220 TRAVELS AMONGST THE GREAT ANDES, chap. xii. 



extremely well with those of the mercurial barometer when 

 observing differences of level, when the observations were made 

 quickly, — that is to say, when only a short interval of time 

 elapsed between the readings at the lower stations and the upper 

 ones, or between the upper and lower ones, as the case might 

 be. At the Ravine of Gruallabamba I expected to descend about 

 3000 feet' in two hours, and looked forward with curiosity to 

 see whether upon this large difference of level I should observe 

 the same satisfactory accordance between the aneroids and the 

 mercurial barometer as had previously been noted upon minor 

 ones. 



At the top of the descent, at mid-day, the reading of the 

 mercurial barometer No. 558 (reduced to 32° Faht.) was 21 '692 

 inches. The two aneroids I carried (marked A, B), at the same 

 place and time, read 21*140 and 19'940 inches respectively. 

 Aneroid A, thus, had an index-error of— 0*552 inch, and B an 

 index-error of — 1*752 inches. At 2.30 p.m., on the bridge at 

 the bottom of the ravine, the reading of the mercurial barometer 

 (red. to 32° Faht.) was 23*929 inches ; of Aneroid A, 23*400 

 inches ; and of Aneroid B, 22*200 inches. The increase in the 

 pressure shewn by the three barometers, therefore, was 



Mercurial Barometer, No. 558 . . 2 '237 inches. 



Aneroid A ..... 2-260 „ 



Do. B . . . . . 2-260 „ 



The result, although in one sense highly satisfactory, was puzzling ; 

 for here were two aneroids, one with an index-error of — 0*552 

 of an inch, and the other with an error more than three times 

 as large, each indicating precisely the same increase in pressure, 

 and differing in the measurement from the mercurial barometer 

 only to the extent of 0*023 of an inch (an error of a shade 

 more than one per cent in the measurement). 



^ From my barometric observations, the depth of the ravine from the commence- 

 ment of the descent to the top of the bridge amounted to 2834 feet. With the 

 addition of the part below tlie bridge, the total depth is a little less than 3000 feet. 



