Aneroid Barometer. S§ 



results to the eye than in a tabular form. The mean index 

 error of the Aneroid deduced from 120 comparisons is 

 + "22 of an inch, the Aneroid being always in defect of the 

 Observatory standard, and it appears to be a constant quan- 

 tity in this ])articiilar instrument on all pressures between 

 29'50 inches and 30"50 inches ; whilst on such comparative 

 observations as have been obtained on pressures below 29"50 

 inches this index error increases to '28 inches and "30 inches, 

 and evidently goes on increasing as pressure diminishes. I 

 attribute this constantly increasing error in low pressures to a 

 change in the density of the air, produced by a diminution in 

 the weight of the incumbent atmosphere ; and it is but rational 

 to infer that it acts with greater force on pressures of 27. 26. 

 and 25. inches. The error which tins defect in principle intro- 

 duces when employing the instrument for the mensuration of 

 heights is therefore evident. 



The greatest test to which I have subjected the 

 instrument was on the occasion of making the ascent of 

 Mount Wellington. From the moment of quitting the 

 town, the Aneroid commenced indicating a diminution in 

 the atmospheric pressure, the steel index gradually moving 

 down towards the low numbers on the dial-plate. On 

 arriving at Mr. Degraves's establishment it stood, corrected 

 for index error, at 29'32 inches; and by the comparative ob- 

 servations wdiich were made at the Observatory, it assigns 

 391 feet as the height above the level of the sea. Pursuing 

 our path up to the Springs it continued to fall rapidly, and 

 its mean indication during our stay there was 26'88 inches; 

 and as a remarkable instance of its accuracy in relative 

 changes, it continued to fall slightly during our stay at the 

 Springs, a change which upon returning to the Observatory I 

 found had taken place there also. The comparative observa- 

 tions with the Aneroid and Observatory standard barometer 

 give 2" 730 feet as the height of the Springs above the level 



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