1S51.] The adaptation of the Aneroid for surveying in India. 347 



much valuable space on the dial-plate of the instrument, which might 

 be valuably occupied otherwise. The brass index or register may be 

 expedient at home where the Aneroid is used as a weather-glass 

 merely, by people too indolent or slovenly to write down their obser- 

 vations. Here it is an incumbrance constantly in the way, and liable 

 to bring about the breaking of the glass, and ought, accordingly, to be 

 discarded. The Aneroid, as used at home, is generally cut from 27.5 

 to 31 inches, so that at altitudes above 2500 feet it is useless. It 

 ought to be graduated all round the scale, or at least as low as 23 

 inches ; and in this case it would suit for the survey of the Neilgher- 

 ries. 



The Dial, as I shall call it, or Index-plate of the Aneroid, is about 

 4 inches in diameter, the scale is engraven about half an inch from the 

 edge of the dial, and is by consequence 9 inches in circumference. 

 When engraven all around it reads from 23 to 31 or over a space of 8 

 inches, each space corresponding to a barometric inch, being therefore 

 in reality 1.125 inches, this is divided into tenths, each tenth being sub- 

 divided into quarters, so that the instrument reads to 0.025, it may 

 be estimated to half this or 0.0125. 



The space between the present scale and the extreme edge of the 

 dial is half an inch in breadth all around, and is occupied by the 

 words "stormy," "much rain," &c, calculated, as already stated, only 

 to mislead us in India. Were these to be omitted and the thermometer 

 which at present occupies the other margin of the dial-plate to be 

 sunk, so as not to interfere with this scale or sweep of the index, a 

 large portion of valuable space would be gained. The scale might 

 now be cut half an inch further out so that one inch of the barometer 

 would be represented by 1.5 instead of by 1.125 as at present. This 

 may easily be subdivided into hundredth parts capable of being read to 

 half this, or 0.005. Troughton's Marine Barometers when meant to 

 be read without vernier are cut to hundredth parts of an inch, each 

 division being a third less than those recommended for the Aneroid. 

 As already stated, the brass index is a mere encumbrance endangering 

 the glass and constantly in the way ; and the steel index ought to be 

 made very much finer than it is at present — as fine in fact as the 

 second hand of a stop-watch. Reducing its dimensions besides im- 

 proving the delicacy of the reading diminishes the mass and momen- 



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