184 Probable Influence of Evaporation on the 
enable its influence on the augmentation of the water supply 
to be definitively determined; and as I would wish to give a 
safe estimate, at any rate, (or less rather than more, than the 
probable supply), I have now excluded any separate allowance 
for dew from the calculation. * 
Supply. 
GALLONS. 
Available rainfall, assumed to be equivalent, 
as already explained, to 5:2 inches, over 
total surface 44,000 acres, whose drainage> 5,175,950,208 
either flows into the Plenty above the cut, 
or else flows direct into the reservoir. 
Rainfall on surface of reservoir,—36 pence is 
on1,300acres cy a 1,058,377,820 
6,284,327,528 
Demand. 
Quantity required to maintain an adequate flow } = 
ehuite dpwer Plenty ne ase f 227,240,000 
Evaporation,—66°6 inches from 1,300 acres of e 
water surface - 1,958,626,618 
Contingent allowance for loss of flood-water, 
during excessive rains, absorption, &c., aI 176,452,848 
inches over 1,300 acres 
Balance, equivalent to the supply, at the rate 
of twenty-five gallons per head, per day, of 
a population 313,880 persons, or a population ‘. 2,872,008,067 
of about 196,000 if the larger rate of a 
gallons per head, per day, be assumed . 
— 
6,234,327,528 
* Dr. Hales, in his Vegetable Statistics, states, from experiment, that “ the 
moister the earth is the more dew falls upon it in the night; and more than 
double the quantity of dew falls upon a surface of water than there does on an 
equal surface of moist earth.” The author of the recent Report to the Board 
of Health, on the supply of water derivable from the Farnham Tertiary for- 
mation, holds the same opinion. Professor Young, in article Dew, in Rural Ency- 
clopedia, states that a water surface condenses more dew than any other 
surface. White considered that the permanent character of the ponds occa- 
sionally met with on the crest of the South Downs was to be attributed to the 
large amount of dew condensed on the surface of such ponds. The assertion 
of Mr. Stirrat, in reference to the influence of dew on the large reservoirs on 
his bleaching grounds, has already been alluded to. It has been supposed by 
some writers, that water has a capacity to assimilate to itself the vapour in the 
atmosphere. under certain conditions, even though the temperature of the sur- 
face of the water should exceed that of the air above it. Unless this capacity 
existed, the surface of water (if its relative temperature to that of the air be 
only considered) could seldom effect the condensation of dew. In Great Britain 
every one must have noticed, on nights otherwise clear and serene, long serpen~ 
o 
