84 



Professor Potter on the Aerometric Balance, 



knife-edge at C rests on another agate-plate held by the frame- 

 work of the instrument. The end D of the lever is formed so 

 as to be graduated as a vernier ; and a graduated arc having its 

 centre at the knife-edge C is fixed firmly in a vertical plane 

 to the brass framework. The readings of the graduated arc 

 by means of the vernier give the position of equilibrium of the 

 lever. At a b is a fine steel screw passing through the arm 

 of the lever and carrying a brass head at b ; this screw, turned 

 by a key, regulates the sensibility of the balance by raising or 

 lowering the centre of gravity of the lever. The spirit-levels 

 for adjustment to the horizontal position, as well as other de- 

 tails, are not drawn in the figure. 



The whole balance is fixed firmly in a glass case or 

 lanthorn, which also contains its accompanying thermometer. 



The instrument evidently acts by the changing weight of 

 the air displaced by the globe with its appendages, and the 

 lever. If these volumes were exactly known, and the place 

 of the centre of gravity of that displaced by the lever for given 

 temperatures, then the density of the air could be calculated 

 from the position of equilibrium ; but since these cannot be 

 easily determined with accuracy, the method of employing the 

 instrument must be that of finding the value of its divisions in 

 terms of the density of the air, by experime?ital methods, for 

 changes of temperature and pressure; such as by examining 

 the instrument under the receiver of an air-pump furnished 

 with a barometer gauge, and by noting the effect of change of 

 temperature in the lanthorn when the atmospheric pressure is 

 stationary. 



In investigating the theoretical sensibility of the bent lever 

 balance, I shall first suppose, in the ordinary wa}', that the 

 weights are absolute weights, and afterwards take the modifi- 

 cation introduced by supposing the balance used, as actually, 

 in a medium of varying density. 



M 



& 



Let c be the fulcrum, b the point from which the globe and 



