MB. W. N. SHAW ON HYOROMETBIC METHODS. 113 



At this time APJOHN'S formula /" =/' c//88 X p/30 for calculating the pressure 

 of vapour from the wet-and-dry-bulb indications was known. UANIELL calls atten- 

 tion to the following disad vantages attending their use : 



(1.) The smallness of the scale of the instrument, so that the probable errors of 

 observation bear a very high proportion to the required result, particularly about 

 the freezing point. 



(2.) The liability of the indication to be affected by causes other than the humidity 

 of the air and its temperature. The suggestion of PELTIER, that it may be affected 

 by the electrical state of the atmosphere, is referred to as a possible explanation of 

 the cases of the temperature of the wet bulb being found above that of the dry 

 " which occur in all long series of observations." (See pp. 129 and 138.) 



(3.) The uncertainty in the results computed from the observations ; instances are 

 given to show that the results deduced by APJOHN'S formula are different from those 

 obtained from the Greenwich table of factors, derived from experimental comparison 

 of the wet-and-dry-bulb readings with direct observations of the dew-point. 



There is another discussion, in English, of the various questions relating to hygro- 

 metric measurements, occupying 22 pages of the seventh edition of the ' Encyclopaedia 

 Britannica,' and published in 1842, from which we extract the following information 

 about Balance Hygrometers: 



" Hygrometers have frequently been formed by suspending from one arm of a 

 balance some substance which strongly attracts moisture from the atmosphere, and 

 nicely counterpoising it by a weight on the other arm. The changes in the humidity 

 of the air are then meant to be indicated by the changes in the position of the beam, 

 arising from the gain or loss of weight in the suspended body. A great variety of 

 substances have been used for this purpose, such as sponge, caustic potash, the 

 deliquescent salts, sulphuric acid, &c. These, like the former instruments, are all too 

 late in their indications, though some of them might scarcely be liable to lose their 

 sensibility were it not that they soon become useless from the accumulation of dust, 

 soot, &c., especially if in or near a large city." 



DE SAUSSURE'S hair and DE Luc's whalebone hygrometers are carefully described, 

 and a comparison of their scales is given, showing very considerable divergence, and 

 indicating very peculiar behaviour of the hair hygrometer, as though it had a 

 maximum near the saturation point. I give half the table of comparison to make 

 the divergence clear. This is DE Luc's comparison, and was repudiated by 

 DE SAUSSURE, because DE Luc saturated the hygrometer by actual contact with 

 water. 



MDi ' I.\\\\ III. A. 



