112 MR. W. N. SHAW ON HT/GROMETRIC METHODS. 



NOTE A. 



SUMMARY OF THE RESULTS OF WORK ON HYGROMETRIC INSTRUMENTS AND METHODS 

 SINCE THE TIME OF PROFESSOR J. F. DANIELL. 



The subject of Hygrometry was considered in an essay by DANIELL, a third edition 

 of which was published in 1845, after the author's death ; it discusses the method of 

 hygrometric observation founded on the deposition of dew upon an artificially cooled 

 surface, and describes the method of using and the advantages of the dew-point 

 hygrometer invented by DANIELL* in 1820. The following paragraphs may be quoted 

 as bearing on the subject which I have now in hand : 



" It was also an important object to ascertain whether any hygrometric property of 

 the glass, or difference between it and the metal in attraction of moisture, would have 

 any appreciable effect upon the condensing power. 



" Long experience has, however, convinced me that the metallic hygrometer 

 possesses no real superiority over the glass one. The visibility of the deposition in 

 the latter is rendered perfect by making the condensing ball of black glass, and 

 viewing it by reflected light in the manner of a mirror ; and I never could perceive 

 any difference in the sensibility of the two instruments." 



The essay also discusses tables of pressure of aqueous vapour, &c., which have been 

 entirely modified by subsequent and more accurate experimental data. Hygrometers 

 founded upon the variation of organic substances are considered, and evidence to show 

 the superiority of the dew-point methods is adduced. The chief reasons against 

 organic hygrometers, as DE SAUSSURE'S and DE Luc's, are quoted from DE SAUSSURE'S 

 essay. Setting aside the difficulty of preparation of the hairs, it would appear that 

 DE SAUSSURE did not expect two hair hygrometers to give identical readings, even 

 when similarly prepared and mounted, and the following instance of a case in which 

 identical readings would not be obtained was taken by DANIELL from DE SAUSSURE. 

 If two otherwise identical hygrometers are both exposed for a long time to very dry 

 air, say at 40 of the hair hygrometer scale, and then one of them is exposed to a 

 humidity of 30 while the other is in an atmosphere of 50, when the two are replaced 

 in the same atmosphere of 40 humidity, neither will return to the 40 indication ; 

 one will read about 37 or 38, and the other 42 or 43. 



DANIELL had used DE Luc's hygrometer, and considered it as unsatisfactory as 

 DE SAUSSURE'S. 



He discusses also the wet-and- dry-bulb method (which was introduced by 

 Dr. HUTTON and modified by Sir JOHN LESLIE) in the form given to it by Dr. MASON. 

 We need not trouble about previous modifications, as MASON'S form is now practically 

 adopted. 



* ' Quarterly Journal of Science,' 1820. 



