53 



diminish during a great pari of the year the di- 

 rect action of the solar rays. The decrement 

 of the heat being nearly the same between the 

 tropics*, and during the summer under the 

 temperate zone, the small difference of level of 

 one hundred toises should produce only a 

 change in the mean temperature of 1° or 1*5°. 

 But we shall soon find, that at Cumanacoa the 

 difference rises to more than four degrees. 

 This coolness of the climate is sometimes the 

 more surprising, as very strong heats are felt in 

 the town of Carthago-^ ; at Tomependa, on the 

 bank of the river of Amazons ; and in the val- 

 leys of Aragua,to the west of Caraccas ; though 

 the absolute height of these different places is 

 between 200 and 480 toises. In plains, as well 

 as on mountains, the isothermal lines [lines of 

 similar heat] are not constantly parallel to the 

 equator, or the surface of the globe %. It is the 

 grand problem of meteorology, to determine the 

 inflections of these lines, and to discover, amid 

 modifications produced by local causes, the con- 

 stant laws of the distribution of heat. 



* See my memoir on horizontal refractions, in my Ast. 

 Obs. Vol. i, p. 129 and 141 . and in the present work, vol. i» 

 p. 146, 185, 261. 



f In the province of Popayan, the heat is caused by the 

 reverberation of the plains. 



% See my Prolegomena de Distributione geograpbica Plan- 

 tarum, secundum Cceli Temperiem et Altitudinem Monti-* 

 urn, in the Nov. Gen. et Spec, Tom. i, p. 28, 4to edition. 



