MR. W. N. SHAW ON HYGROMETRIC METHODS. 123 



himself to determine whether the indications of hair hygrometers are strictly com- 

 parable under the following different conditions : 



1. When they are constructed of the same kind of hair, prepared at the same 



operation. 



2. When they are constructed of different kinds of hair, but prepared at the same 



operation. 



3. When they are constructed of different kinds of hair, and prepared at different 



operations by different processes. 



A considerable number of hygrometers were compared with each other, their " fixed 

 points " being determined by means of saturated air, and ajr dried by sulphuric acid. 

 The results are summed up by REONAULT as follows : 



" Two hygrometers, mounted with similar hair, may be non-comparable solely on 

 account of their not being stretched by equal weights." 



" Hygrometers constructed with hair of the same kind, prepared at the same opera- 

 tion, do not give identical results, but they do not differ to such an extent that, for 

 the majority of observations, they may not be regarded as comparable," The maximum 

 difference between different instruments was about three hygrometric degrees. 



" Hygrometers constructed with hairs of different kinds arid prepared in different 

 ways may show very great differences in their indications, even when they agree at 

 the fixed points." 



It is consequently impracticable to construct a single table for the calculation of 

 results which is applicable to all instruments, and it is desirable that observers should 

 have at their disposal a simple process which enables them to make for themselves a 

 table for their own instrument, and by which they can verify the graduation of the 

 instrument as often as they wish. 



With this object REGNAULT proposes to enclose the hygrometer in a cylindrical 

 vessel at the bottom of which can be placed successive layers of mixtures of sulphuric 

 acid and water of different known strengths ; he gives formula and tables for the 

 pressure of aqueous vapour due to such solutions at different temperatures. The 

 process of forming a scale for the hygrometer is described. REONAULT proposes to 

 abandon the attempt to determine the point of the scale corresponding to absolute 

 dryness, and indeed to use instead the fractional humidity of about '20, as given by a 

 suitable solution of sulphuric acid. The effect of changes of temperature upon hair 

 hygrometers was not determined, in consequence of the breaking of the apparatus 

 intended for that purpose, (See p. 139.) 



REGNAULT gives as his final opinion about hair hygrometers, in his second memoir : 

 " II est & de"sirer que les observateurs renoncent ddfinitivement a un appareil sur le 

 bon e'tat duquel ils ne peuvent jamais compter."* 



* ' Annales de Chimic," vol. 37, 1853, p. 258. (For later information about the hair hygrometer, 

 see p. 139.) 



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