T2 ATMOSPHERIC MOISTURW 



zones, we must endeavour to solve the problem of tt e mean 

 tension of the vapours contained in the atmosphere in dif- 

 ferent latitudes, and at different heights above the surface 

 of the ocean. 



A great number of local circumstances cause the produce 

 of evaporation to vary ; it changes in proportion as more or 

 less shade covers the basin of the waters, with their state 

 of motion or repose, with their depth, and the nature and 

 colour of their bottom ; but in general evaporation depends 

 only on three circumstances, the temperature, the tension of 

 the vapours contained in the atmosphere, and the resistance 

 which the air, more or less dense, more or less agitated, 

 opposes to the diffusion of vapour. The quantity of water 

 that evaporates in a given spot, everything else being equal, 

 is proportionate to the difference between the quantity of 

 vapour which the ambient air can contain when saturated, 

 and the quantity which it actually contains. Hence it 

 follows that the evaporation is not so great in the torrid 

 zone as might be expected from the enormous augmentation 

 of temperature ; because, in those ardent climates, the air ia 

 habitually very humid. 



Since the increase of agricultural industry in the valleys of 

 Aragua, the little rivers which run into the lake of Valencia 

 can no longer be regarded as positive supplies during the 

 six months succeeding December. They remain dried up 

 in the lower part of their course, because the planters of 

 indigo, coffee, and sugar-canes, have made frequent drainings 

 (azequias), in order to water the ground by trenches. AVe 

 may observe also, that a pretty considerable river, the Kio 

 Pao, which rises at the entrance of the Llanos, at the foot of 

 the range of hills called La G-alera, heretofore mingled its 

 waters with those of the lake, by uniting with the Cafio de 

 Cambury, on the road from the town of Nueva Valencia to 

 Gruigue. The course of this river was from south to north. 

 At the end of the seventeenth century, the proprietor of a 

 neighbouring plantation dug at the back of the hill a new 

 bed for the Bio Pao. He turned the river; and, after 

 having employed pirt of the water for the irrigation of hia 

 fields, he caused the rest to flow at a venture southward, 

 following the declivity of the Llanos. In this new southern 

 direction the Kio Pao, mingled with three other rivers, the 



