336 
Experiments on Evaporation 
[Nor. 
Explanations and Remarks. 
A. D. was a tin cylindrical vessel, 5 inches deep and 4,08 diameter, placed on a 
wooden rail, elevated 10 feet above the Azotea. or flat roof of the house I lived in, 
filled up every day with a glass measure, dividing a cubic inch into 50 parts, by 
which I reckoned the evaporation. 
A. S. was the shallow top to the above, placed on the same rail, and filled up 
daily in the same manner. 
W. D. a tin cylinder of 5 inches deep, and inches 4,08 diameter, placed in my 
window, (always open, and opposite to another, also open,) and very rarely reach- 
ed by the sun : it was filled up daily as above. 
-W . S. a canister top, similar to A. S. in depth, and inches 2,40 diameter, placed by 
the side of W. D. ami filled as above. 
B. S. 1 and 2 were two shallow canister tops of diamoter inches 4,35 and 1,65 
placed behind the two shutters of the same window, where the air had little mo- 
tion, and filled as above. As there was every day either a sea breeze, or a Norte, 
the agitation of the water exposed a larger surface than the area of the vessels to 
evaporation, by wetting all the space inside between the edge and the surface of the 
water. This in common Ur is as would average, a quarter of an inch, and would 
make the result about 2 tenths in excess. But in days of A T orte, the winds were so 
violent that it was necessary to reduce the level 1 or 2 or more inches below the 
edge of the deeper vessel, and reject the shallow one as useless. At such times 
the correction for excess would be an average reduction of half the apparent evapo- 
ration. The season of Nortes prevailing through eight months, commencing with 
October, during which they blow about half the number of days, we must take off 
for A T ortes 30 inches, and for Briar. 24 inches, from the aggregate of 180 inches of 
A. D. which reduce the loss to 127 inches, or per diem ,347 inch. This cannot be 
considered accurate, and is probably in excess of the truth ; but, notwithstanding its 
position by the sea shore, and the great quantity of rain, (nearly 100 inches) 
which falls in the season, Vera Cruz must be subject to a higher degree of evapo- 
ration than Calcutta; from two causes, the almost constant sunshine and infre- 
quent dav-showers during the Rainy season ; the heavy falls of rain coming with 
the hind wind of the night ; and the frequency and violence of the Nortes, which in 
a few hours bring down the temperature 15 or 20, and sometimes 25 and cven30 
degrees of Fahrenheit, and produce an extraordinary dryness in the atmosphere. 
W. I). and W. S. were affected in the same way, but in a less degree, especially the 
latter. The corresponding correction would probably be an abatement of 15 per 
cent, from the former, and 10 per cent, from the latter, which makes them respec- 
tively as follows. 
W. D. 59A inches per annum, and ,162 per diem ) , .„ 
W. S.45I „ „ ,124 1 „ } Mean ,143 
Until the middle of April, 1819, IV , S. stood in the open window of a room, which 
had no free current of air: hence the great difference in the corresponding months 
of the two years ; a small evaporation in that room, shut up, gave only 0,917 in June, 
and 1,04 in July, 1819, or per diem ,03 and ,034 respectively ; and ,032 per diem from 
30th September to 2d October. G. 
Note by the Editor. 
The above we consider a valuable addition to our correspondent’s former paper on 
the subject. It is both curious and instructive, as showing the very great variation 
in the quantity evaporated, occasioned by differences of situation, and even by an 
increase or decrease of depth in the fluid to be evaporated, all things else being alike. 
From the meteorological tables kept at Benares, an attempt was made to deter- 
mine the ratio of evaporation at different temperatures, of which an account will be 
found in the Oriental Magazine for 1827. For the sake of comparison with those of 
Calcutta, and V'era Cruz, we here subjoin a few of the Benares results. 
The mean temperature in the shade for four years, was 79°. 
The depression of the wet bulb thermometer, 9°. 
The annual evaporation, was 64,5 inches. 
or monthly, 5,4 ditto, 
and daily, 0,180 ditto. 
Taking the average of four years, and selecting the periods most at variance, or the 
hottest, coldest, driest, and most damp, were obtained the following. 
Mean temperature, 
Depression of wet bulb, 
Monthly evaporation, inches, 
Daily ditto, 
In December 
and January. 
62°, 3 
6°,0 
2 ,55 
0 ,085 
Iu 
March. 
79°,4 
15°,3 
7 ,3 
0 ,243 
In April 
and May. 
91°, 2 
20°, 3 
13,9 
0 ,463 
In July 
and August. 
84°, 4 
2°,0 
3 ,0 
0,1 
