52 Mr. Daniell on Evaporation. 
Notwithstanding the slight irregularity of the above series, we 
can, I think, run no risk in drawing from it the conclusion, that 
the amount of evaporation is c@éeris paribus in exact inverse pro- 
portion to the elasticity of the incumbent air; and that De Saus- 
sure was misled by his hygrometer, when he inferred from its indica- 
tions, that a diminution of one-third the density doubled the rate. 
Before we proceed, it is necessary to say a few words upon the 
apparent discrepancy between the results of Mr. Dalton’s experi- 
ments and mine, as to the amount of evaporation, at the full pres- 
sure of the atmosphere. He found, upon the supposition of no 
previous vapour existing in the air, that the full evaporating force 
of water, of the temperature of 45°. would be 1:26 grains per 
minute, from a vessel of six inches in diameter. This amount re- 
duced in proportion to the squares of the diameters of the two ves- 
sels, would give 7°65 grains in half an hour, from the glass of 2°7 
inches diameter, which I employed. It must, however, be recol- 
lected, that Mr. Dalton’s culculations were founded upon experi- 
ments made at a temperature very considerably above that of the 
surrounding medium, and that consequently a current must have 
been established in the latter which greatly accelerated the pro- 
gress. It is true, that he afterwards subjected his calculations to 
the test of experience, at common atmospheric temperatures; but 
then he expressly states, that ‘‘ when any experiment, designed as 
a test of the theory, was made, a quantity of water was put into 
one of them (vessels), the whole was weighed to a grain; then 7 
was placed in an open window, or other exposed situation, for ten 
or fifteen minutes, and again weighed, to ascertain the loss by 
evaporation.” In this way he ascertained, that with the same 
evaporating force, a strong wind would double the effect. The 
difference, however, even after these considerations, is still very 
striking; but, from several repetitions of the experiment, I have 
no doubt of its exactness. 
Experiment 4, 
The arrangement described in the last experiment, having been 
found adequate to maintain in the receiver a state approaching to 
