32 Professor Mitchel. on the Trade Winds, 



the parallel of 30°. But the heat at the parallel of 30° exceeds 

 that of the parallel of 60° more than it is itself exceeded by the 

 heat of the equator. Both theory and observation lead us to 

 this conclusion. See Halley''s paper in the Philosophical Trans- 

 actions, and Emerson''s Miscellanies, for a mathematical deter- 

 mination of the amount of heat communicated by the sun's rays 

 in different latitudes. Supposing the sun to remain on the 

 equator, it varies as the cosine of the latitude. But the cosine 

 diminishing more rapidly for a given number of degrees in the 

 high than in the low latitudes, so must also the heat ; or the 

 mean temperature at 60° must differ more from that of 30° than 

 this last does from that of the equator. With this agrees the 

 remark of one who had ample opportunity of observation. " Not- 

 withstanding our advanced latitude, and its being the winter 

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

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

 the equal and lasting influence of the stm\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 al- 

 most entirely to the direction of the rays of the sun, indepen- 

 dent of the bare distance, which is by no means equal to the 

 effect *." 



Professor Mayer, of Gottingen, undertook to deduce from a 

 comparison of the meteorological observations, made in different 

 latitudes, an empirical law for determining the mean tempera- 

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

 temperature to change very slowly in the neighbourhood of 

 both the equator and the pole, and rapidly in the intervening 

 space. Thus the mean temperature under the equator he makes 

 84° 2' ; at the parallel of thirty, 71° 1' ; at sixty, 45° ; ,the diffe- 

 rences of which are 13° 1' and 26° 1'. If his numbers are cor- 

 rect, 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°. 



(J).) When the air has once been set in motion by the more 

 elevated temperature of the lower latitudes, the creation of a 

 trade-wind is determined by the increasing magnitude of the pa- 



• Cook's Voyage to the Pacific, 4to. vol. iii. p. 255. 



