tained from observations on the temperature of the air at the 

 equator. The comparatively cold air flowing in from tempe- 

 rate regions has not time to be fully heated by the sun's rays 

 before it rises in an ascending current and returns to the 

 temperate regions from whence it came. More than this, these 

 trades prevent us from being able to determine with accuracy 

 the intensity of the sun's heat from the temperature of the 

 ground ; for the surface of the ground in equatorial regions is 

 kept at a much lower temperature by the air blowing over it 

 than is due to the intensity of the sun's heat."* Certainly no 

 physicist or meteorologist ever thought of a determination of 

 the sun's heat by observations on the temperature of the air or 

 the ground, on the equator or elsewhere. But Dr. Groll evi- 

 dently thinks that this would be possible on the equator, were 

 it not for the influence of cool winds from temperate regions ! 



Let us first consider how far facts corroborate this opinion. 

 According to Ferrei's tables, the mean annual temperature of 

 10° N. is 81° F.; of the equator 80° F.; thus the equator cannot 

 be cooled by winds from the northern hemisphere, as the low- 

 est latitudes of the latter are warmer than the equator. The 

 cool winds must come from the south. Yet the mean annual 

 temperature of 10° S. is 78'7, that is, only 14 lower than the 

 equator. Even in July, the winter of the Southern hemisphere,, 

 the difference amounts to but 3*8, that is, in the annual mean 

 to OM F. for a degree of latitude, in July to 0*38° for the 

 same. The greater part of the equatorial regions consists of 

 ocean and islands, where, at least south of the equator, the 

 trade-winds are prevailing. They are not strong winds, a few 

 ocean regions excepted, they have a direction from the east- 

 ward and, blowing over very large extents of sea, they bring to 

 the equatonial regions the temperature which prevails over the trop- 

 ical 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° 1ST. to 5° S. at least. Two exam- 

 ples 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 northeast monsoon blows as a strong, 

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

 Saigon, but 12-| south of the tropic, the mean temperature of 

 January is not below the normal of the parallel. 



* Climate and Cosmology, Ch. IV, and ' Nature,' vol. sxi, p. 129. 



