472 SUBTERRANEAN WATERS. 
the aqueous vapors of the air are the ultimate source of supply, 
and all these hidden stores are again returned to the atmos- 
phere by evaporation. 
The proportion of the water of precipitation taken up by 
direct evaporation from the surface of the ground seems to 
have been generally exaggerated, sufficient allowance not being 
made for moisture carried downwards or in a lateral direction, 
by infiltration or by crevices in the superior rocky or earthy 
strata. According to Wittwer, Mariotte found that but one- 
sixth of the precipitation in the basin of the Seine was 
delivered into that sea by the river, “so that five-sixths re- 
mained for evaporation and consumption by the organic 
world.” * Maury estimates the annual amount of precipitation 
in the valley of the Mississippi at 620 cubic miles, the discharge 
of that river into the sea at 107 cubic miles, and concludes that 
“this would leave 513 cubic miles of water to be evaporated 
from this river-basin annually.” + In these and other like com- 
putations, the water carried down intothe earth by capillary 
and larger conduits is wholly lost sight of, and no thought is 
bestowed upon the supply for springs, for common and artesian 
wells, and for underground rivers, like those in the great caves 
of Kentucky, which may gush up in fresh-water currents at the 
bottom of the Caribbean Sea, or rise to the light of day in the 
far-off peninsula of Florida. 
* Physicalische Geographie, p.286. It does not appear whether this infer- 
ence is Mariotte’s or Wittwer’s. I suppose it is a conclusion of the latter. 
According to Vallés, the Seine discharges into the sea thirty per cent. of the 
precipitation in its valley, while the Po delivers into the Adriatic two-thirds 
and perhaps even three-quarters of the total down-fall of its basin. The 
differences between the tributaries of the Mississippi in this respect are remark- 
able, the Missouri discharging only fifteen per cent., the Yazoo not less than 
ninety. The explanation of these facts is found in the geographical and 
geological character of the valleys of these rivers. The Missouri flows with 
a rapid current through an irregular country, the Yazoo has a very slow flow 
through a low, alluvial region which is kept constantly almost saturated by 
infiltration. 
+ Physical Geography of the Sea. Tenth edition. London, 1861, § 274. 
t In the low peninsula of Florida, rivers, which must have their sources 
