578 HISTORY OF THERMOTICS. 



less of vapour in comparison with that which its 

 temperature and pressure enable it to contain, it 

 is more or less humid; and an instrument which 

 measures the degrees of such a gradation is a hygro- 

 meter. The hygrometers which were at first in- 

 vented, were those which measured the moisture by 

 its effect in producing expansion or contraction in 

 certain organic substances; thus De Saussure devised 

 a hair-hygrometer, De Luc a whalebone-hygrometer, 

 and Dalton used a piece of whipcord. All these 

 contrivances were variable in the amount of their 

 indications under the same circumstances; and, 

 moreover, it was not easy to know the physical 

 meaning of the degree indicated. The dew-point, 

 or constituent temperature of the vapour which 

 exists in the air, is, on the other hand, both constant 

 and definite. The determination of this point, as 

 a datum for the moisture of the atmosphere, was 

 employed by Le Roi, and by Dalton (1802), the 

 condensation being obtained by cold water 20 : and 

 finally, Mr. Daniell (1812) constructed an instru- 

 ment, where the condensing temperature was pro- 

 duced by evaporation of ether, in a very convenient 

 manner. This invention (DanieWs Hygrometer) 

 enables us to determine the quantity of vapour 

 which exists in a given mass of the atmosphere at 

 any time of observation (u A). 



Clouds. When vapour becomes visible by being 

 cooled below its constituent temperature, it forms 

 30 Daniell, Met. Ess. p. 142. Manch. Mem. vol. v. p. 581. 



