82 CORRECT CHAMBER DAROMETER. 



tlie necessaiy instmctioris will accordingly be found in the 

 annexed figure, and description accompanying it ; which 

 explains the cause, that renders common barometers ini})er- 

 fect measures of the changes in the weight of the atmo- 

 sphere, and at the same time points^ out an easy and satis- 

 factory apparatus for counteracting this source of errour. 

 A correct Suppose A a (PI. IIT, fig. 1) to be the frame or outline of 



chamber baro- ^]jg barometer, the ornamental part of which may be left to 

 meter describ- ,,..„, , , * 



cd. the discretion of the workman : moreover let 28,31 repre- 



sent a scale of 3 or more inches properly divided and fur- 

 ^ nished with a nonius : let B C D E F be the compound 



tube or inverted siphon containing the mercury ; it is her- 

 metically sealed at B, and open at E F ; the bore of the 

 longer leg B C is -^^ of an inch in diameter, and that of the 

 shorter D E F f |-, in the instrument from which the descrip- 

 tion is taken. When the siphon has been filled, it is to be 

 fi^ed to the frame A a, in such a manner, that the two legs 

 )5 C^ and D E F may have a vertical position. This being 

 ^ done, a circle parallel to the horizon is to be cut on the ex- 

 ternal surface of the leg D E F at the distance of 31 inches 

 from the top of the scale, or 29f inclies from its middle, the 

 place of which is denoted in the figure by the line O o. It 

 The cause of will be perceived immediately that my barometer, as far as 



maccuiacy in -^^ j^^^^ ^jg^^^ described, differs in nothing from the common 

 the common " 



weather glass, weather glass: the imperfection of which ought to be ex- 

 plained to the less scientific reader, before the method of 

 correcting the instrument is described. In order to do this 

 !with the greater perspicuity, suppose, that when the tube is 

 first filled, the surface of the mercury in the leg D E F 

 coincides, as it ought to do, with the circle O o, and let the 

 surface on the other leg B C fall exactly on some division of 

 the scale, for instance 29 '5 : the weather glass will in this 

 case give the true weight of the atmosphere ; because the 

 ■ length of the column of mercury in B C is exactly 29*5 

 .inches. But an instrument thus constructed will give the 

 Weight of the atmosphere falsely in all other instances : for 

 * let the length of the column in B C increase in consequence 

 of an increasing pressure in the atmosphere, it is evident) 

 that the surface of the mercury in the leg D E F will de- 

 scedd below the circle Oo; because the augmentation ii 



tl)e 



