12 
ATMOSPHERIC MOISTUR" 
zones, we must endeavour to solve the problem of tl 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 is 
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 mouths 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 ltio 
Pao, which rises at the entrance of the Llanos, at the foot of 
the range of hills called La Gfalera, heretofore mingled its 
waters with those of the lake, by uniting with the Cano de 
Cambury, on the road from the town of Knew a Valencia to 
G-uigue. 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 llio Pao. He turned the river; and, after 
having employed part of the water for the irrigation of his 
fields, he caused the rest to flow at a venture southward, 
following the declivity of the Llanos. In this new southern 
direction the Eio Pao, mingled with three other rivers, tha 
