ter.

ter.

e

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

a

are

ter.

ter.

a

a

a

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.

a

If

a

a

a

ter.

ler.

a

a

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.