HYGROMETER, an instrument for measuring the degrees of dryness or moisture of the atmosphere, in like manner as the barometer and thermometer measure its different degrees of gravity or warmth.
Though every substance which swells in moist, and shrinks in dry weather, is capable of becoming an hygrometer; yet this kind of instrument is far from being as yet arrived at such a degree of perfection as the barometers and thermometers. There are three general principles on which hygrometers have been constructed. 1. The lengthening and shortening of strings by dryness and moisture, or their twisting and untwisting by the same. 2. The swelling and shrinking of solid substances by moisture or dryness; and, 3. By the increase or decrease of the weight of particular bodies whose nature is to absorb the humidity of the atmosphere.
1. On the first of these principles Mr Smeaton hath constructed an hygrometer greatly superior to any that had appeared before, and of which the following account is given in the 62d volume of the Philosophical Transactions.
“Having some years ago attempted to make an accurate and sensible hygrometer by means of a hempen cord of a considerable length, I quickly found, that, though it was more than sufficiently susceptible of every change in the humidity of the atmosphere, yet the cord was upon the whole in a continual state of lengthening. Though this change was the greatest at first, yet it did not appear probable that any given time would bring it to a certainty; and, furthermore, it seemed, that as the cord grew more determinate in mean length, the alteration by certain differences of moisture grew less. Now, as on considering wood, catgut, paper, &c. there did not appear to be a likelihood of finding any substance sufficiently sensible of differences of moisture that would be unalterable under the same degrees thereof; this led me to consider of a construction which would readily admit of an adjustment; so that, though the cord whereby the instrument is actuated may be variable in itself, both as to absolute length and difference of length under given degrees of moisture, yet that, on supposition of a material departure from its original scale, it might be readily restored thereto; and, in consequence, that any number of hygrometers similarly constructed, might,
like thermometers, be capable of speaking the same language.
“The two points of heat the more readily determinable in a thermometer, are the points of freezing and boiling water. In like manner, to construct hygrometers which shall be capable of agreement, it is necessary to establish two different degrees of a moisture which shall be as fixed in themselves, and to which we can have recourse as readily and as often as possible.
“One point is given by making the substance perfectly wet, which seems sufficiently determinable; the other is that of perfectly dry, which I do not apprehend to be attainable with the same precision. A readiness to imbibe wet, so that the substance may be soon and fully saturated, and also a facility of parting with its moisture on being exposed to the fire to dry; at the same time, that neither immersion, nor a moderate exposition to the warmth of the fire, shall injure its texture; are properties requisite to the first mover of such an hygrometer, that in a manner exclude all substances that I am acquainted with, besides hempen and flaxen threads and cords, or substances compounded of them.
“Upon these ideas, in the year 1758, I constructed two hygrometers as nearly alike as possible, in order that I might have the means of examining their agreement or disagreement on similar or dissimilar treatment. The interval or scale between dry and wet I divided into 100 equal parts, which I call the degrees of this hygrometer. The point of 0 denotes perfectly dry; and the numbers increase with the degrees of moisture to 100, which denotes perfectly wet.
“On comparing them for some time, when hung up together in a passage or staircase, where they would be very little affected by fire, and where they would be exposed to as free an air as possible in the inside of the house, I found that they were generally within one degree, and very rarely differed two degrees; but as these comparisons necessarily took up some time, and were frequently interrupted by long avocations from home, it was some years before I could form a tolerable judgment of them. One thing I soon observed, not altogether to my liking, which was, that the flaxen cords made use of seemed to make so much resistance to the entry of small degrees of moisture, (such as is commonly experienced within doors in the situation above-mentioned), that all the changes were comprised within the first 30° of the scale; but yet, on exposing them to the warm steam of a wash-house, the index quickly mounted to 100. I was therefore desirous of impregnating the cords with something of a saline nature, which should dispose them more forcibly to attract moisture; in order that the index might, with the ordinary changes of the moisture in the atmosphere, travel over a greater part of the scale of 100. How to do this in a regular and fixed quantity, was the subject of many experiments, and several years interrupted inquiry. At last I tried the one hereafter described, which seemed to answer my intention in a great measure; and tho’
Hygrometer. upon the whole it does not appear probable that ever this instrument will be made capable of such an accurate agreement as the mercurial thermometers are, yet if we can reduce all the disagreements of an hygrometer within th part of the whole scale, it will probably be of use in some philosophical inquiries, in lieu of instruments which have not yet been reduced to any common scale at all.
"Fig. 1. and 2. ABC is an orthographic delineation of the whole instrument seen in front in its true proportion. DE is that of the profile, or instrument seen edgewise. FG in both represents a flaxen cord about 35 inches long, suspended by a turning peg F, and attached to a loop of brass-wire at A, which goes down into the box cover H, and defends the index, &c. from injury; and by a glass exposes the scale to view.
"Fig. 3. shows the instrument to a larger scale, the upright part being shortened, and the box-cover removed; in which the same letters represent the same parts as in the preceding figures; GI are two loops or long links of brass-wire, which lay hold of the index KL, moveable upon a small stud or centre K. The cord FG is kept moderately strained by a weight M of about half a pound avoirdupois.—It is obvious, that, as the cord lengthens and shortens, the extreme end of the index rises and falls, and successively passes over N 2 the scale disposed in the arch of a circle, and containing 100 equal divisions. This scale is attached to the brass sliding ruler QP, which moves upon the directing piece RR, fixed by screws to the board, which makes the frame or base of the whole; and the scale and ruler NQP is retained in any place nearer to, or further from, the centre K, as may be required by the screw S.
"Fig. 4. represents in profile the sliding piece and stud I (fig. 3.), which traverses upon that part of the index next the centre K; and which can, by the two screws of the stud, be retained upon any part of the index that is made parallel; and which is done for three or four inches from the centre, for that purpose. The stud is filed to the edges, like the fulcrum of a scale-beam; one being formed on the under-side, the other on the upper, and as near as may be to one another. An hook formed at the lower end of the wire-loops CI, retains the index, by the lowermost edge of the stud; while the weight M hangs by a small hook upon the upper edge: by these means the index is kept steady, and the cords strained by the weight, with very little friction or burthen upon the central stud K.
"Fig. 5. is a parallelogram of plate brass, to keep out dust, which is attached to the upper edge of the box-cover H; and serves to shut the part of the box-cover necessarily cut away, to give leave for the wire GI to traverse with the sliding stud nearer to or further from the centre of the index K; and where, in fig. 5. a is an hole of about an inch diameter, for the wire GI to pass through in the rising and falling of the index freely without touching; b is a slit of a lesser size, sufficient to pass the wire, and admit the cover to come off without deranging the cord or index; cc are two small screws applied to two slits, by which the plate slides lengthways, in order to adapt the hole a to the wire GI, at any place of the stud I upon the
index KL.
"1. In this construction, the index KL being 12 inches long, 4 inches from the extreme end are filed so narrow in the direction in which it is seen by the eye, that any part of these four inches lying over the divisions of the scale, becomes an index thereto. The scale itself slides four inches, so as to be brought under any part of the four inches of the index attenuated as above-mentioned.
"2. The position of the directing piece RR is so determined as to be parallel to a right line drawn thro' O upon the scale, and the centre K of the index; consequently, as the attenuated part of the index forms a part of a radius or right line from the same centre, it follows, that whenever the index points to O upon the scale, though the scale is moved nearer to or further from the centre of the index, yet it produces no change in the place to which the index points.
"3. When the divided arch of the scale is at 10 inches from the centre, (that is, at its mean distance); then the centre of the arch and the centre of the index are coincident. At other distances, the extremes of which are eight or twelve inches, the centre of the divisions, and the centre of the index pointing thereto, not being coincident, the index cannot move over the spaces geometrically proportionable to one another in all situations of the scale, yet, the whole scale not exceeding of a circle, it will be found on computation, that the error can never be so great as th part of the scale, or of the hygrometer; which in this instrument being considered as indivisible, the mechanical error will not be sensible.
"The cord here made use of is flax, and between th and th of an inch in diameter; which can be readily ascertained by measuring a number of turns made round a pencil or small stick. It is a sort of cord used in London for making nets, and is of that particular kind called by net-makers flaxen three-threads laid. A competent quantity of this cord was boiled in one pound avoirdupois of water, in which was put two pennyweights troy of common salt; the whole was reduced by boiling to six ounces avoirdupois, which was done in about half an hour. As this ascertains a given strength of the brine, on taking out the cord; it may be supposed that every fibre of the cord is equally impregnated with salt. The cord being dried, it will be proper to stretch it; which may be done so as to prevent it from untwisting, by tying three or four yards to two nails against a wall, in an horizontal position, and hanging a weight of a pound or two to the middle, so as to make it form an obtuse angle. This done for a week or more in a room, will lay the fibres of the cord close together, and prevent its stretching so fast after being applied to the instrument, as it would otherwise be apt to do.
"The hygrometer is to be adjusted in the following manner. The box-cover being taken off to prevent its being spoiled by the fire, and choosing a day naturally dry, set the instrument nearly upright, about a yard from a moderate fire; so that the cord may become dry, and the instrument warm, but not so near as would spoil the finest linen by too much heat, and yet fully evaporate the moisture; there let the instrument stay till the index is got as low as it will go; now and then stroaking the cord betwixt the thumb and
and finger downwards, in order to lay the fibres there-
of close together; and thereby causing it to lengthen
as much as possible. When the index is thus become
stationary, which will generally happen in about an
hour, more or less, as the air is naturally more or less
dry, by means of the peg at top raise or depress the
index, till it lies over the point . This done, remove
the instrument from the fire; and having ready some
warm water in a tea-cup, take a middling camel's-hair
pencil, and, dipping it in the water, gently anoint the
cord till it will drink up no more, and till the index
becomes stationary and water will have no more effect
upon it, which will also generally happen in about an
hour. If in this state the index lies over the degree
marked 100, all is right: if not, slack the screw ,
and slide the scale nearer to or further from the centre,
till the point 100 comes under the index, and then the
instrument is adjusted for use: but if the compass of
the slide is not sufficient to effect this, as may proba-
bly happen on the first adjustment, slack the proper
screws, and move the sliding stud nearer to or fur-
ther from the centre of the index, according as the
angle formed by the index between the two points of
dry or wet happens to be too small or too large for
the scale."
1. On the second general principle, namely, that
of the swelling of solid bodies by moisture, and their
contraction by dryness, Mr De Luc's is the best. He
makes choice of ivory for the construction of his hy-
grometer, because he finds, that, being once wetted,
ivory regularly swells by moisture, and returns exactly
to the same dimensions when the moisture is evaporated,
which other bodies do not. This hygrometer is
represented in fig. 6. is an ivory tube open at the
end , and close at . It is made of a piece of ivory
taken at the distance of some inches from the top of a
pretty large elephant's tooth, and likewise at the same
distance from its surface, and from the canal which
reaches to that point. (This particular direction is
given, that the texture of the ivory in all different hy-
grometers may be the same, which is of great impor-
tance.) This piece is to be bored exactly in the direc-
tion of its fibres; the hole must be very straight, its
dimensions lines in diameter, and 2 inches 8 lines
in depth from to . Its bore is then to be exactly
filled with a brass cylinder, which, however, must pro-
ject somewhat beyond the ivory tube, and thus it is to
be turned on a proper machine, till the thickness of
the ivory is exactly of a line, except at the two ex-
tremities. At the bottom , the tube ends in a point;
and at the top it must for about two lines be left a
little thicker, to enable it to bear the pressure of an-
other piece put into it. Thus the thin or hygrometrical
part of the tube will be reduced to French inches,
including the concavity of the bottom. Before this
piece is used, it must be put into water, so that the
external part alone may be wetted by it; and here it
is to remain till the water penetrates to the inside, and
appears in the form of dew, which will happen in a
few hours. The reason of this is, that the ivory tube
remains somewhat larger ever after it is wetted the first
time.
For this hygrometer, a glass tube must be provided
about 14 inches long, the lower end of which is shewn
in . Its internal diameter is about of a line.
If now the ivory tube is exactly filled with mercury,
and the glass one affixed to it, as the capacity of the
former decreases by being dried, the mercury will be
forced up into the glass one.
The piece is intended to join the ivory with
the glass tube. It is of brass, shaped as in the figure.
A cylindrical hole is bored through it, which holds
the glass tube as tight as possible without danger of
breaking it; and its lower part is to enter with some
degree of difficulty into the ivory pipe. To hinder
that part of the tube which incloses the brass piece
from being affected by the variations of the moisture,
it is covered with a brass vessel represented in .
The pieces must be united together with gum-lac or
mastic.
The introduction of the mercury is the next opera-
tion. For this purpose, a slip of paper three inches
wide is first to be rolled over the glass tube, and tied
fast to the extremity nearest the ivory pipe. A horse-
hair is then to be introduced to the tube, long enough
to enter the ivory pipe by an inch, and to reach three
or four inches beyond the extremity of the glass one.
The paper which has been shaped round the tube must
now be raised, and used as a funnel to pour the mer-
cury into the instrument, which is held upright. The pu-
rest quicksilver is to be used for this purpose, and it will
therefore be proper to use that revived from cinnabar. It
easily runs into the tube; and the air escapes by means
of the horse hair, assisted with some gentle shakes. Fresh
mercury must from time to time be supplied, to pre-
vent the mercurial tube from being totally emptied; in
which case, the mercurial pellicle which always forms
by the contact of the air would run in along with it.
Some air-bubbles generally remain in the tube;
they may be seen through the ivory pipe, which is thin
enough to have some transparency. These being col-
lected together by shaking, must be brought to the
top of the tube, and expelled by means of the horse-
hair. To facilitate this operation, some part of the
mercury must be taken out of the tube, in order that
the air may be less obstructed in getting out, and the
horse-hair have a freer motion to assist it. Air, how-
ever, cannot be entirely driven out in this manner. It
is the weight of the mercury with which the tube is
for that reason to be filled, which in time completes
its expulsion, by making it pass through the pores of
the ivory. To hasten this, the hygrometers are put
into a proper box. This is fixed nearly in a ver-
tical direction to the saddle of a horse, which is set a-
trotting for a few hours. The shakes sometimes divide
the column of mercury in the glass tube, but it is
easily re-united with the horse-hair. When, upon sha-
king the hygrometer vertically, no small tremulous
motion is any longer perceived in the upper part of
the column, one may be sure that all the air is gone
out.
The scale of this hygrometer may be adjusted, as
soon as the air is gone out, in the following manner.
The instrument is to be suspended in a vessel of water
cooled with ice, fresh quantities of which are to be ad-
ded as the former melts. Here it is to remain till it
has sunk as low as it will sink by the enlargement of
the capacity of the ivory tube, owing to the moisture
it has imbibed. This usually happens in seven or
eight hours, and is to be carefully noted. In two or
three
Hygrometer. three hours the mercury begins to ascend, because the moisture passes into the cavity, and forces it up. The lowest station of the mercury is then to be marked 0; and for the more accurate marking the degrees on the scale, M. De Luc always chose to have his hygrometrical tube made of one which had formerly belonged to a thermometer. The reason of this is, that in the thermometer the expansion of the mercury by heat had been already determined. The distance between the thermometrical points of melting ice and boiling water at 27 French inches of the barometer was found to be 1937 parts. The bulb of this preparatory thermometer was broke in a basin, in order to receive carefully all the mercury that it contained. This being weighed in nice scales amounted to 1428 grains. The hygrometer contained 460 grains of the same mercury. Now it is plain, that the extent of the degrees on the hygrometer, ought to be to that of the degrees on the preparatory thermometer as the different weights of the mercury contained in each; consequently 1428 : 460 :: 1937 : 624 nearly; and therefore the corresponding intervals ought to follow the same proportion: and thus the length of a scale was obtained, which might be divided into as many parts as he pleased.
Fig. 7. is a representation of De Luc's hygrometer when fully constructed. In elegance it far exceeds Smeaton's or any other, and probably also in accuracy; for by means of a small thermometer fixed on the board along with it, the expansion of the mercury by heat may be known with great accuracy, and of consequence how much of the height of the mercury in the hygrometer is owing to that cause, and how much to the mere moisture of the atmosphere.
3. On the third principle, namely, the alteration of the weight of certain substances by their attracting the moisture of the air, few attempts have been made, nor do they seem to have been attended with much success. Sponges dipped in a solution of alkaline salts, and some kinds of paper, have been tried. These are suspended to one end of a very accurate balance, and counterpoised by weights at the other, and shew the degrees of moisture or dryness by the ascent or descent of one of the ends. But, besides that such kinds of hygrometers are destitute of any fixed point from whence to begin their scale, they have another inconvenience (from which indeed Smeaton's is not free, and which has been found to render it erroneous), namely, that all saline substances are destroyed by long continued exposure to the air in very small quantities, and therefore can only imbibe the moisture for a certain time. Oil of vitriol hath therefore been recommended in preference to the alkaline or neutral salts. See CHEMISTRY, n° 105; and, indeed, for such as do not chuse to be at the trouble of constructing a hygrometer on the principles of Mr Smeaton or De Luc, this will probably be found the most easy and accurate. Fig. 8. represents an hygrometer of this kind. A is a small glass cup containing a small quantity of oil of vitriol, B an index counterpoising it, and C the scale; where it is plain, that as the oil of vitriol attracts the moisture of the air the scale will descend, which will raise the index, and vice versa. This liquid is exceedingly sensible of the increase or decrease of moisture. A single grain, after its full increase, has varied its
equilibrium so sensibly, that the tongue of a balance, only an inch and a half long, has described an arch one third of an inch in compass, (which arch would have been almost three inches if the tongue had been one foot), even with so small a quantity of liquor; consequently, if more liquor, expanded under a large surface, were used, a pair of scales might afford as nice an hygrometer as any kind yet invented.—A great inconvenience, however, is, that as the air must have full access to the liquid, it is impossible to keep out the dust, which, by continually adding its weight, must render the hygrometer false: add to this, that even oil of vitriol itself is by time destroyed, and changes its nature, if a small quantity of it is continually exposed to the air. So that of all the inventions that have hitherto appeared, De Luc's alone seems capable of being brought to the requisite perfection.