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



its utility. A letter from the eminent instrument-maker, Mr. Adie, 

 read before a late meeting of the Society, has tended very much to 

 increase previously existing doubts of its usefulness in ascertaining high 

 altitudes, for which its portability and cheapness would have made it 

 particularly suitable. This Society also having ordered a supply from 

 England, it is of great importance not only to have those doubts set at 

 rest, but also to have some means of testing their correctness in order to 

 inform purchasers of the limits within which they can be trusted. In 

 order to do so, I obtained two Aneroids, one belonging to Mr. Treacher, 

 graduated to 27.5 inches, and one belonging to the Society, graduated 

 to 23 inches, and subjected them to the following experiment. In the 

 neck of a flask containing a small quantity of mercury, I inserted a 

 small bent tube, and when the flask was inverted, the mercury of 

 course stood at the same level in the flask and in the tube. 



" The flask was properly supported on a small retort stand, and the 

 Aneroids were then placed under the receiver of an air-pump, and a 

 few strokes given to the pump. When the air became a little rarified 

 in the receiver, the elastic force of the air in the flask pressed down 

 the mercury, and the degree of exhaustion was measured by the alti- 

 tude to which the mercury rose in the tube. Therefore, neglecting for 

 the present the diminution of the elastic force of the air in the flask 

 arising from the increase of volume and neglecting also the temperature 

 under the receiver, the rise of mercury in the tube should be exactly 

 equal to the fall indicated by the Aneroid, and vice versa. And this 

 was the case in each of the experiments, as will be seen from the read- 

 ings given below. The air was first pumped out, and the receiver, not 

 being perfectly air-tight, it re-entered gradually, and readings were 

 taken at the same instant by myself and Mr. Ardaseer Framjee. 

 "Teacher's Aneroid. — No. 1. 



Aneroid. 



Height of 



Mercury in tube. 



Inches. 





Inches. 



27 -5 





2.55 



28 -0 





2.25 



28 -5 





1-55 



29 -0 





1.05 



29 -4 





0-55 



3005 





0.00 



