Section III, 1917 [121] Trans. R.S.C. 



Relative Humidity. 

 By A. Norman Shaw, B.A. (Cantab.) D.Sc. (McGill) 



Presented by Dr. H. T. Barnes, F.R.S., F.R.S.C. 



(Read May Meeting, 1917.) 



1. Introduction. The apparent dryness or dampness of a 

 sample of gas has been said to depend upon its fraction of saturation 

 with aqueous vapour, and not upon its actual moisture or vapour 

 content. This quantity, the fraction of saturation, has appeared 

 extensively in meteorological records, and in scientific papers and 

 text books, with the name relative humidity; and, considered in con- 

 junction with the temperature and pressure, it has been accepted as a 

 standard quantity for gauging atmospheric comfort in general, and 

 humidity in particular. Many objections have been offered to its use 

 in this way, and the point has been reached where well known investiga- 

 tors have gone so far as to refer to the most common of the uses of the 

 term as a ''superstition" with "wo experimental foundation,'" and have 

 recorded their surprise at what they call the ''unaccountable affection 

 and reverence for the term."^ 



The confusion which arises in connection with humidity is espec- 

 ially apparent in some industrial and in some physiological problems, 

 and it has been due very largely to the unsatisfactory terminology of 

 the subject. This has often been noted and the continued acceptance 

 of the quantity, relative humidity, for use in standard records is 

 therefore, in view of recent work, a matter which seems to deserve 

 considerable attention. It is the object of these notes: — (a) to review 

 the arguments in favour of an alteration in the method of making 

 humidity records, pointing out that an improvement can now be 

 urged insistently as both necessary and practicable; (b) to refer to a 

 scheme of observation and record which could be adopted; and (c) 

 to report briefly on some experimental work of the writer, dealing with 

 improved methods of hygrometry. 



2. The Use of the Term "Relative Humidity." Since the 

 relative humidity is a function of both the temperature and the 

 absolute humidity, it is apparent that a prominent use of the term as a 

 standard quantity requires justification. For, if we call relative 



>Hill, Griffith and Flack, PIul. Trans. Roy. Soc. Lon., Ser. B. Vol. 207, p. 204, 

 (1916). 



Sec. Ill, Sig. 9 



