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



this method authors nearly always refer, as being the ultimate standard of reference 

 for the other methods. There is no evidence to show that, speaking generally, the 

 various absolute hygrometers, such as those of SCHWACKHOFER, EDELMANN, NEESEN, 

 DINES, and others, can be relied upon to that degree of accuracy, whatever may be 

 done by one particular observer after laborious trial and testing with one particular 

 instrument. 



2. It is possible by the use of suitable desiccating tubes to absorb the moisture 

 from air passed through them, and thereby to determine the weight of water 

 contained in the air ivhich actually passes over the desiccating substances. The 

 degree of accuracy attainable in this measurement is limited only by the un- 

 certainty of the weight of the drying tubes. As the drying tubes are somewhat 

 bulky and their weights are liable to alterations in consequence of variations in the 

 state of their external surface, this uncertainty is quite appreciable. No data are 

 given with regard to it by REGNAULT or other observers who have employed the 

 method. From a number of experiments of my own I may, however, conclude that 

 it is not safe to assign an accuracy to the weighings greater than that of 

 1 milligramme, but that with due precautions this limit need not be exceeded. 

 This may, of course, be the same, whatever the total amount of moisture absorbed 

 may be, and the fractional error will therefore depend upon that total amount of 

 moisture. If 1 gramme be taken up, the limit of error will be one part in one 

 thousand, so that an accuracy of 1 per cent, is well within the reach of an observer. 



3. Air may be saturated by vapour arising from water in a vessel with glass sides 

 so that the vapour pressure reaches a value agreeing to within about 2 per cent, with 

 the vapour pressure in vacuo at the same temperature. There are no observations to 

 show whether a still closer approximation to the vacuum saturation-pressure would 

 be obtained by using air drawn from an enclosure surrounded by wet muslin. 

 REGNAULT'S experiments bearing upon this question do not give a decisive answer, in 

 consequence of complications arising from the uncertainty as to the density of 

 saturated vapour, and the effect of the glass. 



4. The dew-point instruments, in the hands of skilled observers, give readings of 

 the so-called dew-point which are sensitive to within 0'l C., but the reading may 

 depend on the skill of the observer, and there is no evidence to show within what 

 limits of accuracy the temperature so observed may be regarded as the true satura- 

 tion temperature of the air. Suggestions have been made as to causes of error, but 

 no measurements of the effects of those causes have been made which would enable 

 an observer to specify the degree of accuracy of the inferences from his observations. 



5. With reference to the wet-and-dry-bulb method, two points are clear : 

 (i.) Different observers use different tables* for the reduction of their observations, 

 and in certain cases these different reductions lead to very serious differences of 

 results; (ii.) The ordinary method of exposure when the two thermometers are 



* See note B., p. 146. 



