1851.] The adaptation of the Aneroid for surveying in India. 327 



state of tension. The clotted line marks the position of the diaphragms 

 after the introduction of the gas, which effects compensation for changes 

 in the capacity of the case by alterations of temperature. Without 

 this gas the capacity of the case would be diminished by heat, and 

 increased by cold, but the changes in the elasticity of the gaseous fluid 

 by varying temperatures, effect compensation. In using the Aneroid 

 Barometer for scientific purposes, a certain thermometrical correction 

 is required. This is made by carefully noticing the indication of the 

 instrument in the external atmosphere, then placing it before a fire till 

 the thermometer indicates 100° F., and watching the change which 

 has taken place. The variation of the hand, divided by the degrees 

 of the thermometer, gives the quantity for each degree. The amount 

 will be sometimes in excess, occasionally in defect." — Dr. D. P. Thom- 

 son's Introduction to Meteorology, pp. 447 — 452. 



The following are the readings of various comparisons taken by the 

 Aneroid and Barometer made at different elevations up to 2000 feet 

 above the level of the sea. Further than this I have not gone. 



The survey station at Neat's Tongue, between Trombay and Mehal, 

 exactly 1000.6 feet above the mean level of the sea, as ascertained by 

 theodolite, afforded a very suitable place for experiment ; and the col- 

 lection of instruments in possession of the Geographical Society offered 

 a most convenient opportunity for determining the point. The beau- 

 tiful standard barometers by Adie, 2, 3, and 5 were with three Ane- 

 roids now selected for comparison. Barometer No. 4, was left at 

 Balcairn, about seventy feet above the level of the sea, and No. 1 in 

 the Geographical Society's Rooms, thirty-five feet lower, for reference. 

 The first observation was made at 5 p. m., about half way up the hill, 

 where barometer No. 4, stood at 19,600, temperature 84°; at Bal- 

 cairn it had stood at 29,874 at 3 p. m., temperature 86° : it had thus 

 fallen 00.274. The three Aneroids stood as follows — 



No. 3187 No. 1942 No. 1737 



Aneroid, 70 feet above sea, 29.945 29.860 29.850 



Neat's Tongue, 29.626 29.552 29.560 



Difference 319 308 290 



Mean. 306. There was no time to try more than one barometer 



here. On the top of the hill three barometers were made use of 



