

HYGROMETER. 



tion as the hair lengthens or shortens, it 

 causes the cylinder to turn in'pne 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 includ ed the hygrometer whh 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 to the 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 heat, by penetrating-it, 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. 



De Luc, who devoted his attention to 

 the same object has followed a different 

 method. This philosopher employed for 

 the construction of his hygrometers, a 

 very thin slip of whale -bone, which per- 

 forms the same office as ,the hair in the 

 hygrometer of Saussure. He kept this 

 whale-bone bent by means of a spring, 

 the action of which he preferred to that of 

 a weight : he determined the degree of 

 extreme humidity, by immersing the slip 

 of whale-bone entirely under water; and 

 to fix the opposite limit, which is that of 

 extreme dryness, he made use of calcined 

 lime, which he inclosed with the hygro- 

 meter under a glass bell. The choice of 

 lime is founded on this, that the calcina- 

 tion having produced a higher degree of 

 dryness, if it be afterward left to cool, so 

 far that it may be placed without incon- 

 venience under the glass belJ destined for 

 the experiment, it will be still found, as 

 to sense, in the same state of dryness, 

 since it is very slow in acquiring- humidi- 

 ty ; and thus all its absorbent faculty will 

 be employed to dry up, by little and little, 

 the air contained under the receiver, and 

 to make the hygrometer itself pass to a 

 state which approaches the nearest pos- 

 sible to extreme dryness. The hygrome- 

 ter has been long neglected in meteoro- 

 logical observations ; it is necessary to as- 

 sociate with it the thermometer and the 

 barometer, to be in a state to unravel the 

 complication of different causes which in- 

 fluence the variations of the atmosphere ; 

 and it is only by the aid of a long series of 

 observations, made by these various in- 

 struments, together with all the indica- 

 tions which are deduced from the state 

 of the heavens, that we can obtain such 

 data as will enable us to prognosticate, 

 with great probability, the temporary 

 changes, and to arrive at a plausible the- 

 ory upon this subject, so interesting, and 

 so naturally calculated to excile our curi- 

 osity. 



Mr. Marshall says, that a simple instru- 

 ment of this sort may be formed by means 

 of "a flaxen line (large well manufactur- 



