86 Aneroid Baromeier. 



of the sea. On a former occasion I had measured the same 

 spot with two very excellent mountain harometers belonging 

 to H.M.S. Fhj, and they gave a mean result of 2,500 feet. 

 The great difference in the two results must therefore be 

 attributed to the imperfections of principle in the Aneroid 

 before-mentioned, and which appear insuperable in the 

 present state of the instrument. Leaving the Springs, we 

 continued to ascend the mountain, until the Aneroid 

 indicated 25-87 inches (about 3,500 feet), at which height it 

 ceased to act, and the index became perfectly stationary; it 

 remained so during the remaining portion of the ascent : 

 and when we reached the highest part of the mountain 

 (which I had previously ascertained to be 4,190 feet), 

 the index steadily pointed to ^5"87 inches, although we had 

 in the meantime ascended at least 500 feet more. It now 

 became interesting to ascertain the cause of its having 

 ceased to act ; and we were much gi'atified to find on our 

 descent that on arriving just below the spot where it had 

 previously become stationary, the index began to move 

 towards the higher numbers, and continued to do so steadily 

 as we increased our distance from the summit. It was 

 therefore evident that we had been in an atmosphere more 

 rarified than the gas contained in the cylinder, and that, in 

 consequence, its corrugated surface was dilated to its full 

 extent, and ceased to exercise any influence upon the levers 

 and index of the dial-plate. It was not until we had 

 increased the weight of the incumbent atmosphere by our 

 descent that the index would move ; and from that time its 

 indications were the same as they had been during the 

 ascent. On reaching the Springs it showed 2G'90 inches, the 

 same as it had done previously ; and I found afterwards 

 that the Observatory barometer had returned at that time 

 to the same reading which it had in the morning at the time 

 of our ascent. 



