346 The adaptation of the Aneroid for surveying in India. [No. 4. 



results, however, are always too small, because in his investigation, he 

 was only anxious to obtain an approximation, and neglected systema- 

 tically all but round numbers, and all the omissions tended to reduce 

 the apparent height. Near the equator the diminution of the force of 

 gravity is another source of error, which still more diminishes the 

 height deduced from the usual formula. I have therefore used, in the 

 formation of the table given above, the number 52416, deduced from 

 Poisson's formula, in preference to 52000 used by Leslie. Besides the 

 thermometers in general use being graduated according to Fahrenheit's 

 scale, it is inconvenient to be obliged to convert the degrees into those 

 of the centigrade. As some persons may prefer the use of his rule, I 

 add it, with the example given above worked out. 



" Leslie's Rule. — As the sum of the mercurial columns is to their 

 difference, so is the constant number 52000 feet to the approximate 

 height. Correct the approximate elevation by shifting the decimal 

 point three places back to the left, and multiply by twice the sum of 

 the degrees of the detached centigrade thermometer ; this product 

 being now added, will give the true height. 



" Taking the former example, we have — 55.64 : 4.06 : 52000 : 3793, 

 the approximate height and the correction is 3.798 ft. + 99.7 = 378, 

 which gives for the true height, 4171, differing from the former by 27 

 feet. 



" Of the more minute daily variations, and the corrections, if any, 

 that are to be applied. I hope to be able to have some account for 

 the next meeting of the Society. 



" 2\st November, 1850. Joseph Patton." 



It appears to me that at home the value of the Aneroid has been 

 greatly underrated ; and that it has been looked on notwithstanding 

 all the noise that has been made about it, as little better than a house 

 weather-glass fit enough to take the place of the wheel barometer, but 

 fit for little more. Nothing certainly can be more ridiculous than the 

 legends "set fair," "change," " rain," "much rain," "stor- 

 my," &c, with which the instruments have been marked when they 

 are meant to be employed for survey purposes. In the Dekhan or 

 wherever an elevation of 2000 feet is obtained, the Aneroid indicates 

 throughout the year, a state of perpetual tempest. And not only 

 does this tend to bewilder and mislead, but it occasions the loss of 



