Mr Foggo on the Dew-Point Hygrometer. 4S 



scale, and made of black enamel blown so thin that in a strong 

 light the mercury appears through it, of an elongated, shape, 

 about one-third of an inch in diameter, and three-fourths in 

 length. The bulb is covered with muslin, leaving the black 

 enamel exposed at the part next to the scale. The exposed part 

 should be as small as consistent with facility of observation. 

 The use of the ring is to confine the ether to the covered 

 surface. This instrument, therefore, is as easy of construc- 

 tion as a common thermometer ; it is very portable ; and its 

 sensibility is unlimited. 



I. have subjoined the tables to be used with the dew-point 

 instruments. I cannot here enter into any account of the 

 principles on which they are constructed. These will be found 

 detailed in Mr Anderson's Essay on Hygrometry in the Edin- 

 burgh Encyclo])(Bdia^ (vol. xi.) or in Mr DanielFs Meteorolo- 

 gical Essays. In my Elements of Meteorology I shall give 

 my reasons for preferring Mr Dalton's elastic forces to those 

 given by Dr Ure. 



Problem. — The temperature of the atmosphere and of the 

 dew-point being given, to find the quantity of vapour in a 

 cubic foot of air. 



If the temperature of the air and the dew-point be coinci- 

 dent, then in the first table opposite to the temperature will 

 be found the corresponding elastic force, and in the third 

 column is the weight of a cubic foot of vapour expressed in 

 grains. Let the temperature of the air be 70®, and the dew- 

 point the same, then from the first table we find the elastic 

 force corresponding to this temperature = .726, and the 

 weight of a cubic foot 8.082. But if the temperature of the 

 air be different from the dew-point, a correction is necessary 

 to find the exact weight. Suppose the dew-point to be 70° 

 as before, but the temperature to be 80% then the vapour has 

 suffered an expansion due to an excess of 10°. We find 

 in the second table the correction for 10° is 1.0208, and 

 8.082 divided by this = 7.917. To find the relation of these 

 conditions on the natural scale of humidity, the weight of 

 vapour at the dew-point being divided by the weight at the 

 temperature of the air, the quotient gives in parts of 1.000 



the decree of saturation, thus -i 00*^ ~ .7S5. 



