274 Observations oil the Hygrometer. 



readily measured by means of the barometer and thermometer ; 

 but with respect to the instrument best adapted for finding the 

 dew-point, and to which the term Hygrometer has been gene- 

 rally confined, meteorologists are by no means so well agreed. 



Various inventions have been proposed for this purpose, such 

 as those of Leslie, Daniell, Saussure, De Luc, and many others; 

 but it is intended to confine the observations in this paper to two 

 of these, the hygrometers of Professor Leslie and Mr Daniell, 

 and more particularly to the theory of the former instrument. 



Professor Leslie's hygrometer indicates simply the degree of 

 cold produced by the evaporation of water, and from this datum, 

 in connexion with those of the density and temperature, both 

 the absolute and relative dampness of the air are most ingeni- 

 ously deduced. Mr DanielPs hygrometer, on the other hand, 

 enables us to find the dew-point directly by experiment. It is 

 assumed, indeed, in using this instrument, that the air would 

 be saturated with moisture, if reduced to the temperature at 

 which a cold surface becomes covered with dew ; but the truth 

 of this supposition is so nearly certain, that the dew-point may 

 still be said to be experimentally determined. 



Much difference of opinion exists respecting these hygrome- 

 ters. As far as regards the ease and accuracy with which ob- 

 servations may be made, that of Professor Leslie is certainly 

 better adapted to general use. But, as has been stated, Mr 

 DanielPs hygrometer possesses one, and perhaps only one, de- 

 cided advantage, in that it indicates the dew-point without the 

 intervention of theory. 



Among the objections which are urged against the latter in- 

 strument, none seems more important than the frequent want of 

 coincidence between the real dew-point and the temperature in- 

 dicated, at the corresponding instant, by the interior thermo- 

 meter. The result of the observation is often rendered doubt- 

 ful from this cause, even when the aether has been applied with 

 the utmost caution. The explanation of the want of coinci- 

 dence alluded to, evidently is, that the thermometer is entirely 

 a relative measure, as well with respect to the time in which any 

 change of temperature takes place, as to the degree of the 

 change ; and although the rate at which the temperature varies 

 may be so adjusted as to coincide very nearly with that of the 

 expansion or contraction of the thermometric fluid, thus render- 



