a new Air-therrmometer. 299 



column of mercury B C is a constant measure of the pressure of 

 the air in the globe G ; E F a scale of degrees, the graduation of 

 which will be hereafter described. A mark is placed at A, so 

 that the air in the globe may be always brought to a constant 

 pressure by means of the adjustment screw S. Suppose the 

 temperature of the air to change, then a slight change will 

 be observed in the level of the mercury at A; by relaxing or 

 tightening the screw S, as the case may require, the mercury is 

 brought on a level with the mark A (which may be done with 

 mathematical precision), and then the number on the scale E F 

 coincident with the surface of the mercury in the thermometer- 

 tube will give the temperature of the surrounding air. The 

 scale E F is graduated in the following manner : — 



The tube A K C is filled with mercury in the same manner as 

 that of an ordinary barometer. The globes and tubes being 

 filled with dry air (by a process not necessary to describe here), 

 and a sufficient quantity of mercury being introduced at M, the 

 air is exhausted (by means of an air-pump attached to the tube 

 of the globe N) until the mercury falls for a short space in the 

 tube B C ; then after allowing the external air to euter the globe 

 N, the mercury will rise in the tube ME, say to J, so that the 

 sum of the columns B C and N J will be equal to the column of 

 mercury balancing the pressure of the atmosphere at the time ; 

 that is to say, if this column be 30 inches, and B C be 18 inches, 

 then N J will be 12 inches. The india-rubber ball, in a some- 

 what flaccid state, being attached to the globe N, and the screw' 

 bar I H fixed, the screw-head S is turned so as to compress the 

 air in the ball until the mercury in the tube M E stands at D 

 about the middle of the tube E J when the air is at or near to 

 mean temperature ; a mark is then made at A coincident with 

 the level of the mercury. The ball of a delicate standard ther- 

 mometer being placed beside the ball G, two extreme tempera- 

 tures, / and t v are noted, and marks placed coincident with the 

 corresponding levels of mercury in the tube L E ; then the space 

 between these marks will give a reading of degrees equal to the 

 difference between the two temperatures; hence the scale is 

 readily formed. 



Let V = the volume of the air at / temperature. 



P = its corresponding pressure measured by the column 



of mercury B C. 

 p = the length of the column of A K B. 

 a = section of the tube D. 

 b — section of the tube ABC. 



V, = the volume of the air at t l temperature, P A being- 

 its corresponding pressure. 



X2 



