44 



THE ANEROID BAROMETER. 



A new instrument, the Aneroid Barometer, has lately 

 been invented by M. Vidi of Paris, for ascertaining the 

 variations of the atmosphere : its action depends on the 

 effect produced by the pressure of the atmosphere on a 

 metallic box from which the air has been exhausted and 

 then hermetically sealed. It has already been explained 

 that the weight of the column of the mercurial baro- 

 meter is conterpoised by the weight of the atmosphere, 

 and that the variations in the weight of the atmosphere 

 are shown by the variations in the length of this column, 

 and measured in inches and tenths ; but in the Aneroid 

 an index traversing a dial records the changes in the 

 weight or pressure of the atmosphere on a given surface, 

 suppose a square inch ; it would therefore have greatly 

 facilitated the comprehension of the action of the instru- 

 ment had the dial been graduated to show the difference 

 of the atmospheric pressure, in absolute weight or pounds. 

 Though for purely scientific purposes the Aneroid is at 

 present far removed from competition with the mercurial 

 barometer, it nevertheless has some advantages in its 

 extreme sensibility and its portability. Much has been 

 urged against its variations from temperature; the 

 writer has made experiments with his own instrument 

 and with many sent him for comparison by Messrs. Wat- 

 kins and Hill : in a range of temperature from 28 to 80, 

 the variations have seldom exceeded a tenth of an inch ; 

 and it must be borne in inind, that had the mercurial 

 barometer been subjected to the same range, it would 

 have been equally affected, only in the latter case the 



