36 TEAVULS A3I0NGST THE GREAT ANDES, chap. ii. 



field with the same accuracy, or in the same manner, as they 

 had done when tested under the air-pump, and that their 

 behaviour was perplexing. Upon leaving Guayaquil (just above 

 the level of the sea), one of the seven read higher than the 

 mercurial barometer, and the six others all read lower than it, 

 though not to a large extent. The mean of the readings of the 

 whole of the aneroids was 0'055 of an inch lower than the 

 mercurials, and the seven instruments differed amongst each 

 other to the extent of 0*346 of an inch. It was to be expected 

 that they would continue to differ amongst each other, and that 

 the greatest difference would increase, as this is what is com- 

 monly found to occur in most assemblages of aneroids. The 

 object of comparison in the field was to determine whether 

 increase of the differences amongst the aneroids would affect 

 the mean error of the whole when compared with the mercurial 

 barometer ; that is to say, would the mean error of the aneroids 

 remain 0*055 of an inch, or would it become materially altered ? 



It became apparent at an early stage of the journey that 

 the means of the aneroids shewed larger and larger depart- 

 ures from the mercurial barometer. After a little time, each 

 individual instrument indicated loiver pressures than the mer- 

 curial barometer.^ By the time we arrived at Guaranda the 

 mean error of the aneroids had increased from —0*055 to 

 — 0*520 of an inch, and it augmented daily. ^ In the course 

 of the narrative I shall point out from time to time the 

 exceedingly serious errors which would have been fallen into 

 in determination of altitudes if I had been obliged to rely 

 upon aneroids alone. ^ 



AVhen the weather was favourable I took out the camera, 



1 Aneroid D, the one which read higher than the mercurials at Guayaquil, by 

 the time we arrived at Tambo Gobierno had a minus error of 0'359 of an inch. 



"^ See Appendix C, § 5, and the tables showing the constant growth of the 

 "greatest difference," and the "mean error of aneroids." 



3 Those who desire to pursue this subject are referred to the pamphlet How 

 to use the Aneroid Barometer, which is published simultaneously with this volume. 



