of the Equatorial Regions. 139 



quantity of the absorbed or heating rays (a quantity which 

 augments rapidly when the angle of incidence, reckoned from 

 the level of the surface, increases ;) and lastly, the number of 

 solar rays which a given horizon embraces. The law of 

 Mayer, with all the modifications which have been introduced 

 into it for thirty years, is an empirical law, „which represents 

 the generality of the phenomena by approximation, and often 

 in a satisfactory manner ; but it cannot be employed against 

 the testimony of direct observations. If the surface of the 

 globe, from the equator to the parallel of Cumana, was a 

 desert like that of Sahara, or a savanna uniformly covered with 

 grasses like the Llanos of Calobozo or of Apure, there would 

 undoubtedly be an increment of mean temperature from 10^°. 

 of latitude to the equator, but it is very probable that this 

 increase does not amount to 2J° of Fahrenheit. M. Arago, 

 whose important and ingenious researches extend to all the 

 branches of meteorology, has found, from direct experiments, 

 that from a perpendicular incidence to 20° of zenith distance 

 the quantity of reflected light is nearly the same. He has 

 found also that the photrometrical effect of solar light varies 

 extremely little at Paris in the month of August, from noon 

 to three o'clock, in spite of the changes in the length of the 

 path described by the rays which traverse the atmosphere. 



If I have fixed the mean temperature of the equator in 

 round numbers at 81 J ., it was to attribute to the equatorial 

 zone, properly so called, from 8° N. to 3° S., the mean tem- 

 perature of Cumana, 81°.86. This city, surrounded with 

 arid sands, situated under a sky always serene, and whose 

 thin vapours almost never resolve themselves into rains, pos- 

 sesses a more burning climate than all the places which sur- 

 round it, and which are like it on the level of the sea. In 

 advancing southward in America, and to the equator, by the 

 Orinoco and Rio Negro, the heat diminishes, not on account 

 of the elevation of the soil, which from the Fort of St Carlos 

 is very little, but on account of the forests, the frequency of 

 rains, and the transparency of the atmosphere. It is to be 

 regretted that travellers, even the most laborious, should be 

 so little in a state to advance the progress of meteorology, by 

 adding to our knowledge of mean temperatures. They do 



