180 



DANIELIi'S HYGROMETER. 



air and of the dew-point. Thus, if the ring of dew is formed as soon 

 as ether is applied, and only one degree of difference is observable, the 

 air is nearly saturated ; if the difference is 5° to 10°, the dryness is very 

 moderate, while 15° to 20° of difference indicate excessive dryness, and 

 beyond this the air is parching. 



^ 



Fig. XXIX. 



The objection to this instrmnent consists in its not being well suited to 

 the hands of a person unaccustomed to use philosophical apparatus. In 

 order to overcome this difficulty, Mr. Harris has proposed the following 

 contrivance. ' ' It consists of an old-fashioned instrument commonly sold 

 in the opticians' shops as Leslie's differential thermometer, (Pig. 

 XXX. in the opposite page). It is arranged so that when not in use the 

 fluid stands at zero in the stem, A ; over the bulb of the opposite stem, 

 I, place a piece of muslin, C, which has been well soaked in a strong 

 solution of common salt in water ; the muslin having been out into a 

 circular shape, is laid on the bulb whilst wet, and the moisture wiU 

 make it adhere sufficiently. A shelf, or bracket, with sides, top, and 

 back, is made for it to stand on, to seclude it from the sunshine, an 

 essential precaution, and .also to prevent the damp wall from having 

 effect upon the muslin, so that it may draw all its moisture from the 

 atmosphere alone. It will be found convenient to have a thermometer 

 hung on the same stand, as in all hygrometric observations the state of 

 the thermometer must be attended to. The rationale of its action is 

 simple. If the absorption of moisture exceeds the evaporation from the 

 muslin, heat wfll be generated which wiU expand the air in the bulb, 

 C, and drive the fluid up the opposite stem, indicating the degree by 



