268 Proximate Causes of certain Winds and Storms. 



" Notwithstanding our advanced latitude, and its being the winter 

 season, we had only begun for a few days past to feel a sensation 

 of cold in the mornings and evenings. This is a sign of the equal 

 and lasting influence of the sun's heat at all seasons to thirty degrees 

 on each side of the line, the disproportion is known to become very 

 great after that. This must be attributed almost entirely to the di- 

 rection of the rays of the sun, independent of the bare distance, which 

 is by no means equal to the effect."* 



Professor Mayer, of Gottingen, undertook to deduce from a com- 

 parison of the meteorological observations, made in different lati- 

 tudes, an empirical law for determining the mean temperature of 

 different points in the earth's surface. He found this temperature 

 to change very slowly in the neighborhood of both the equator and 

 the pole, and rapidly in the intervening space. Thus the mean tem- 

 perature under the equator he makes 84° 2' ; at the parallel of thir- 

 ty, 71° y ; at sixty, 45° ; the differences of which are 13° V and 

 26° 1'. If his numbers are correct, it is apparent that the causes 

 tending to create a movement of the air towards the equator, operate 

 at the parallel of 60° with just about double the force they do at 30°. 



(6.) When the air has once been set in motion by the more ele- 

 vated temperature of the lower latitudes, the creation of a trade wind 

 is determined by the increasing magnitude of the parallels over which 

 it passes in its progress towards the equator, and the rapidity of the 

 current flowing westward, will be greater in proportion as the differ- 

 ences in the circumferences of the successive parallels is greater. 

 But these differences depending upon the differences of the cosines 

 of the latitudes, must be greatest in the high latitudes ; and suppos- 

 ing the movement of the air towards the equator to be the same as 

 within the parallel of 30°, the trade winds should not only exist, but 

 blow more violently there. 



It appears therefore that both of the causes on which the trade 

 winds are made by Hadley's theory to depend, operate with greater 

 energy between the parallels of 30° and 60°, than within the actual 

 limits of the trades, and yet fail of producing any wind. Not only is 

 there no trade wind there, but there is in both the nordiern and 

 southern hemispheres a decided predominance of winds from the west. 

 It is generally regarded as a sound maxim in philosophy, that when 



** Cook's Voyage to tlic Pacific, 4to. Vol. iii. page 255. 



