HYGROMETERS. 



81 



scends into the ball b> at the extremity of 

 this aim, and is placed, not in the centre 

 of the ball, but as near as possible to some 

 point of its circumference. A zone of this 

 ball is gilt and burnished, so that the de- 

 position of dew may easily be perceived 

 upon it. The other ball a, is covered with 

 muslin. When an observation is to be 

 made, this last ball is moistened with ether 

 which is supplied slowly by a drop or two 

 at a time. It is cooled by the evaporation of 

 the ether and becomes capable of condensing 

 the vapour of the included fluid, and there- 

 by occasions evaporation in the opposite 

 ball b, containing the thermometer. The temperature of the 

 ball 6, should be thus reduced in a gradual manner, so that 

 the degree of the thermometer at which dew begins to be depo- 

 sited on the metallic part of the surface of the ball may be ob- 

 served with precision. The temperature of b being thereafter 

 allowed to rise, the degree at which the dew disappears from 

 its surface may likewise be noted. It should not differ much 

 from the temperature of the deposition, and will probably give the 

 dew point more correctly, although, strictly speaking, the mean 

 between the two observations should be the true dew point. It 

 is convenient to have a second thermometer in the pillar of 

 the instrument, for observing the temperature of the air at the 

 time. 



A less expensive instrument is constructed by Mr. Jones of 

 London, which appears to indicate the dew point with tolerable 

 accuracy. It consists of a delicate mercurial thermometer, of 

 which the whole bulb, with the exception of about one-fourth of 

 its surface, is covered with muslin. The bulb is cooled by the 

 application of ether to the coated surface, and the temperature 

 observed at which dew first makes its appearance upon the 

 naked part of the bulb. Mr. Foggo, of Leith, finds the indica- 

 tions of this instrument to be trustworthy,* but a preference is 

 given, by most observers, to the original instrument of Daniell. 

 The indications of the wet-bulb hygrometer first described are 

 discovered by simple inspection. It is, therefore, a problem of 



firewater's Journal, VII, 36, 



