232 Dr. A. Woeikof s Examination of 



55°, i. e. give it a mean temperature of 135°." Now such a 

 temperature is not only above anything known on the globe 

 as the mean temperature even of a single month, but the 

 absolute maximum known by exact observations does not 

 exceed 131° F. (55° C.). 



Though Dr. Croll repeatedly, and quite rightly, points out 

 the far greater influence of ocean -currents as compared with 

 air-currents in modifying the temperature of the globe, he 

 quite unexpectedly expresses an opinion as to the power of 

 air-currents in cooling the temperature of the ground and 

 air at the equator which ought not to remain unchallenged. 

 " No knowledge whatever as to the intensity of the sun's heat 

 can be obtained from observations on the temperature of the 

 air at the equator. The comparatively cold air flowing in 

 from temperate 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 tem- 

 perature 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 teat by observations on the tem- 

 perature of the air or the ground at the equator or elsewhere. 

 But Dr. Croll evidently thinks that this would be possible at 

 the equator were it not for the influence of cool winds from 

 temperate regions ! 



Let us first see how far facts corroborate this opinion. 

 According to Ferrers tables, the mean annual temperature of 

 10° N. is 81° F., of the equator 80°'l F.; thus the equator 

 cannot be cooled by winds from the northern hemisphere, as 

 the lowest 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 0, 7, L e. only 1 0, 4 lower than 

 the equator. Even in July, the winter of the southern hemi- 

 sphere, the difference amounts to but 3' 8, i. e. in the annual 

 mean to o, 14 F. for a degree of latitude, in July to o, 38 

 for the same. 



The greater part of the equatorial region consists of ocean 

 and islands, where, at least south of the equator, the trade- 

 winds prevail. They are not strong winds, a few ocean regions 

 excepted, they have a very easterly direction, and blowing 



* ' Climate and Cosmology,' chap, iv., and ' Nature/ vol. xxi. p. 129. 



