1888.] 



Report on Hyyrometric Methods. 



333 



III. "Report on Hygrometric Methods. First Part, including 

 the Saturation Method and the Chemical Method, and 

 Dew-point Instruments." By W. N. Shaw, M.A. Commu- 

 nicated by R. H. Scott, F.R.S., Secretary to the Meteoro- 

 logical Council. Received January 17, 1888. 



(Abstract.) 



"With the exception of certain " absolute hygrometers," the beha- 

 viour of which has not yet been sufficiently tested, the determination 

 of the pressure of water- vapour in the air is indirect and requires a 

 formula of reduction. The formulas in use are based upon assump- 

 tions which are at present not so completely verified by experiment that 

 any hygrometric method can be relied upon to give measures of the 

 pressure of aqueous vapour trustworthy to within 01 mm. of mer- 

 cury. The authority for these statements is given in detail in an 

 account of the hygrometric work done since 1830. This account is 

 appended to the report as Note A. 



In the report, the chemical hygrometric method is provisionally 

 regarded as a standard. The formula of reduction applicable in this 

 case is 



_ 760(1 + «Q 

 6 ~ Ad h 



where e is the pressure of aqueous vapour in millimetres ; /, the 

 number of grammes of moisture per cubic metre in the air at tempe- 

 rature t°C. ; a. the coefficient of expansion of air per degree C. ; A the 

 density of dry air at 0° and 760 mm., i.e., 1293 grammes per cubic 

 metre ; and d is the specific gravity of the moisture referred to air at 

 the same temperature and pressure. 



The assumptions upon which the formula is based are — (1) That it 

 is possible to absorb the whole of the moisture from air by passing it over 

 desiccating substances ; and (2) that a numerical value can be assigned 

 to d. The first assumption has been discussed by Regnault and others, 

 and is sufficiently nearly accurate for all hygrometric calculations. 

 With regard to the second, Regnault's direct observations upon steam 

 (free from air) and other evidences point to the value 0*622. The 

 assumption can, moreover, be tested, by applying the chemical method 

 to air saturated at a known temperature, assuming the value 0*622 

 for d, and comparing the results with the table of saturation pressures 

 in vacuo. This, however, assumes Daltons law to be strictly accurate, 

 an open question, upon which opinion is reserved until further experi- 

 mental investigation is concluded. Regnault made the comparison in 

 sixty-eight experiments, in fifty-nine of which the air was practically 

 saturated when it entered the drying tubes. For these he found that 



