1851.] The adaptation of the Aneroid for surveying in India. 323 



Aneroid Barometer.* "Since writing the preceding paragraph, the 

 author has inspected^ this new and beautiful instrument, invented by 

 M. Vidi. It was described by Professor Lloyd to the British Associa- 

 tion^ and reported to have stood the test of being placed under the 

 receiver of an air-pump, when the indications corresponded with those 

 of the mercurial gauge to less than 0.01 inch. The principle upon 

 which the instrument depends, is the pressure of the atmosphere upon 

 a metallic chamber partially exhausted, and so constructed, that by a 

 system of levers a motion is given to an index-hand which moves upon 

 a dial. 



"The principle of the vacuum-case was formerly applied by M. 

 Conte § in Egypt, but from the faulty mode of constructing his instru- 

 ment, it was rejected and neglected. 



" Upon comparison of indications made with the Aneroid Barometer 

 — not corrected for the particular temperature — and a very perfect 

 mercurial barometer, given by Mr. Dent, we find that from forty-nine 

 observations made between the 6th January and 23rd February, 1848, 

 the mean difference was 0.037 inch, the aneroid being in excess : and 

 from sixty similar observations made with a standard barometer, dur- 

 ing December, 1848, and between the 3rd and 3 1st January, 1849, the 

 mean difference amounted to 0.026 inch, the mercurial being, in this 

 case, in excess over the aneroid barometer. Combining these observa- 

 tions (109 in number) a mean difference amounting to 0.0025 inch is 

 found to exist, the indications of the aneroid being in excess. || For 

 general use, the instrument is thus shewn to be well suited ; for the 

 measurement of heights it is peculiarly adapted, from its portability 

 and comparative strength ; and for nautical purposes we know of no 

 better instrument. 



* a privative, vy]p6s and €?5os — a form without moisture. See Dent on the 

 Aneroid Barometer ; Mech. Mag. No. 1307. 



f At Mr. Abraham's, Lord Street, Liverpool. The price is £3. It is 4f inches 

 in diameter, and If inches thick. The scale is divided to 0.025 inch. 



X At Swansea, in 1848. 



§ Bulletin des Sciences. Floreal, An. 6, p. 106. 



|| The sum of all these observations gave 3239. 712 inches for the aneroid, and 

 3239.44 inches for the mercurial barometer, the difference being 0.272 inch, which 

 divided by 109, = 0.00249. 



2 U 



