HYGRO 
other end of the balance, and bring the balance into equi¬ 
librium by fcrewing up or down the leaden bullet. 3. 
Mark now the place to which the leaden bullet is brought 
by the fcrew, as accurately as poffible, for the point of 
the greateft drynefs. 4. Then take away the glafs-plate 
from the balance, dip it completely in water, give it a 
lhake that the drops may run off from it, and wipe them 
carefully from the edge. 5. Apply the glafs-plate thus 
moiltened again to the balance, and bring the latter into 
equilibrium by fcrewing the leaden bullet. Mark then 
the place at which the bullet Hands as the higheft degree 
of moifture. 6. This apparatus is to be fufpended in a 
fmall box of well-dried wood, fufficiently large to fuffer 
the glafs-plate to move up and down. An opening muft 
be made in the lid, exactly of fuch a fize as to allow the 
tongue of the balance to move freely. Parallel to the 
tongue apply a graduated circle, divided into a number 
of degrees at pleafure from the higheft point of drynefs 
to the higheft degree of moifture. The box muft be 
pierced with fmall holes on all the four Tides, to give a 
free pafifage to the air; and to prevent moifture from pe¬ 
netrating into the wood by rain, when it may be requiiite 
to expole it at a window, it muft either be lackered or 
painted. To fave it at all times from rain, it may be co¬ 
vered, however, with a fort of roof fitted to it in the 
moft convenient manner. But all thefe external appen¬ 
dages may be improved or altered as may be found ne- 
ceftary. 
Such are the moft approved inftruments for meafuring 
the moifture and drynefs of the air; from which the ju¬ 
dicious reader will difcern, that a perfect hygrometer is 
yet a defideratum ; and, as the whole atmofpheric eco¬ 
nomy, as far at leaft as relates to the weather, depends 
upon, or is connected with, the ftate of vapour it contains, 
it is rather furprifing that we find fo few hygrometrical 
obfervations among the many meteorological diaries that 
have been publifhed. From time immemorial, the eftefts 
of moifture have been confidered as a prognoftic of the 
weather, as is evident by the confidence the houfewife 
places in her falt-box, the carter in his whip leather 
thong, and the failor in his Ihrcuds. But, whether the 
hygrometer be a prognoftic or not of the weather, it is 
certainly of the utmoft importance to the natural philo- 
fopher, and would probably prove a valuable oracle to the 
farmer, which is fully evinced by the following obferva- 
tion of Mr. Marfhall, in his Minutes of Agriculture : 
“ Yefterday morning, fays he, while the hygrometer flood 
at two degrees moift, the peafe were by no means fit for 
carrying ; the balm was green, and the peafe foft. About 
ten o’clock the hygrometer fell to one degree dry; before 
one, the peafe were in good order; I went into the field, 
merely on the word of the hygrometer, and found the 
peafe fit to be carried.” It is plain therefore, that on a 
fcattered farm, in hay-time and harveft, an hygrometer 
muft be peculiarly ufeful. 
In confidering the aflion of hygrometers, it will be ne- 
ceflary to recur to the phenomena of Evaporation, as ex¬ 
plained under that article in vol. vii. p. 46, 7. Thus va¬ 
pours are decompofed not only by the mutual approach of 
the particles, but all'o in virtue of the affinity of water to 
thofe fubftances that are called hygrofcopic, of which fire 
may be reckoned one. The principal law of this affinity is, 
that the water diftributes itfelf to all the fubftances of the 
elafs that are within its reach, to every one alike, propor¬ 
tional to its capacity of retention. If new fire be introduced 
into a given fpace, where there is no fuperabundant water, 
it will take away fome of the water from all the hygrof¬ 
copic fubftances, and diminsfh their humidity. If fome of 
the fire be taken away, the water that was united thereto 
will be divided among all the reft ; and if any other hy¬ 
grofcopic fubftances be introduced, containing a greater 
or lei's quantity of humidity than thofij already there, 
the furplus of humidity will be divided among them. 
It is by fire that this diftribution is effected ; the particles 
of this element, being always in motion, take up the wa- 
M E T II Y. 635 
ter from one that has more than its ftiarc, and give it 
out to another that has lefs. Thus hygrofcopic fubftances 
have their humidity always proportional to the places 
they are in. 
Hygrofcopic fubftances are of three diftinct daffies : 1. 
Thofe that feize on the water of vapour by a chemical 
affinity with that liquid ; among thefe are acids, falts, 
and calces. 2. Thole that imbibe the water, by the ten¬ 
dency it has to propagate itfelf in capillary pores, but 
from their nature receive no fenfible increafe of bulk by 
its introduction ; fuch are porous Hones. 3. Thofe that, 
imbibing a certain quantity of water, are thereby ex¬ 
panded ; and thefe are moft of the folids of the vegetable 
and animal kingdoms. M. de Luc, by a long feries of 
experiments, fhows, that the fubftances of the laft clafs 
are'the only ones proper for hygrometers, and that even 
in this clafs, to avoid fallacy in refpeft to the moft im¬ 
portant phenomena, we muft ufe thofe that ceale to 
lengthen only when they can be penetrated with more 
water. 
Here, however, it will be neceflary to define in what 
fer.fe we ufe the words moifture and humidity; for, in the 
manner they are commonly ufed, they fometimes imply a 
caufe, fometimes an effect. Moifture, in a general fenfe, 
may be confidered asinvifible v,'ater, producing oblervable 
phenomena. Thus in hygrofcopic bodies, the quantity of 
water which expands them, and increafes their weight, is 
concealed within their pores; and in the ambient medium, 
that water which affects hygrofcopic bodies, being there 
under the form of vapour, is as invifible as the air itfelf. 
But in refpefl to hygrometry, where moifture is confidered 
as having correfpondent degrees in the medium, the 
word requires a more particular determination. Moifture 
may be either totally abfent or abfolutely extrenie, both 
in the hygrofcopic bodies and in the ambient medium ; 
hence, both in the whole and in correfpondent parts, 
moifture affumes in the medium the character of a caufe, 
and in hygrofcopic bodies that of an effect. Thefe two 
circumltances furnilh us alfo with a fixed module for de¬ 
termining correfpondent degrees. 
Moifture is totally abfent, firft, in the medium when it 
contains no vapour; and then as a confequence in hy¬ 
grofcopic bodies, becaufe they contain no more water 
that can evaporate, without a decompofition of their 
component parts. The cafe here fuppofed is, that when, 
by fome adequate caufe, no fenfible quantity of vapour is 
permitted to remain in the medium, as in the lime-veffel 
ufed by M. de Luc to obtain the point of extreme dry- 
nels. Moifture is extreme, firft, in the medium, whether 
air or a fpace free from air, when no more vapour can be 
introduced therein without a part being decompofed ; 
and then, as a confequence, in hygrofcopic bodies, becaule 
no more w’ater can be admitted into their pores. 
Here it is to be obferved, that from the nature of the 
laft of thefe maxima the quantity of water which pro¬ 
duces it, i. e. extreme moifture, in a given body, is fixed, 
becaufe it is determined by the actual capacity of its 
pores; but the quantity of water which produces extreme 
moifture in a medium of a given extent, is as variable as 
the temperature. The equilibrium, therefore, between 
the medium and hygrofcopic bodies in different ftages of 
moifture, which equilibrium is the object of hygrometry 
as a fcience, does not depend on certain quantities of 
water contained in the medium of w'hich bodies may re¬ 
ceive their lhare ; it depends on different aptitudes of 
the vapour contained in the medium, to communicate 
water to thofe bodies ; which aptitudes vary not only with 
the different denfities of that fluid, but alfo in vapour of 
the lame denfity according to the temperature. 
From the hygrometer we have learned, that, in the phe¬ 
nomenon of dew, the grafs often begins to be wet when 
the air a little above it is Hill in a middle ftate of moif¬ 
ture ; and that extreme moifture is only certain in that 
air, when every folid expofed thereto is wet. It has 
taught us, that the maximum of evaporation, in a clofe 
lpace 
