On Taking the Dew-puint. 177 



capillary bore, and an oblong bulb /^ 

 to -^^ of an inch long, by /^ wide ; a 

 very light and narrow scale, four inches 

 long, and ranging from 100° Fahr. to 

 near zero, is attached from 9.^ to 3 

 inches from the extremity of the bulb ; 

 and a cork capable of fitting into the 

 neck of the bottle slides on the bare 

 part of the stem. The more similar the 

 circumstances, the more constant the re- 

 sults are likely to be. 



As the cooling material, equal parts of 

 nitre and sal ammoniac, are employed, 

 the salts being rather finely pounded, 

 well dried, and thoroughly mixed toge- 

 ther ; and a stock of this mixture is kept 

 ready prepared in a close bottle. A little glass tube is provided, 

 capable of holding, when filled to a mark, 40 grains of the 

 mixed salts. This quantity is called a measure of the salts. 

 Another tube is in like manner provided, holding, when filled 

 to a mark, \ oz. measure. This is called a measure of water. 



Suppose, then, that we introduce into the brass bottle one 

 measure of the salts. If we next, taking care that the neck is 

 not obstructed by salt, pour into the bottle very gently one 

 measure of water, at the temperature of the surrounding air, 

 and immediately, but cautiously, introduce the thermometer 

 into the bottle, so that the bulb shall reach the bottom, and the 

 cork fit into the neck, the mercury will fall a few degrees, rarely 

 exceeding five or six; and the farther reduction of temperature 

 we have entirely at our command. By an agitation of the 

 bottle, at first very gentle and cautious, and afterwards slowly 

 and gradually increased, holdmg it by the upper part of the 

 neck, the farther fall of the thermometer will be so regular and 

 gradual, that we can easily mark the temperature at which the 

 deposition of moisture occurs on the polished external surface of 

 the brass ball. 



With the proportion of salts and water just indicated, we can 

 usually obtain a reduction of temperature of from IS'' to 20° 

 below that of the water used. This will inrlude by far the 



VOT,. XIX. NO. XXXVII JULY I880. M 



