METEOROLOGICAL INSTRUMENTS. 131 



is evidently that to which, if the air were cooled down under 

 the same pressure, it would be completely saturated with 

 moisture, and ready to deposit dew on any body in the least 

 degree colder than itself. The difference, therefore, between 

 the temperature of the air, and the temperature of the water 

 in the vessel when the dew begins to be formed, will afford 

 an indication of the dryness of the air, or of its remoteness 

 from the state of complete saturation. 



But the observation which has now been described is ca- 

 pable of affording far more interesting and precise results 

 than a mere indication of the comparative dryness or mois- 

 ture of the atmosphere. With the help of tables of the 

 elastic force of aqueous vapor at different temperatures, it 

 gives the means of determining the absolute weight of the 

 aqueous vapor diffused through any given volume of air, 

 the proportion of vapor existing in that volume to the quan- 

 tity that would be required to saturate it, and of measuring 

 the force and amount of evaporation. 



Daniell's Hygrometer. (Fig. 618, next page.) The cut 

 represents Daniell's hygrometer, in which a and b are two thin 

 glass balls of one and a quarter inch in diameter, connected 

 together by a tube, having a bore of about one-eighth of an 

 inch. The tube is bent at right angles over the two balls, 

 and the arm, b c, contains a small thermometer, d , whose 

 bulb, which should be of a lengthened form, descends into 

 the ball, 5. This ball, having been about two-thirds filled 

 with ether, is heated over a lamp till the lluid boils, and the 

 vapor issues from the capillary tube, f, which terminates 

 the ball, a. The vapor having expelled the air from both 

 balls, the capillary tube,/, is closed hermetically by the 

 flame of a lamp. This process is well known to those who 

 are accustomed to blow glass, and may have been known to 

 have succeeded, after the tube has become cool, by revers- 

 ing the instrument and taking one of the balls in the hand, 

 the heat of which will drive all the ether into the other ball, 

 and cause it, to boil rapidly. The ball, a, is now to be 

 covered with a piece of muslin. The stand, g h, is of brass, 

 and the transverse socket, i, is made to hold the glass tube, 

 in the manner of a spring, allowing it to turn and be taken 

 out with little difficulty. A small thermometer, 7t /, is in- 

 serted into the pillar of the stand. 



