Hygrometry . 41 
dire&ed me from the beginning, which now are as follows. 
1 ft, Fire , as caufe of heat , is a fure, and the only fure, means 
of obtaining extreme drynefs: this is produced by white heat in 
every hygrofcopic fubftance that can bear it ; and it may be thus 
tranftnitted to the "hygrometer. 2d, Water , in its liquid ftate, 
is a fure, and the only fure, means of determining the point of 
extreme moijlure on that inftrument. 3d, It is not to beexpedted, 
a priori , of any hygrofcopic fubftance, that its changes be pro- 
portional to thofe of moijlure ; but it may be affirmed, that 
no fibrous or vafcular fubftance, taken lengthwife , is proper for 
the hygrometer. 4th, A means of throwing light on the march 
of a chofen hygrometer , may be, to compare it with the cor- 
refpondent changes in weight of many hygrofcopic fubftances. 
61. From thofe determinations in hygrometry feme great 
points are already attained in hygrology , meteorology , and che- 
mi/try, of which I (hall only indicate the moft important, ift, 
In the phenomenon of dew , the grafs often begins to be wet 
when the air , a little above it, is ftill in a middle ftate of 
moifture ; and extreme moifture is only certain in that air , when 
every folid expofed to it is wet (§ 28.). 2d, The maximum of 
evaporation in a clofe fpace, is far from identical with the maxi- 
mum of moijlure ; this depending confiderably, though with the 
conftant exiftence of the other, on the temperature common to 
the fpace and to the water that evaporates (§ 30.). 3d, The 
cafe of extreme moifure exifting in the open tranfparent air, in 
the day, even in time of rain, is extremely rare : I have ob- 
ferved it only once, the temperature being 39 0 . 4th, The air 
is dryer and dryer as we afeend in the atmofphere ; fo that in 
the upper attainable regions, it is conftantly very dry , except in 
the clouds . This is a fafl: certified by M. de Saussure’s ob- 
fervations and mine. 5th, If the whole atmofphere paffed 
Vol, LXXXI. G from 
