HYGROMETER. 



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 centraction 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 ; puper, 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 Saus- 

 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 inveloped 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 m'mutes; the 

 hairs are then passed through two ves- 

 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 of 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 proper- 



