APPENDIX. COMPARISONS OF THE ANEROID BAROMETER. 405 



C— COMPARISONS OF THE ANEROID AGAINST THE 

 MERCURIAL BAROMETER. 



Aneroids were carried to Ecuador to endeavour to ascertain whether the 

 means of the readings of several, or of a number, would or would not accord 

 with the mercurial barometer at low pressures. 



It has long been known that the indications afforded by a single aneroid 

 are apt to be of a very deceiving nature, even at moderate elevations ; but it 

 seemed to me possible if several, or if a number were employed, that one 

 might, by inter-comparison, discriminate between those which went most 

 astray and those which held closely together ; and that, by adoption of the 

 means of the readings of the latter, a decent approximation might be obtained 

 to the truth, possibly even at great altitudes. It may be added that I wished 

 this might prove to be the case ; for the portability of aneroids, the facility 

 with which they can be read, and the quickness of their action, would render 

 them valuable for many purposes, if their indications could be relied upon. 



Eight aneroids were taken. One of these, by Casella, marked No. 580, 

 had been made for an earlier journey ; and, through being only graduated to 

 20 inches, was of no service for comparison at great heights.^ The seven 

 remaining aneroids were constructed expressly for the expedition, and were 

 under trial and examination for nearly twelve months before our departure. 

 They were selected from picked instruments, and only those were taken 

 such as were, so far as one could tell, in all respects as perfect as could be 

 produced.^ These seven aneroids were marked A-G. A, B, and C were 

 graduated from 31 down to 15 inches, and D, E, F, G were graduated from 

 31 down to 13 inches, — a range which I thought would be sufficient for my 

 purposes.^ 



It became apparent at an early stage of the journey, a. that the whole of 

 the aneroids had acquired considerable errors ; h. that they differed amongst 

 each other to a very large extent ; and c. that neither means of the whole, 

 nor means of those which held closest together, nor means of any combination, 

 would give decent approximations to the truth. The more evident this 

 became the greater importance I attached to the preservation of the mer- 

 curials. Comparisons of the aneroids against the mercurials were neverthe- 



1 This was left with Mr. Chambers at Guayaquil, as a reserve for him, in case accident 

 befell the Standard Mercurial ; and he read both instruments during the whole of our 

 absenre in the interior. 



2 I abstain, however, from mentioning the names of the makers (to whom I am much 

 indebted), lest the remarks which follow should be construed to their disadvantage. 



s It proved to be inadequate. 



