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Hygrome. ing opon it should not increase the diameter of the arbor round which it winds. But this sveight cannot Hygrome

arbor, and never take a situation too oblique and vari. be sensibly increased without still greater inconvenien-
able. The slip is fixed to the arbor by a small pin F. ces : he therefore observes, that this bygrometer is well
The other extremity of the arbor D is shaped like a calculated for a fixed situation in an observatory, and
pulley, flat at the bottom so as to receive a fine supple for various hygrometrical experiments ; since, instead
silken string, to which is suspended the counterpoise of the bair, there may be substituted any other sub-
g in the large figure, and G in the side one. This stance of which a trial may be wanted ; and it may be
counterpoise is applied to distend the hair ; and acts in kept extended by a counterpoise more or less heavy as
a contrary direction to that of the hair, and the move. they may require : but the instrument will not admit
able pincers to which the bair is fixed. If then the of being moved, nor serve even for experiments which,
bair should be loaded with the weight of four grains, may subject it to agitation.
the counterpoise must weigh four grains more than the To obviate the objection above mentioned, M. Saus. Portable
pincers. The arbor at one end passes through the cen- sure has contrived another apparatus mure portable byrome.
tre of the dial, and turns therein, in a very fine bole, and convenient, and which, if not so extensive in its va-

ter by M.

Saussure. on a pivot made very cylindrical and well polished : at riations, is in fact very firm, and not in the least liable the other end is also a similar pivot, which turns in a to be deranged by carriage and agitation. Fig. 8. is a Fig. 8. hole made in the end of the arm h of the cock hi, representation of this hygrometer, wbicli he calls the HI. This cock is fixed behind the dial by means of portable hygrometer, in distinction from the preceding, the screw I.

which he calls the great hygrometer or the hygrometer
The dial keek, divided into 360 degrees, is sup- with the arbor. The material part of this instrument
ported by two arms ll; these are soldered to two tubes, is its index abce; an horizontal view of which, and
which inclose the cylindrical columns mm. The the arm that carries it, is seen in the separate figure
setting screws n n move upon these tubes, and serve GBDEF. This index carries in its centre D a thin
thereby to fix the dial and arbor to any height requi- tube bollow throughout, and projects out on each side
red. 'l'he two columns which support the dial are firm- of the needle. The axis which passes through it, and.
ly fastened to the case of the hygrometer, which rest round which the index turns, is made thin in the
upon the four screws 0000; by the assistance of these middle of its length and thick at the ends ; so that
screws, the instrument is adjusted, and placed in a ver- the cylindrical tube which it passes through touches it
tical situation.

only at two points, and acts upon it only at its extre-
The square column pp, which rests upon the base of mities.
the bygrometer, carries a box q, to which is fixed a The part de DE of the index serves to point out
kind of port-cragon r, the aperture of which is equal to and mark on the dial the degrees of moisture and dry-
the diameter of the counterpoise g. When the hygro- ness; the opposite part db DB serves to fix both the
meter is to be moved from one place to another; to pre. bair and counterpoise. This part which terminates in
vent a derangement of the instruments from the oscil- a portion of a circle, and is about a line in thickness,
lations of the counterpoise, the box q and the port. is cut on its edge in a double vertical groove,

which
crayon r must be raised up so as the counterpoise may makes this part similar to the segment of a pulley with
fall into and be fixed in it, by tightening the screws a double neck. These two grooves, which are portions
and the box and counterpoise together by the screw t. of a circle of two lines radius, and have the same cene
When the hygrometer is intended for use, the counter- tre with that of the index d, serve in one of them to
poise must be disengaged by lowering the box, as may contain the hair, and in the other the silk, to the end of
be conceived from the figure.

which the counterpoise is suspended. The same index

.
Lastly, at the top of the instrument is a curved piece carries vertically above and below its centre two small
of metal x, y, %, which is fastened to the three columns Screw.pincers, situated opposite to the two grooves: that
just described and keeps them together. It has a square above at a, opposite to the hindmost groove, serves lo
hole at y, which serves to bang up the hygrometer by fix to the silk to which the counterpoise is suspended;
wben required.

and that below at b, opposite to the hithermost groove,
The variations of which this hygrometer is capable, serves to hold one of the ends of the-, hair. Each of
are (all things besides equal) as much greater as the these grooves has its partitions cut, as seen in the sec-
xrbor round which the slip of silver winds is than a tion B, and its bottom made fiat in order that the hair
smaller diameter, and as the instrument is capable of and silk may have the greatest freedom possible. The
receiving a longer hair. M. Saussure has bad hygro. axis of the needle DD goes through the arm g SGF,
meters made with ba'z's 14 inches long, but he finds and it is fixed to this arm by the tightening screw fF.
one foot sufficient. The arbor is three-fourths of a All the parts of the index should be in perfect equili-
line in diameter at the base between the threads of brium about its centre; so that when it is on its pivot
the screw or the part on which the slip winds. The without the counterpoise, it will rest indifferently in any
variations, when a hair properly prepared is applied to position it may be placed in.
it, are more than an entire circumference, the index It must be understood, that when the hair is fixed;
describing about 400 degrees in moving from extreme by one of its extremities in the pincers.e, and by the
dryness to extreme humidity. M. Saussure mentions other end on the pincers y at the top of the instrument,
an inconvenience attending this hygrometer, viz. its it passes in one of the necks of the double pulley b..
not returning to the same point when moved from one whilst the counterpoise to which the silk is fixed in *
place to another ; because the weight of three grains passes in the other ceck of the same pulley : the coun-
that keeps the silver slip extended, cannot play so ex- ierpoise serves to keep the hair extended, and acts al-
actly as to act always with the samo precision against the ways in the same direction and with the same force,

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Hygrome. whatever the situation of the index may be. When All the instrument should be made of brass : though Higrome. therefore the dryness contracts the bair, it overpowers

the axis of the index and its tube work were pleasantly
the gravity of the counterpoise, and the index descends : together if made of bell-metal.
when, on the contrary, the humidity relaxes the bair, The extent of this hygrometer's variations is not
it gives way to the counterpoise, and the index ascends. more than the fourth or fiftb part of the hygrometer
The counterpoise should weigh but three grains; so with the arbor. It may be augmented by making the
that the index should be made very light and very easy segment of the pulley to which the bair' is fixed of a
in its motion, in order that the least possible force may smaller diameter ; but then the hair, in moving about
move it and bring it back again to its point when it, would fret and contract a stiffness, which would cause
drawn aside.

it to adhere to the bottom of the neck. M. Saussure
The dial h e h is a circular arch, the centre of which is of opinion, that the radius of this pulley should not
is the same with that of the index. This arch is di. be less than two lines, at least that there should be
vided into degrees of the same circle, or into the hun- ada pted a plate of silver or some other contrivance ;
dredths of the interval which is found between the li- but then the hygrometer would be too difficult to con-
mits of extreme dryness and extreme humidity. The struct, and it would require too much attention and
interior edge of the dial carries at the distance h i a care on the part of ibose who use it: bis object was,
kind of projecting bridle or stay ii, made of brass wire, to make an instrument generally useful, and easy and
curved to the arch, and fixed in the points i i. This convenient in its use. The bygrometer with the arbor
bridle retains and guards the index, at the same time may be used for observations which require au extreme
leaving it to play with the requisite freedom. The sensibility.
screw.pincers y, in which is fastened the upper extre. The variations of this instrument may be augment-
mity of the hair, is carried by a moveable arm, which ed by making it higher, because in that case longer bairs
ascends and descends at pleasure the length of the frame might be adapted : but it would be then less portable.
Kk. This frame is cylindrical everywhere else, ex- Besides, if tbe bair is too long, when observations are
cept its being here flattened at the hinder part to made in the open air, the wind has too great an effect
about half its thickness, in order that the piece with upon it, and thus communicates to the index in-
the screw which carries the arm should not project out convenient vibrations. It is not proper therefore to
underneath, and that the arm may not turn. The make it niore than a foot in height. When it is of
arm may be stopped at any desired height by means of this dimension, an hair properly prepared can be ap-
the pressing screw x. But as it is of use sometimes to plied to it, and its variations from extreme dryness to
be able to give the instrument a very small and accurate extreme humidity are 80 or even 100 degrees; which
motion, so as to bring the index exactly to the part that on a circle of 3 inches radius forms an extent sufficient
may be wanted, the slide piece l, which carries the for observations of this kind. M. Saussure has even
pincers y, to which the hair is fixed, is to be moved by made smaller instruments that may be carried corve-
the adjusting screw m.

niently in the pocket, and to make experiments with
At ihe base of the instrument is a great lever nopq, under small receivers : they were but seven inches high
which serves to fix the index and its counterpoise when by two inches of breadth ; which, notwithstanding their
the hygrometer is to be moved. The lever turns an variations, were very sensible,
axis n, terminated by a screw which goes into the Thus much for the construction of the various parts
frame; in tightening this ecrew, the lever is fixed in of the instrument. The limits of this work will not
the desired position. When the motion of the index admit of our inserting the whole of M. Saussure's eub-
is to be stopped, the intended position is given to this sequent account of the preparation of the hair, the man-
lever, as represented in the dotted lines of the figure. ner of determining the limits of extreme humidity and
The long neck p of the lever lays hold of the double of extreme dryness, the pyrometrical variations of the
pulley b of the index, and the short neck o of the coun- hair, and the graduation of the bygrometer. The fol-
terpoise : the tightening screw q fastens the two necks lowing extract must therefore suffice.
at once. In confining the index, it must be so placed, In the preparation of the bair, it was found de-
that the hair be very slack; so that, if whilst it is cessary to free it of a certain unctuosity it always
moved the bair should get dry, it may bave room to has in its natural state, which in a great measure de-
contract itself. Afterwards, when the instrument is prives it of its hygrometrical sensibility. A number
placed for use, the first thing to be done is to relax of hairs are boiled in a ley of vegetable alkali; and
the screw n, and turn back the double lever with great among these are to be chosen for use such as
caution, taking equal care at the same time not to most transparent, bright, and soft ; particular pre-
strain the hair. It is better to apply one hand to the cautions are necessary for preventing the straining of
index near its centre, whilst the other band is disenga- the hair, which renders it unfit for the intended pur-
ging the pulley and the counterpoise from the lever

pose.
that holds them steady. The book r serves to suspend The two fixed points of the hygrometer are the ex-
a thermometer upon ; it should be a mercurial one, tremes both of moisture and dryness. The former is
with a very small naked bulb or ball, so as to sbow in the obtained by exposing the instrument to air completely
most sensible manner the changes of the air : it should saturated with water; and this is effected by placing
he mounted in metal, and guarded in such a manner as it in a glass receiver standing in water, the sides of
not to vibrate so as to break the hair. Lastly a notch which are kept continually moistened. The point on
is made under the top of the frame s, to mark the point the dial, at which the hand after a certain interval re-
of suspension, about which the instrument is in equili. mains stationary, is marked 100. The point of ex-
brium, and keeps a vertical situation.

treme dryness, not absolute dryness, for that does not

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Higgrome. exist, but the greatest degree of it that can be obtain they adhere, and then bend towards the back, where Hygrone

ed, is produced by introducing repeatedly into the same they form a great many folds. The part of these ves-
receiver containing the instrument, and standing now sels next the stomach is of a cylindric form, and about
upon quicksilver, certain quantities of deliquescent al- a line in diameter. These vessels contain a gummy sort
kaline salts, which absorb the moisture of the air. The of matter from which the worm spins its silk; and,
highest point to which the band can be brought by though they are exceedingly tender, means have been
this operation, not only when it will rise no higher, devised to extract them from the insect, and to prepare
but when it becomes retrograde from the dilatation them for the above purpose. When the worm is
'occasioned by heat, is called o; and the arch between about to spin, it is thrown into vinegar, and suffered to
these two points is divided into 100 equal parts, being remain there twenty-four hours ; during which time
degrees of the hygrometer. The arch P p, upon which the vinegar is absorbed into the body of the insect, and
the scale is marked in the instrument (represented in coagulates its juices. The worm being then opened,
fig. 2.) being part of a circle of three inches diameter; both the vessels, wbich have now acquired strength, are
hence every degree measures about one-third of a line. extracted ; and, on account of their

pliability, are ca-
In the stationary hygrometer, fig. 1. the scale upon the pable of considerable extension. That they may not,
complete circular dial is so much larger, that every however, becoine too weak, they are stretched only to
degree measures about five lines ; but this M. Saussure the length of about fifteen or twenty inches. It is
considers so far from being a perfection, that it is ra- obvious that they must be kept sufficiently extended
ther an inconvenience ; since the instrument becomes till they are completely dry. Before they attain to
thereby so very susceptible of the least impression, that that state, they must be freed, by ineans of the pail of
there is even no approaching it without a sensible the finger, from a slimy substance which adheres to
variation. The thermometer, adapted as before men- them. Such a thread will sustain a weight of six pounds
tioned, serves to correct the changes of tempera- without breaking, and may be used for an hygrometer
ture: towards the extreme of dryness, 1° of the thermo- in the sanie manner as cat-gut; but we confess that we
meter produces on the hair an effect of half a degree of do not clearly perceive its superiority.
the hygrometer, but towards the extreme of moisture, the II. On the second general principle, namely, that of De Luc's.
same difference of temperature causes an effect no les the swelling of solid bodies by moisture, and their con-
than 3° on the hygrometer. He constructed two traction by dryness, M. de Luc's instrument is the best.
tables, that gave the intermediate hygrometrical varia- He makes choice of ivory for the construction of his
tions for single degrees of the thermometer at different hygrometer, because he finds that, being once wetted,
parts of the scale.

ivory regularly swells by moisture, and returns exactly The whole range of the atmospherical variations to the same dimensions when the moisture is evaporatakes in about 75° of this scale; a dryness of more ted, which other bodies do not. This hygrometer is than 25° being always the effect of art. The sensibi- represented in fig. 9. where a ab is an ivory tube open Fig. 9. lity of this instrument is so very great, that being ex- at the end a a, and close at b. It is made of a piece of posed to the dew, he mentions that it varies above 40° ivory taken at the distance of some inches from the top in about 20 minutes of time. Being removed from a of a pretty large elephant's tooth, and likewise at the very moist into a very dry air, it varied in one instance same distance from its surface and from the canal no less than 35° in three minutes. He says that which reaches to that point. (This particular direcits variations were always found uniform in different tion is given, that the texture of the ivory in all dif. instruments suspended in different parts of the same at- ferent hygrometers may be the same, which is of great mosphere. This hygrometer is considered by the au- importance). This pir.ce is to be bored exactly in the thor as possessed of all the properties requisite in such direction of its fibres; the hole must be very straight, an instrument. These are, 1. That the degrees in the its dimensions 2 lines in diameter, and 2 inches 8 lines scale be sufficiently large, and to point out even the in depth from a a lo c. Its bore is then to be exactly least variation in the dryness or moisture of the at- filled with a brass cylinder, which however, must promosphere. 2. That it be quick in its indications. ject somewhat beyond the ivory tube ; and thus it is to 3. That it be at all times consistent with itself; viz. be turned on a proper machine, till the thickness of that in the same state of the bair it always points to the ivory is exactly as of a line, except at the two the same degree. 4. That several of them agree with extremities. At the bottom b the tube ends in a point; one another. 5. That it be affected only by the aque- and at the top a a it must for about two lines be left a ous vapours. 6. That its variations be ever propor- little thicker, to enable it to bear the pressure of anotionate to the changes in the air.

ther piece put upon it. Thus the thin or hygrometri-
But after all it must be observed, that a considerable cal part of the tube will be reduced to 2 French
degree of trouble and delicacy is requisite in the pre- inches, including the concavity of the bottom. Before
paration of the bair, and it is very fragile; circum- this piece is used, it must be put into water, so that
stances wbich may prevent it from coming into general the external part alone may be wetted by it; and here
use among common observers, although probably it may it is to remain till the water penetrates to the inside,
be the best in principle of any yet made.

and appears in the form of dew, which will happen
Instead of hairs or cat.gut, of which hygrometers in a few hours. The reasou of this is, that the ivory
of the first kiud are commonly made, Casse bois, a Be. tube remains somewhat larger ever after it is wetted
nedictine monk at Mentz, proposed to make such hiy- the first time.
grometers of the gut of a silk-worm. When that in. For this hygrometer, a glass tube must be provided
sect is ready to spin, there are found in it two vessels about 14 inches long, the lower end of which is shown
proceeding froin the head to the stomach, to which in ddee. Its internal diameter is about of a line.

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Hygrome. If now the ivory tube is exactly filled with mercury, hours the mercury begins to ascend, because the moi. Hygrom

and the glass one affixed to it, as the capacity of the sture passes into the cavity, and forces it up. The former decreases by being dried, the mercury will be lowest station of the mercury is then to be marked o; forced up into the glass one.

and for the more accurate marking the degrees on the The piece figs is intended to join the ivory with scale, M. de Luc always chose to have his hygromethe glass tube. It is of brass, shaped as in the figure. trical tube made of one which had formerly belonged

A cyclindrical hole is bored through it, which holds to a thermometer. The reason of this is, that in the the glass tube as tight as possible without danger of thermometer the expansion of the mercury by beat had breaking it; and its lower part is to enter with some been already determined. The distance between the degree of difficulty into the ivory pipe. To hinder thermometrical points of melting ice and boiling water that part of the tube which incloses the brass piece at 27 French inches of the barometer was found to be from being affected by the variations of the moisture, 1937 parts. The bulb of this preparatory thermomeit is covered with a brass verrel represented in hhii. ter was broke in a bason, in order to receive carefully The pieces must be united together with gum-lac or all the mercury that it coutained. This being weigbed mastich.

in nice scales amounted to 1428 grains. The hygroThe introduction of the mercury is the next opera- meter contained 460 grains of the same mercury. Now tion. For this purpose, a slip of paper three inches it is plain, that the extent of the degrees on the hygrowide is first to be rolled over the glass tube, and tied meter, ought to be to that of the degrees on the prefast to the extremity nearest the ivory pipe. A horse- paratory thermometer as the different weights of the

hair is then to be introduced into the tube, long enough mercury contained in each; consequently 1428 : 460 : to enter the ivory pipe by an inch, and to reach three 1937: 624 nearly; and therefore the corresponding in, or four inches beyond the extremity of the glass one. tervals ought to follow the same proportion : and thus

which has been shaped round the tube must the length of a scale was obtained, which might be dinow be raised, and used as a funnel to pour the mer. vided into as many parts as he pleased. cury into the instrument, which is held upright. The Fig. 10. is a representation of De Luc's bygrome-rig. 1., purest quicksilver is to be used for this purpose, and it ter when fully constructed. In elegance it far exceeds will therefore be proper to use that revived from cinna. Smeaton's or any other, and probably also in accuracy; bar. It easily runs into the tube ; and the air escapes for by means of a small thermometer fixed on the board by means of the horse-hair, assisted with some gentle along with it, the expansion of the mercury by beat shakes. Fresh mercury must from time to time be may be known with great accuracy, and of consequence supplied, to prevent the mercurial tube from being to- how much of the height of the mercury in the hygrotally emptied ; in which case, the mercurial pellicle meter is owing to that cause, and how much to the which always forms by the contact of the air, would run mere moisture of the atmosphere. in along with it.

M. de Luc having continued his inquiries further inSome air-bubbles generally remain in the tube ; to the modifications of the atmosphere, mentions in his they may be seen through the ivory pipe, which is tbin Idée sur la Météorologie another hygrometer, which he enough to have some transparency. These being col. finds to be the best adapted to the measure of local bulected together by shaking, must be brought to the midity. Of all the bygroscopic substances which be top of the tube, and expelled by means of the horse. tried for this purpose, that which answers the best is a hair. To facilitate this operation, some part of the slip of whalebone cut transversely to the direction of mercury must be taken out of the tube, in order that the fibres, and made extremely thin ; for on this de. the air may be less obstructed in getting out, and the pends its sensibility. A slip of 12 inches in length and horse-hair have a free motion to assist it. Air, bow- a lime in breadth, he has made so thin as to weigh only ever, cannot be entirely driven out in this manner. It half a grain ; and it may be made still thinner, but is is the weight of the mercury with which the tube is then of too great sensibility, being affected even by the for that reason to be filled, which in time completes approach of the observer. This slip is kept extended its expulsion, by making it pass through the pores of by a small spring, and the variations in its length are the ivory. To hasten this, the hygrometers are put measured by a vernier division, or by, which is perhaps into a proper box. This is fixed nearly in a vertical better, an index on a dial plate : the whole variation direction to the saddle of a horse, which is set a trot. from extreme dryness to extreme moisture is about ; of ting for a few hours. The shakes sometimes divide its length. the column of mercury in the glass tube, but it is easily In these hygrometers, which are made by the instrure-united with the horse-hair. When upon shaking the ment-makers in London, the slip of whalebone is mount. hygrometer vertically, no small tremulous motion is ed in a frame very similar to that belonging to M. any longer perceived in the upper part of the column, Saussure's hygrometer before described (see fig. 7.). one may be sure that all the air is gone out.

The only material difference is, that a small concentric The scale of this hygrometer may be adjusted, a. wire spring is used, instead of a counter poise, to keep soon as the air is gone out, in the following manner. the slip of whalebone extended. M. Saussure had tried The instrument is to be suspended in a vessel of water such a spring applied to his bairs; but the weakest cooled with ice, fresh quantities of which are to be add. spring be found too strong for the hair ; and he was ed as the former melts. Here it is to remain till it further apprehensive, that the variations which the has sunk as low as it will sink by the enlargement of cold, heat, and the weather infallibly make, would sufthe capacity of the ivory tưrle, owing to the moisture it fer from the force of the springs. has imbibed. This usually happens in seren or eight M. de Luc, in the hygrometers he formerly made, as hours, and it is to be carefully noted. lu two or three before described (made of ivory), had graduated them

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Hyzroine. from one fixed point only, that of extreme moisture, which has been found to render it erroneous), namely, Hygrone.

which is obtained by soaking them in water. He has that all saline substances are destroyed by long conti-
now very ingeniously contrived to fix the other ex- nued exposure to the air in very small quantities, and
treme, that of dryness : but this being producible only therefore can only imbibe the moisture for a certain
by means of strong fires, such as hygrometers cannot time. Sulphuric acid has therefore been recommended
support, he uses an intermediate body, quicklime; in preference to the alkaline or neutral salts, and, in-
which after having been deprived, by force of fire, deed, for such as do not choose to be at the trouble of
of all its own bumidity, has the property of slowly constructing a hygrometer on the principles of Mr
imbibing humidity again from the bodies in its neigh- Sneaton or De Luc, this will probably be found the
bourhood; and whose capacity is such that all the most easy and accurate. Fig. 11. represents an hygro. Fig. 11.
vapour that can be contained in a quantity of air equal meter of this kind. A is a small glass cup containing
to its own bulk, can give it no sensible humidity. These a small quantity of oil of vitriol, B an index counter
hygrometers, inclosed with a large quantity of fresh poising it, and C the scale ; where it is plain, that as
burnt lime in lumps, acquire in three weeks the sanie the oil of vitriol attracts the moisture of the air, the
degree of dryness with the lime, which cannot differ scale will descend, which will raise the index, and vice

,
sensibly from extreme dryness.

tersa. This liquor is exceedingly sensible of the in-
M. de Saussure makes choice of bairs, prepared by crease or decrease of moisture. A single grain, after
maceration in alkaline lye. M. de Luc shows that hairs, its full increase, has varied its equilibrium so sensibly,
and all other animal or vegetable substances, taken that the tongue of a balance, only an inch and a half
lengthwise, or in the direction of their fibres, undergo long, has described an arch one-third of an inch in
contrary changes from different variations of humidity: compass (which arch would have been almost three
that, when immersed in water, they lengthen at first, inches if the tongue had been one foot), even with so
and afterwards shorten ; that when they are near the small a quantity of liquor ; consequently, if more li-
greatest degree of humidity, if the moisture is increased, quor, expanded under a large surface, were used, a
they shorten themselves; if it is diminished, they pair of scales might afford as nice an hygrometer as
lengthen themselves first before they contract again. any kind yet invented. A great inconvenience, how-
These irregularities, which obviously render them in- ever, is, that as the air must have full access to the
capable of being true measures of humidity, he shows liquid, it is impossible to keep out the dust, whicb, by
to be the necessary consequence of their organic reticu- continually adding its weight, must render the hygro-
lar structure.

meter false ; add to this, that even oil of vitriol itself
M. de Saussure takes his point of extreme moisture is by time destroyed, and changes its nature, if a small
from the vapours of water under a glass bell, keeping quantity of it is continually exposed to the air.
the sides of the bell continually moistened : and af. The best hygrometer upon this principle, and for
firns, that the humidity is there constantly the same ascertaining the quantity as well as the degree of
in all temperatures; the vapours even of boiling water moisture in the variation of the hygrometer, is of the
having no more effect than those of cold. M. de contrivance of Mr Coventry, Southwark, London.

.
Luc shows, on the contrary, that the diff ences of The account he has favoured us with is as follows.
humidity under the bell are very great, though M. " Take two sheets, of fine tissue paper, such as is used by
Saussure's hygrometer was incapable of discovering batters ; dry them carefully at about two feet distance
them; and that the real undecomposed vapour of boil. from a tolerably good fire, till after repeatedly weigh-
ing water has the directly opposite effect to that of ing them in a good pair of scales no moisture remains.
cold, the effect of extreme dryness: and on this point le When the sheets are in this perfectly dry state, reduce
mentions an interesting fact, communicated to him them to exactly 50 grains; the hygrometer is then fit
by Mr Watt, viz. that wood cannot be employed in

The sheets must be kept free from dust, and
the steam engine for any of those parts where the va- exposed a few minutes in the open air ; after which it
pour of the boiling water is confined, because it dries

may be always known by weighing them the exact
so as to crack, just as if exposed to the fire. In M. de quantity of moisture they have imbibed.
Luc's work above mentioned there are striking instances For many years the bygrometer has (says Mr Co-
related, in which the imperfection of M. Saussure's ventry) engrossed a considerable share of my attention;
hygrometer led him into false conclusions respecting and every advantage proposed by others, either as it
phenomena, and into erroneous theories to account respected the substances of which the instrument was
for them.

composed, or the manner in which its operations were
III. On the third principle, namely, the alteration of to be discerned, has been impartially examined. But
the weight of certain substances by their attracting the (adds he) I have never seen an hygrometer so simple
moisture of the air, few attempts have been made, nor in itself, or that would act with such certainty or so
do they seem to have been attended with mach success. equally alike, as the one I have now described. The
Sponges dipped in a solution of alkaline salts, and materials of which it is composed being thin, are
some kinds of paper, have been tried. These are sus- easily deprived wholly of their moisture ; which is a
pended to one end of a very accurate balance, and circunistance essentially necessary in fixing a datum
counterpoised by weights at the other, and show the from which to reckon, and which, I think, cannot be
degrees of moisture or dryness by the ascent or descent said of any substance hitherto employed in the con-
of one of the ends. But, besides that such kinds of struction of hygrometers ; with equal facility they im-
hygrometers are destitute of any fixed point from bibe or impart the humidity of the atmosphere, and
whence to begin their scale, they have another incon. show with the greatest exactness when the least alte-
venience (from which indeed Smeaton's is not free, and ration takes place."
VOL. XI, Part I.

+

B

When

for use.

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