470 



CATALOGUE. — METEOROtOG V, HYGIiOMETER*, 



of Saussure, 20° to 54° ; 30° to 65°; 40° to 80° ; 45° to 86° ; 

 in atmospherical observations : the greatest heights were 

 Deluc's 56°, when Saussure's was 85° ; and Saussure's 00°, 

 T.hen Deluc's was 4 8°. 



Height of Deluc's hygrometer in London, from the Journal 

 of the R. S. 



It is observable, that in this table the mean height of the 

 hygrometer was gradually increased 10° or more in three 

 years, from 1794 to 1797, and thatthesame happened from 

 1800 to 1803, the instrument having been repaired, and a 

 new slip probably inserted. It would therefore he advise- 

 able that every hygrometer shouhi be annually submitted to 

 the tests of extreme dryness and extreme moisture, other- 

 wise an allowance must be made for the expansion probably 

 produced by exposure to the air, which appears to amount, 

 in the beginning at least, to three or four degrees annually. 

 If we apply this correction, the mean heights will become 

 66.8, 67.8, 66.6, 67.2, and 69.2, 68.5, 68.3, 67.8, for 

 the eight years that are compared, and the mean of these is 

 67.8. We may therefore call the mean height of the hygro- 

 meter in London 68°, or at most 70", and not 79". Accord- 

 ing to Deluc's comparison, this corresponds to 95.6° of 

 Saussure's hair hygrometer ; but it does not seem probable 

 that Saussure's hygrometer would make the mean moisture 

 of London so near the extreme, much less that it would 

 stand above 97°, which would be inferred from 79" of Deluc. 



Mr. Deluc seems to consider the weight acquired by any 

 hygrometrical substance as the most natural test of the de- 

 gree of moisture ; but it does not appear that this is a very 



correct criterion ; theproportions may vary greatly in different 

 substances, and they certainly do vary greatly with the time 

 of exposure. The true natural scale appears to be that 

 which expresses the proportion of moisture in any space to 

 that which would so far saturate it as to begin to be depo- 

 sited : and Saussure's experiments show, that his hygrome- 

 ter indicates this proportion inasimilar manner at two differ- 

 ent temperatures, yet not correctly with respect to either, 

 except in particular parts of the scale. Deluc's hygrometer 

 indicates the proportion pretty accurately through half the 

 scale, but the mean between the height of his hygrometer 

 andthat of Saussure, agrees tolerably well throughout. It is 

 obvious, that the height of the natural hygrotjieter may be 

 found directly on Mr. Dalton's principles, by ascertaining 

 the point of deposition, since it is expressed by the elasticity 

 of vapour at the point of deposition divided by the elasticity 

 at the actual temperature, or by the seventh power of the 

 quotient of the temperatures reckoned from a point 133" be- 

 low the zero of Fahrenheit; and, from the height of the na- 

 tural hygrometer, we may deduce the depression necessary 

 to produce a precipitation, by multiplying its seventh root 

 by tlie temperature expressed in the same way. For Saus- 

 sure's experiments on the moisture in air, the degrees of the 

 natural scale appear to be obtained pretty correctly, by taking 

 2 I — U, I being the degree of Deluc divided by 100; but 

 this seems to make the mean moisture of London too great, 

 since it would become .91 of the natural scale, which im- 

 plies adepression of 2.4° only to produced deposition of m«is- 

 ture. 



Saussure found, that a cubic foot Fr. of air at 15.16° R. 

 absorbed 11. 069 gr. Fr. of moisture, expanding 5'j ; at 

 6.18°, 5.655 grains; the hair hygrometer standing at 98°; at 

 other heights of the hygrometer, the quantities of moisture 

 were in the proportions expressed in the table here deduced 

 from tlicse experiments, which consequently show the 

 degrees of the natural hygrometer : the degrees of Deluc's 

 hygrometer are inferred from his experiments, and to these 

 degrees the approximation 2 I — U is also applied. 



