288 



METEOROLOGY. 



Sect. IX. 



Sect 



---r-. 



•^ 





i 



lass, removable at pleasure. 



The cylinder is filled with a deep blue liquid (am- 

 monio-sulphate of copper), and the ball at the top being 

 purposely let!:, full of air, and the point closed with 

 melted wax, it becomes, in any given position of the 

 screw, a thermometer of great delicacy, capable of beino* 

 read off on a divided scale attached. The cylinder is 

 enclosed in a chamber blackened on three sides, and on 

 the fourth, or face, defended from currents of air by a 

 thick 



The action of the screw is to diminish or increase at 

 pleasure the capacity of the hollow of the cylinder, and 

 thus to drive, if necessary, a portion of the liquid up into 



the ball, which acts as a reservoir, or^ if necessary, to 

 draw 



just fill it, leaving no bubble of air in the cylinder. The 

 interior thermometer indicates the temperature of the 

 blue liquid approximately for the subsequent reduction 

 of the observation. 



To use the instrument^ examine first whether there be 

 any air in the cylinder, which is easily seen by holding it 

 levelj and tilting it, when the air, if any, will be seen to 

 run alon 



back from the reservoir such a quantity as shall 



it 



If there be any, hold it upright in the left 

 hand, and the air will ascend to the root of the ther- 



mometer-tube. 



.'The 



n, by alternate screwing 



and un- 



screwing the screw with the right hand, SiS the case may 



require, it will always be practicable to drive the air out 

 of the cylinder into the ball, and suck down the liquid, 

 if any, from the ball, to supply its place, till the air is 

 entirely evacuated from the cylinder, and the latter, as 

 well as the whole stem of the thermometer-tube, is full 

 of the liquid in an unbroken column. Then, holding it 



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