THE SYMPIESOMETER. 



487 



the oil in the tube of the Sympiesometer correspond- 

 ing to these points being marked on its scale, and 

 the spaces between being divided into an hundred 

 parts, these parts correspond with hundredths of an 

 inch, on the scale of the mercurial barometer. 



As the bulk of the gas is altered by any change 

 that takes place in the temperature of the atmos- 

 phere, it is necessary to apply a correction on this 

 account. For this purpose the principal or barome- 

 tric scale m n, is made to slide upon another scale 

 o p, placed either below it or on one side of it, 

 which is divided into degrees and parts, so as to re- 

 present the change of bulk in the gas produced by 

 a change of temperature under the same pressure, 

 and corresponding to the degrees of a common Ther- 

 mometer attached to the instrument. 



This scale is constructed in the same manner as 

 the scale of a common thermometer, by changing 

 the temperature of the bulb while the pressure is 

 the same, and noting the range of the oil occasion- 

 ed by it. 



In using the instrument, observe the temperature 

 by the thermometer, and set the index which is up- 

 on the sliding Sympiesometer scale, opposite to the 

 degree of temperature upon the fixed scale ; and 

 then the height of the oil, as indicated on the slid- 

 ing scale, will be the pressure of the air required. 



When the height of one place above another is 

 to be measured by the diminution of the pressure 

 of the atmosphere, another correction is necessary to 



