EXPERIMENTS on EVAPORATION. — 1rrg: 
experiment I was convinced that it never could be known 
with much accuracy by either of thefe methods, what 
quantity of water does really evaporate from the furface 
of feas, lakes and rivers. For in the one cafe, after 
about an inch is exhaufted the furface of the water is too 
much fheltered from the wind, which greatly retards the 
evaporation. In the other, as the water has all the advan- 
tage of the wind, and is heated by the fun, and atmof- 
phere, to a confiderable greater degree than the water in 
feas, lakes and rivers, the quantity of evaporation comes 
out too much. And therefore nothing certain as to the 
real quantity of evaporation from watery fluids, can be de- 
' termined by fuch experiments, however carefully they 
may be made. 
EXPERIMENT JIL 
To meafure with more certainty the real quantity of 
evaporation, I attempted in the next place to examine 
what it was in fact from the furface ofa river. This ex- 
periment was made in the following manner: I filled one 
of the veflels with river water, and placed it as before. 
The other I fixed in the centre of a circular board of three 
feet diameter. This inftrument, by means of a line 
faftened to a tree on a fmall ifland, was placed fo as to 
float near the middle of Menimack river. To defend the 
tube avain{t the dews and rain, a circular piece of glafs, 
fifteen inches diameter, was fupported by wires fixed to 
the board, eight inches above the tube; and the whole 
was fo balanced by weights as to leave half an inch of the 
tube above the furface of the water. When thus afloat I 
filled the tube with water, propofing to let it remain in 
this fituation a week, to fee how much would evaporate in 
that fpace of time. After repeated difappointments by 
the rain, wind and waves, for three months, I at laft fuc- 
ceeded in trying the experiment from Augu/? 26th, to 
September 2d. During that time there was little wind, 
fill: 
