<334 II Y G R 0 
taken off, leaves that fide of the vefiel quite open. The 
fecond veffel has the fame xiimenfions as the firft, but its 
back is foldered, and its front is of woven brafs wire. 
This veffel may be applied to the back of the former, in 
Inch a manner as to make of both one fingle vefiel ; which, 
when the Aider of the fore-part is taken off, is only di¬ 
vided by a vertical partition of the woven brafs wire. 
The ufe of that fecond veffel is to produce extreme dry- 
iids in the other, for which purpofe it is filled with 
large pieces of quicklime taken from the kiln. When 
that veffel is not tiled, it is kept in a tin-box which it 
fills entirely ; and when it is in, as well as while it is out 
foT ufe, that box is kept Ihut with putty, by which means 
the lime may ferve many times, in the following manner: 
When I want to produce extreme drynefs in the firft vef¬ 
fel, (fays M. de Luc,) I apply it to the fecond, fattened 
with hooks ; I then pull out the Aider of the firft, and 
ftop with putty the chinks between them. When that 
firft operation is completed, I put again the Aider to the 
fore vefiel, and take off the other. In this,laft operation, 
lome moifture might be introduced through the chinks 
of the Aider before they are again flopped with putty, 
efpecially as the deftruclion of the air in the vefiel has 
' made room for more air to come in ; but I prevent it, by 
making firft the apparatus fenfibly warmer than it was 
when I put on the lime-veflel ; by which means, in the 
little time employed for the operation, the motion of the 
air is from the infide to the outfide, which prevents all 
accefs of moifture in the vefiel. On the top of this vefiel 
may be made fquare openings with clofe flutters, and 
withinfide, juft underneath a wire fattened acrofs with fe- 
veral hooks, upon which are to be fufpended hygrometers 
to be adjufted to extreme drynefs. A few hours is the 
time generally neceffary; but of this the praiftitioner can 
very readily judge. This vefiel was conftrufted by M.de 
Luc to aflilt in his experiments on the comparative changes 
of weight and dimenfions of l’ome hygrofcopic fubftances; 
but it is equally ul’eful for the purpofe of adjufting the 
hygrometers. 
The belt hygrometer for afcertaining the quantity as 
well as the degree of moifture in the variation of the hy¬ 
grometer, is of the contrivance of Mr. Coventry, South¬ 
wark, London. The account he has favoured us with is 
as follows: “ Take two ftieets of fine tiffue paper, fuch 
as is ufed by hatters; dry them carefully at about two 
feet diftance from a tolerably good fire, till, after repeat¬ 
edly weighing them in a good pair of fcales, no moifture 
remains. When the Aieets are in this perfeblly dry ftate, 
reduce them to exablly fifty grains ; the hygrometer is 
then fit for ufe. The ftieets mull be kept free from dull, 
and. expofecl a few minutes in the open air; after which 
it may be always known by- weighing them the exaft 
quantity of moifture they have imbibed. For many years 
the hygrometer has (fays Mr. Coventry) engrofied a 
confiderable (hare of my attention ; and every advantage 
propofed by others, either as it refpedled the fubftances 
of which the inftrument was compofed, or the manner in 
which its operations were to be difcerned, has been im¬ 
partially examined. But (adds he) I have never feen an 
hygrometer fo fimple in itfelf, or that would act with 
fuch certainty or fo equally alike, as the one I have now 
defcribed. The materials of which it is compofed being 
thin,are eafily deprived wholly of their moifture ; which is 
a circumftance effentially neceffary in fixing a datum from 
which to reckon, and which, I think cannnot be laid of 
any fubftance hitherto employed in the conftrufilion of hy¬ 
grometers ; with equal facility they imbibe or impart the 
humidity of the atmofphere, and (how with the g'reateft 
exabtnels when the'leaft alteration takes place.” 
When the paper is prepared, as already defcribed, it 
will ferve, without the trouble of drying, as a ftandard 
for any number of ftieets intended for the fame purpofe. 
But then the ftieets rnuft be kept together in. the open air 
for a few hours; becaufe, whatever alteration may take 
place by tins expofure, the paper already weighed mult 
have undergone the fame; being conlequently in the fame 
M E T R Y. 
ftate, they mull be cut to the fame weight. For eafier weigh¬ 
ing the paper, take a piece of round tin or brafs, the fize 
of a crown-piece, through the centre of which drill a hole, 
and alfo three others round it at equal diftances; then 
cut about one hundred papers ; and after putting them 
under the tin or brafs, drive through each hole a ftrong 
pin into a board, in order to round them to the Ihape of 
the plate; the papers mull be then feparated and expofed 
to the air a few' hours with that already weighed, and fo 
many of them taken as are equal to the weight already 
fpecified. This done, threadle them together through 
thofe holes made by the pins, putting between every pa¬ 
per on each thread a fmall bead, in order to prevent the 
papers from touching each other, and alfo that the air 
may be more readily admitted. The top of 4116 hygro¬ 
meter is covered with a card cut to the fame fize ; and 
which, by reafon of its ftiffnefs, fupports all the papers, 
and keeps them in proper fhape. Before the papers are 
threaded, the beads, filk, card, and a thin piece of brafs 
about the fize of a fixpence, which mult be placed at the 
bottom, and through which the centre firing paffes, mull 
be weighed with the greateft exaftnefs, in order to bring 
them to a certain weight, fuppofe fifty grains ; now, the 
paper in its drieft ftate being of equal weight, they will 
weigh together one hundred grains, confequently what 
they weigh more at any time is moifture. 
The fubftances of which hygrometers may be conftruft- 
ed are fo numerous, that.w'e cannot notice them all. We 
ftialljult ftate the improvements of Meffrs. Lowitz and 
Hockheimer, and then conclude with a few general re¬ 
marks. 
Mr. Lowitz had obferved, at Dmitrieufk, in Aftracan, 
on the banks of the Wolga, a thin bluifn kind of ftate 
which attracted moifture remarkably foon, but again fuf- 
fered it as foon to efcape. A plate of this ftate weighed, 
w’hen brought to a red heat, 175 grains, and, when fatu- 
rated with water, 247 ; it had therefore imbibed, between 
complete drynefs and the point of complete moifture, fe- 
venty-two grains of water. Lowitz fufpended a round 
thin plate of this ftate at the end of a very delicate ba¬ 
lance, faftened within a wooden frame, and fufpended at 
the other arm a chain of filver wire, the end of which 
was made faft to a Aiding nut that moved up and down 
in a fmall groove on the edge of one fide of the frame. 
He determined-, by trial, the pofition of the nut when 
the balance was in equilibrio and when it had ten degrees 
of over-weight, and divided the fpace between thefe two 
points into ten equal parts, adding fuch a number piore 
of thefe parts as might be neceffary. When-the ftone 
was fufpended from the one arm of the balance, and at 
the other a weight equal to 175 grains, or the weight of 
the ftone when perfectly dry, the nut in the groove 
ihowed the excels of weight in grains when it and the 
chain were fo adjufted that the balance Hood in equilibrio. 
A particular apparatus on the fame principles as a ver¬ 
nier, applied to the nut, Ihowed the excefs of weight to 
ten parts of a grain. Lowitz remarked that this hygro¬ 
meter in continued wet weather gave a moifture of more 
than fifty-five grains, and in a continued heat of 113 de¬ 
grees of Fahrenheit, only i-lr degree of moifture. 
The hygrometer thus invented by Lowitz was, how¬ 
ever, attended with this fault, that it never threw off the 
moifture in the fame degree as the atmofphere became 
drier. It was alfo fometimes very deceitful, and an¬ 
nounced moifture when it ought to have indicated that 
drynefs had again begun to take place in the atmofphere. 
To avoid thefe inconveniences, M. Hockheimer propofes 
the following method : 1. Take a fquare bar of Heel 
about two lines in thicknefs, and from ten to twelve 
inches in length, and form it into a kind of baiance, one 
arm of which ends in a ferew. On this ferew let there 
be ferewed a leaden bullet of a proper weight, inftead of 
the common weights that are lufpended. 2. Take a glals 
plate about ten inches long, and feven inches in breadth, 
dellroy its polifli on both lides, free it from all moifture 
by rubbing it over with warm allies, fulpend it at the 
3. other 
