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Art. II. Description of a Differential Thermometer. By 

 W. HOWAKD, M.D. 



This instrument is an imitation of Mr. Leslie's differential 

 thermometer, but is on a different principle. In his, the de- 

 gree of heat is measured by the expansion of air, but in the 

 present one by the increase of expansive force of the vapour 

 of ether or spirit of Avine in vacuo, which affords a test of 

 great delicacy, and is easily constructed. 



A tube (A) being first made with a 

 ball at each extremity, in one of which 

 is left a small orifice, a portion of ether 

 or spirit of wine is then introduced, and 

 heat being applied, is brought to a state 

 of active ebullition. At this moment the 

 orifice is closed with a piece of wax, and 

 finally hermetically sealed by the blow- 

 pipe. The tube may then be carefully 

 bent in the form of a hook, and the scale 

 and foot being adapted, the instrument 

 is finished. (B). 



This thermometer is intended to be 

 used in the same cases as that of Mr. 

 Leslie, but I conceive it to possess some 

 advantages. It is more delicate. When 

 a heated body, as the hand, is ap- 

 proached to one of the balls, the liquid 

 sensibly ascends or descends, and as 

 soon as this cause is removed, begins 

 instantly to return to its former level. 

 Whereas in the air thermometer, the 

 impulsion to the liquid is not instan- 

 taneous, and it continues to move in 

 the" same direction a moment after the 

 moved. 



If the two balls were freed entirely from air, the liquid would 

 always remain at the same level in each branch of the tube, ex- 

 cept a trifling difference caused by capillary attraction. This 



heatino: cause is re- 



