1836.] of the Wet-bulb Hygrometer. 397 



considering that lie had not the means of referring to the original 

 memoir of the French philosopher, and that he had only the erroneous 

 views of the Edinburgh Encyclopediato guide, or rather to misguide, him. 

 In Captain Herbert's first paper*, he reviewed Avith unsparing- 

 criticism the paralogistic reasonings of the Encyclopedist, Mr. Ander- 

 son, and pointed out the true basis of the wet-bulb depression so 

 nearly in accordance with the views of Dr. Apjohn, of Dr. Hudson 

 his coadjutor, and of M. Gay Lussac, that it establishes the general 

 correctness of all, although the particular formula which he proceeded 

 to build upon it, naturally agreed best with the data that my own 

 experiments, published also in the Gleanings of March 1829, had 

 furnished to him. He had fortified himself for the investigation by 

 previous study of the doctrine of the latent heat of gaseous bodies, 

 upon which subject he had published a brief but luminous essay in 

 the Oriental Magazine for September 1827 ; and certainly no subject 

 has so much needed a sprinkling of rationality to lay the dust of 

 unphilosophical hypothesis which even yet remains to obscure a plain 

 question ; so much so, that Dr. Hudson, one of our Dublin competi- 

 tors, while he acknowledges the dependence of the problem on the 

 relative capacity for heat of air and aqueous vapour, " will not dwell 

 on this method nor the corrections it would require, placing no reli- 

 ance on the truth of the requisite assumptionsf." 



But before entering into a review of the various theories that have 

 been adopted by others, it may be preferable to describe in as succinct 

 a manner as is consistent with clearness, the course I originally pur- 

 sued to supply the experimental requisites for calculation, and upon 

 which I ventured to form a table} for the reduction of wet-bulb indica- 

 tions to hygrometric degrees in 1828-9§. I have recently concluded a 

 second and even more extended series of similar experiments, with the 

 advantage of superior means and apparatus, which have enabled me 

 to prosecute some branches of the inquiry that I believe have not 

 before engaged sufficient attention. 



In all hygrometric speculations it is usual to consider the state of 

 extreme moisture, or the point of aqueous saturation of the air, as 



* Gleanings, Vol. I. p. 45. 



f Phil. Mag. Vol. VII. p. 259. 



t Gleanings, Vol.1, p. 81. 



§ Before this period in 1827, I furnished a " table of multipliers" for reducing 

 the depressions into aqueous tensions, calculated from three years meteorolo- 

 gical observations at Benares with this instrument and the hair hygrometer. The 

 Royal Society, who did me the unexpected honor to publish my registers, retrench- 

 ed this table, and the notes which accompanied it. They had been, however, in 

 the mean time printed in the Calcutta Oriental Magazine for March, 1827. 



