120 EXPERIMENTS on EVAPORATION. 
fill water, no rain, nor any thing to difturb the experi- 
ment. ‘The event was, that at the end of the feventh day, 
the tube was exhaufted 1.15 inch. And that no water 
had got into the tube in that time, I was certain from this 
circumftance; all that part of the furface of the board 
which was within half a foot of the tube was dry every 
morning and evening. In the other tube, the evaporation 
in the fame time was 1.50 inch; which gives 35 decimal 
parts of an inch difference between the real evaporation 
trom the furface of the river, and that of the water when 
fufpended in the air, as in the other veflel. All the eva- 
porations therefore meafured the latter of thefe ways, 
ought to be diminifhed in this proportion, to have the true 
quantity fuch as it is in nature. 
EXPERIMENT II. 
Thefe experiments on watery fluids put me upon en- 
quiring what the evaporation was from the furface of the 
earth. To determine this, Sept. 14, two days after there 
had been any rain, I funk one of the veffels into the earth 
in a light foil, fo as to take up all the earth contained in 
a {pace equal to the contents of the veflel. Having. care- 
fully weighed the veffel with the earth it contained, I fix- 
ed it in the ground in a plain open field, where it was ex- 
pofed to the fun and wind, but defended from the dew and 
rain, as in the former experiment. At the end of /even 
days \ took it up, and weighing it again found it had loft 
783 grains, troy. The diameter of the veffel being three 
inches, its furface expreffed in whole numbers was equal 
to nine fquare inches. Dividing the number of grains that 
evaporated, 783, by the number of fquare inches contain- 
ed in the furface of the veffel, 9, we fhall have 87 grains 
for the evaporation from one fquare inch; and this, (af- 
fuming 254 grains as the weight of a cubic inch of water) 
will give .*. parts of an inch, as the depth of water that 
paffed off by evaporation. In the other veflel filled with 
water, 
