Hygrometry . 1 7 
extrem t moijiure on hygrometers ; and this is a moft important 
queftion, both of hygrometry and hygrology, which remains 
to be examined. 
On the maximum of evaporation, and its correfpondence with the 
maximum of moifture in a medium. 
27. Since moijiure confifts in invijible water, an excefs of 
Water is the only immediate means of ascertaining that the 
maximum exifts ; as, if a refervoir is above our reach, the only 
means of knowing if it b t full of water, is when it overflows. 
From that principle, a fog gives the point of extreme moijiure 
on hygrometers , like water itfelf; becaufe it covers very 
foon the hygrofcopic fubftance with a coating of water : fome- 
times even it expands it a little more than an immediate appli- 
cation of water ; but this belongs to an object that I have 
waved before, as relating to fome modifications of elajlic fo- 
lids (§ 8.). No other means then but an excefs of water over 
the furface of the hygrofcopic fubftance of the hygrometer , can 
afcertain that it is arrived at its point of extreme moijiure ; and the 
firft immediate demonftration I (hall give of it, will be afforded 
by dew ; a very uncertain, though apparently certain fign of 
extreme moijiure in the air. We fay, that there is dew, when 
fome folids expofed in the open air in a clear evening are wet ; 
but if that was the effect of a precipitation of water happening 
in the air, all the folids thus expofed w'ould be wet ; which is 
far from being the cafe , confequently, that phenomenon muft 
proceed from fome particular caufes, by which, though no 
water is yet difpofed to abandon the medium, it gathers on fome 
particular folids. It is very long fince the phaenomena of dew 
have perplexed natural philofophers ; and they were the firft 
Vol. LXXXI. D which 
