116 MR. W. N. SHAW ON HYGEOMETRIC METHODS. 



will sometimes be sufficient to keep the acid in a liquid state at a temperature which 

 would freeze it in a close vessel ; and whenever it happens that sulphuric acid remains 

 liquid on the bulb of one thermometer, while water is frozen on that of another, a 

 comparison of the two instruments might throw some light on the influence of frost 

 on the temperature of the latter. We presume, therefore, that the most important 

 use likely to be derived from this hygrometer of M. DE LA RIVE would be to assist 

 in perfecting the theory of the moist-bulb hygrometer ; and possibly some other 

 absorbent substances might answer even better for this purpose than sulphuric acid 

 does." 



The British Association Report of 1832 includes a report on meteorology by 

 Professor J. D. FORBES, the chief points of which are recapitulated in the Encyclopaedia 

 article already cited ; it is, however, mentioned that DE SAUSSURE'S " views of hygro- 

 metry were, in some respects, so very imperfect that he was not aware of the fact that 

 the coolness produced by the evaporation of water from porous bodies was independent 

 of the rate at which the moisture was carried off by currents of air a want of know- 

 ledge which gave him much trouble." 4 There is in addition a passing reference to the 

 labours of GAY-LUSSAC and MELLONI upon the scale of DE SAUSSURE'S hygrometer. 

 These, I think, it is needless to discuss, as the subject has since been taken up by 

 REGNAULT. 



A supplementary report of the same authority appears in the British Association 

 Report of 1840, in which the advantages of the wet-and-dry-bulb method, as compared 

 with the dew-point method, are pointed out. A good deal of attention had been paid 

 to the former method about that time, and from the results of the work FORBES 

 considered that " we may now consider the moist bulb problem as practically solved." 

 The solution appears to be as follows : 



Theoretical considerations lead to the assumption of a formula : 



e"= e'-m(t-t') b ~ e '. 



Where e" represents the actual pressure of vapour in the air. 



e' the saturation pressure at the temperature t'. 



t and t' the readings of the dry and wet bulbs respectively. 



b the height of the barometer. 



B a standard barometric height. 



m is a constant to be determined, either by calculation or by auxiliary hygrometric 

 determinations. 



" This formula, employed by AUGUST and BOHNENBERGER, coincides essentially with 

 that of IVORY ('Phil. Mag.', vol. 60, 1822, p. 81,), who first gave a proper theory of 



* 'Brit. Assoc. Report,' 1832, p. 239. 



