332 Prof. Bischoff" on the Temperature of 



ther longer than that caused by the rains, and consequently cause 

 a more considerable depression of the mean temperature of the 

 soil below that of the air. The extent of the depression caused 

 by these two circumstances might be determined, by observing 

 the temperature of the air, the earth and the rain water, during, 

 and a short time after, the fall of cold winter rains. 



If the mean depression of temperature during the rainy sea- 

 son is equal to the difference between the mean temperature of 

 the air and that of the rain water, that depression cannot, at 

 Cumana, exceed 1°.73. The difference then between the mean 

 temperatures of the air and of the soil, during this season, can 

 be but a small fraction of that quantity, and must, therefore, be 

 exceedingly small. And, although single showers may lower 

 the temperature considerably, (Von Humboldt*) for example, 

 observed a fall of 48°.20F. during a shower at Cumana, the 

 cooling of the air and of the soil will nearly keep equal pace to- 

 gether. 



It seems strange that rains falling from cold regions of the at- 

 mosphere, should only cause a yearly depression of temperature 

 in the air of at the utmost 2°.92.-f- But when we consider that 

 rain-drops, at the moment of their condensation, assume a 

 higher temperature than that of the surrounding medium, by 

 the release of latent heat, and that, in their descent, aqueous 

 vapours are condensed upon them, the latent heat of which, be- 

 coming free, also tends to raise their temperature ; we can no 

 longer be surprised at that phenomenon. Besides, the smaller 

 quantity of heat received from the sun, during the tropical win- 

 ters, has also some share in causing that difference of 2°.92. 



According to Chap. V, the temperature of springs is a func- 

 tion of the temperatures of the meteoric waters, and of the soil 

 through which they flow. The modifications caused by the 

 temperature of the earth on that of the meteoric waters, depends 

 upon— 



1. The capability of the strata to receive the percolating 



waters, which causes a longer or shorter detention of them 

 in those strata. 



2. The capacity of the channels through which the waters 



flow ; and 



" Voyage ix. p. 20. 



t This is the difference between the mean temperatures of the liottest and 

 of the coldest month at Cumana. 



