Ml!. W. N. SHAW ON IirGKOMKTRIC METHODS. 



143 



SERIES V. North-west to north, variable. Barometer 751. Psychrometer 



humidity 0'405. 



These differences in the dew-jx>int readings by the two methods are very serious, 

 as, if this be the real state of affairs, the work done with REGNAULT'S hygrometer as 

 a standard instrument is so far invalidated. 



It will be noticed that the methods of finding the pressure of aqueous vapour in 

 air hitherto described are all indirect : that is to say, when the instrument has done its 

 work, a calculation has to be gone through in order to obtain the value of the vapour 

 pressure. The dew-point method approaches, perhaps, the nearest to a direct method, 

 but it must be remembered that what is actually observed, or rather what is supposed 

 to be observed, is the temperature at which a deposit takes place on a certain cooled 

 surface. It has always been assumed that this temperature corresponds to the 

 temperature of saturation of the air, and that in consequence we may take the corre- 

 sponding pressure of vapour from REONAULT'S table. The experiments of REGNAULT 

 referred to above, page 120, do not completely justify this assumption, although the 

 error is but small, but those of CROVA just described throw doubt, for different 

 reasons, upon this fundamental point. 



The calculation of the pressure of vapour from the weight of water contained in a 

 given volume of air the chemical method depends in like manner upon assumptions 

 for which there is at present no absolute experimental verification. Such experiments 

 as there are show discrepancies between observation and calculation which are beyond 

 the limit of experimental errors. And yet a direct experimentsU determination seems 

 in principle extremely easy. Assuming only that the pressure of dry air in moist air 

 is independent of the pressure of the vapour, which must be assumed if we are to 

 assign any meaning at all to the " pressure of aqueous vapour in the atmosphere," 

 it follows that, if we abstract the moisture without altering the volume, and measure 

 the resulting pressure of the dry air, the pressure of the vapour is simply the difference 

 between the initial and final pressure of the air. Moreover, the dry air is known to 

 obey the law of BOYLE with extreme exactitude for small variations of pressure and 

 volume, and hence the measure of the diminution of volume of air produced by the 



