ITS HISTORY OF THER1IOTICS. 



the other hand, both constant and definite. The determination of 

 this point, as a datum for the moisture of the atmosphere, was en> 

 ployed by Le Roi, and by Dalton (1802), the condensation being 

 obtained by cold water :" and finally, Mr. Daniell (1812) constructed 

 an instrument, where the condensing temperature was produced bv 

 evaporation of ether, in a very convenient manner. This invention 

 (DanieWs Hygrometer} enables us to determine the quantity of vapor 

 which exists in a given mass of the atmosphere at any time of observ- 

 ation. 



[2nd Ed.] [As a happy application of the Atmological Laws 

 which have been discovered, I may mention the completion of the 

 theory and use of the Wet-bulb Hygrometer ; an instrument in which, 

 from the depression of temperature produced by wetting the bulb of a 

 thermometer, we infer the further depression which would produce 

 dew. Of this instrument the history is thus summed up by Prof. 

 Forbes : " Hutton invented the method ; Leslie revived and extended 

 it, giving probably the earliest, though an imperfect theory ; Gay-Lus- 

 sac, by his excellent experiments and reasoning from them, completed 

 the theory, so far as perfectly dry air is concerned ; Ivory extended 

 the theory ; which was reduced to practice by Auguste and Bohnen- 

 berger, who determined the constant with accuracy. English observ- 

 ers have done little more than confirm the conclusions of our indus- 

 trious Germanic neighbors ; nevertheless the experiments of Apjohn 

 and Prinsep must ever be considered as conclusively settling the value 

 of the coefficient near the one extremity of the scale, as those of 

 Ksemtz have done for the other." 24 



Prof. Forbes's two Reports On the Recent Progress and Present 

 State of Meteorology given among the Reports of the British Associa- 

 tion for 1832 and 1840, contain a complete and luminous account of 

 recent researches on this subject. It may perhaps be asked why I 

 have not given Meteorology a place among the Inductive Sciences ; 

 but if the reader refers to these accounts, or any other adequate view 

 of the subject, he will see that Meteorology is not a single Inductive 

 Science, but the application of several sciences to the explanation of 

 terrestrial and atmospheric phenomena. Of the sciences so applied, 

 Thermotics and Atmclogy are the principal ones. But others also 

 come into play ; as Optics, in the explanation of Rainbows, Halos, 



23 Daniell, Met. Ess. p. 142. Manch. Mem. vol. v. p. 5S1. 



24 Second Report on Meteorology, p. 101. 



