HYGROMETER, a machine or instru ment to measure the degrees of dryness or moisture of the atmosphere.
There are divers sorts of hygrometers; for whatever body either swells or shrinks by dryness or moisture is capable of be ing formed into an hygrometer. Such are woods of most kinds, particularly ash, deal, poplar, &c. Such also is catgut, the beard of a wild oat, &c.
All bodies that are susceptible of imbib ing water have a greater or less disposi tion to unite themselves with that fluid, by the effect of an attraction similar to chemical affinity. If we plupge into wa ter several of these bodies, such as wood, a sponge, paper, &c. they will appropriate to themselves a quantity of that liquid, which will vary with the bodies respec tively; and as, in proportion as they tend towards the point of saturation, their affi nity for the water continues to diminish, when those which have most powerfully attracted the water have arrived at the point, where their attractive force is found solely equal to that of the body 'which acted most feebly upon the same liquid, there will be established a species of equilibrium between all those bodies, in such manner, that at this term the im bibing will be stopped. If there be brought into contact two wetted or soak ed bodies, whose affinities for water are not in equilibrio, that, whose affinity is the weakest, will yield'of its fluid to the other, until the equilibrium is established; and it is in this disposition of a body to mois ten another body that touches it, that what is called humidity properly consists. Of all bodies, the air is that of which we are most interested to know the different degrees of humidity, and it is also towards the means of procuring this knowledge that philosophers have principally direct ed their researches ; hence the various kinds of instruments that have been con trived to measure the humidity of the air. A multitude of bodies are known, in which the humidity, in proportion as it augments or diminishes, occasions divers degrees of dilatation or of contraction, according as the body is inclined to one or other of these effects, by reason of its organization, of its texture, or of the disposition of the fibres, of which it is the assemblage. For example, water, by introducing itself within cords, makes the fibres twist, and become situated obliquely, produces be tween those fibres such a separation, as causes the cord to thicken or swell, and, by a necessary consequence, to shorten. The twisted threads, of which cloths are fabricated, may be considered as small cords, which experience, in like manner, a contraction by the action of humidity ; whence it happens, that cloths, especially when wetted for the first time, contract in the two directions of their intersecting threads ; paper, on the contrary, which is only an assemblage of filaments, very thin, very short, and disposed irregularly in all directions, lengthens in all the di mensions of its surface, in proportion as the water, by insinuating itself between the intervals of those same filaments, acts by placing them further asunder, pro ceeding from the middle towards the edges. Different bodies have been em ployed successively in the construction of hygrometers, chosen from among those in which humidity produces the most sen sible motions. Philosophers have sought also to measure the humidity of the air by the augmentation of weight undergone by certain substances, such, as a tuft of wool, or portions of salt, by absorbing the wa ter contained in the air. But, besides that these methods were in themselves very imperfect, the bodies employed were subject to alterations, which would make them lose their hygrometic quali ty more or less promptly; they had, there fore, the double inconvenience of be ing inaccurate, and not being of long ser vice. To deduce from hygrometry real advantages, it must be put in a state of rivalry with the thermometer, by pre senting a series of exact observations, such as may be comparable in the differ ent hygrometers. The celebrated Sans sure, to whom we are indebted for a very estimable work on hygrometry, has attained the accomplishment of this ob ject by a process of which we shall at- , tempt to give some idea. The principal piece in this hygrometer is a hair, which Saussure first causes to undergo a prepa ration, the design of which is to divest it of a kind of oiliness that is natural to it, and that secures it to a certain point from the action of humidity. This prepara
tion is made at the same time upon a cer tain number of hairs forming a tuft, the thickness of which need not exceed that of a writing pen, and contained in a fine cloth serving them for a case. The hairs thus inve]oped are immersed in a long necked phial full of water, which holds in solution nearly a hundredth part of its weight of sulphate of soda, making this water boil nearly thirty minutes; the hairs are then passed through two yes- 4 sels of pure water while they are boiling ; afterwards they are drawn from their wrapper, and separated ; then they are suspended to dry in the air ; after which there only remains to make choice of those which are the cleanest, softest, most brilliant, and most transparent. It is known that humidity lengthens the hair, and that the process of drying short ens it. To render both these effects more perceptible, Saussure attached one of the two ends of the hair to a fixed point, and the other to the circumference or a move able cylinder, that carries at one of its extremities a light index or hand. The hair is bound by a counter-weight of a bout three grains, suspended by a deli cate silk, which is rolled in a contrary way about the same cylinder. In propor tion as the hair lengthens or shortens, it causes the cylinder to turn in'one or the other direction, and, by a necessary con sequence, the little index turns likewise, the motions of which are measured on the circumference of a graduated circle, about which the index performs its revo lution as in common clocks. In this man ner a very small variation in the length of the hair becomes perceptible, by the much more considerable motion that it occasions in the extremity of the index ; and it will be easily conceived, that equal degrees of expansion, or of contraction in the hair, answer to equal arcs describ ed by the extremity of the index. To give to the scale such a basis, as may establish a relation between all the hygrometers that are constructed upon the same prin ciples, Saussure assumes two fixed terms, one of which is the extreme of humidity, and the other that of dryness: he deter mines the first by placing the hygrometer under a glass receiver, the whole interior surface of which he had completely moist ened with water ; the air being saturated by this water acts by its humidity upon the hair to lengthen it. He moistened anew the interior of the receiver, as often as it was necessary ; and he knew that the term of extreme humidity was attained, when, by a longer continuance under the receiver, the hair ceased to extend itself. To obtain the contrary limit of extreme dryness, the same philosopher made use of a hot and well dried receiver, under which he included the hygrometer with a piece of iron plate, likewise heated and covered with a fixed alkali. This salt, by exercising its absorbent faculty upon the remaining humidity in the surround ing air, causes the hair to contract itself, until it has attained the ultimate limit of its contraction. The scale of the instru ment is divided into a hundred degrees. The zero indicates the limit of extreme dryness, and the number one hundred that of extreme humidity. The effects of moisture and of dryness upon the hair are modified by those of heat, which act upon it, sometimes in the same sense, and sometimes in a contrary one ; so that, if it be supposed, for example, that the air is heated about the hygrometer on one part, this air, whose dissolving faculty with regard tothe water will be augment ed, will take away from the hair a portion of the water, which it had imbibed, thus tending to shorten the hair; while, on the other part, the beat, by penetratingit, will tend, though much more feebly, to length: en it ; and hence the total effect will be found to consist of two partial and contra ry effects, the one hygrometric, the other pyrometric. In observations which re quire a certain precision, it is therefore necessary to consult the thermometer at the same time with the hygrometer ; and, on this account, the inventor has con structed, from observation, a table of cor rection, which will put it in the power of philosophers always to ascertain the de gree of humidity of the air, from the ef fect produced by the heat.