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



unscrewing, admit an alteration in the distance of leverage, and there- 

 by enable the index-hand to move over a space corresponding with the 

 scale of a mercurial barometer. To the lever (3) is attached a light 

 rod terminating with the watch-chain, which is attached to the drum 

 fastened to the axis. The handle is kept firmly fixed, when not in 

 motion, by a delicate flat spiral spring attached to the axis, acting 

 against the force of the levers, and always in a state of tension. F is 

 the exhausting tube ; and A, at the back of the instrument, is a screw, 

 which upon being turned, alters the position of the index-hand, and 

 thus enables the observer to adjust the aneroid to any mercurial baro - 

 meter. The atmospheric pressure increasing on D, will cause a slight 

 depression of the corrugated surface to which K is attached, and a 

 corresponding inclination of the lever C ; but as this lever is resting 

 upon unmoveable fulcra at BB, the motion will take place chiefly over 

 the spiral spring S, the increased distance of the lever being as six to 

 one. The metallic chamber being 25 inches in diameter, the pressure 

 of the atmosphere should be about 73 lbs. upon the corrugated 

 diaphragms, but owing to various causes it is not more than 44 pounds. 

 " Figs. 4, and 5. represent the vacuum-case, separated from the levers. 

 The former shows the case before exhaustion ; the latter after the air 

 has been withdrawn, a a a a indicate the lapping over of the thin 

 corrugated metallic diaphragms, where they are soldered to the rim ; 



.tUi 



D is the vacuum chamber, with F the exhausting tube ; and L the 

 screw part fixing D to the metallic plate N below. In Fig. 5, the 

 vacuum-case is in a state of compression after being exhausted, and M 

 represents the socket, which being pulled by the pin K, places D in a 



