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 (Danielts Hygrometer) 

 enables us to determine the quantity of vapour 

 which exists in a given mass of the atmosphere at 

 any time of observation (UA). 



Clouds. When v&pour becomes visible by being 

 cooled below its constituent temperature, it forms 



20 Darnell, Met. Ess. p. 142. Manch. Mem. vol. v: p. 581 . 



