MR. W. N. SHAW ON HTGROMKTRIC METHODS. 130 



PERNTER has also made some comparisons between the results of observations with 

 WILD'S ventilated-psychrometer and REONAULT'S hygrometer on the Obir (2048 metres 

 high) ; these observations are 30 in number, the dry-bulb temperature lying between 

 6 0< 2 C. and 14'4 C., the temperature difference between 0'6 and 3'6, and the 

 barometer between 591 mm. and 600 mm., so that they all refer to states of great 

 humidity. The values of REGNAULT'B constant A are computed for each observation ; 

 they vary between '0008432 and '0014946. 



In spite of the recommendation of REGNAULT and others, the hair hygrometer has 

 not been entirely abandoned. In 1852 KREIL used a hair hygrometer as a hygro- 

 graph and found a difference of 4 per cent, between its indications and those of a 

 psychrometer, but he laid some part of the errors at the door of the psychrometer. In 

 1858 PICHOT suggested the graduation of a hair hygrometer by using always the same 

 air, and producing changes in the humidity by changing the pressure or temperature. 

 An improved form of hair hygrometer was introduced by HERMANN and PFISTER* 

 in 1870, and during the same year R. WOLF compared the indications of two such 

 instruments with those of a psychrometer ; the results are in favour of the hair hygro- 

 meter, especially below the freezing point, inasmuch as the two hair hygrometers are 

 nearer together than either and the psychrometer, but no comparison with a standard 

 method was made, so that the evidence of superiority is not quite complete. In 1878 

 Dr. KABL KOPPE of Zurich published a small bookt in which the whole question of 

 hygrometric measurement is briefly discussed and the advantages of a hair hygro- 

 meter with provision for adjustment are more elaborately described. The adjustment 

 is carried out by means of a screw turned by a watch key, and in order to get a fixed 

 point the hygrometer is enclosed in a metal case provided with a glass front and back. 

 The front can be removed for observations, but for adjusting it ia put in its place and 

 the interior of the case is saturated by a wet cloth. KOPPE regards hair hygrometers 

 as quite sufficiently comparable, and gives the following particulars as to the 

 expansion of hair by heat alone. DE SAUBSURE found an expansion for 1 R. of 

 19 millionths of the length of the hair, and KOPPE, by observing the readings in 

 completely saturated air at various temperatures, found no change of length indicated 

 for changes of temperature' up to 20 C. 



REGNAULT has taken up the subject of hygroraetric methods again in CARL'S 

 ' Repertorium,' vol. 8, 1872, p. l.J He, however, only recapitulates his former con- 

 clusions, adding some practical directions for applying the chemical method con- 

 tinuously in meteorological observatories so as to obtain the mean quantity of 

 moisture for every three hours. VON BAUMHAUEB, referring to REGNAULT'S paper, 



' Carl, Repertorium,' vol. 6, p. 117. 



t ' Die Messnng des Feuchtigkeitsgehaltes der Luft.' Zurich, 1878. 



I Translation of part of a paper entitled " Instructions pouvant servir a 1'etablisscment des observa- 

 toires mc'te'orologiques " in the Geneva 'Archives des Sciences,' vol. 40, 1871. 



' Poggendorff's Annalen,' vol. 148, 1873, p. 448. (Translation of a paper in the ' Archives Necr- 

 Inndnises,' vol. 6, 1881.) 



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