CHAP. II. COMPARISONS OF TILE ANEROIDS. 35 



tested before departure under the air-pump, and compared with the 

 Makers' Standard, as well as compared with my own standards 

 which had been, as I have said, verified at Kew. The prime 

 reason for carrying so many aneroids was the apprehension that, 

 despite the care which was taken of the mercurial barometers, 

 I might be suddenly deprived of them by some unhappy smash ; 

 and, having a number, the occasion was favourable for comparing 

 the actual working of the two classes of barometers. 



AVhilst aneroids are much recommended by some persons, by 

 others they are much condemned. Though it is common to hear 

 them spoken of as unreliable it is certain that differences of 

 pressure can be determined by them with marvellous accuracy. 

 When I sought for information or instruction how and why 

 they were unreliable I obtained no satisfaction, and I was 

 unable to learn that any one had ever taken the trouble to 

 compare the actual w^orking in the field of aneroids against the 

 mercurial barometer at low pressures.^ The recommendations in 

 favour of aneroids have been made, it is to be presumed, on the 

 assumytion that they do read against the mercurial barometer 

 with the same degree of accuracy when employed in the field 

 as they do when tested against it under the air-pump. 



This seemed to be a fit subject for investigation, and I 

 entered upon the enquiry without prepossessions either for or 

 against aneroids, cherishing the hope that the means of several 

 would closely accord with the mercurial barometer, — a hope that 

 I entertained because these instruments seemed to be pretty 

 equally divided between those which had a tendency to read too 

 high and those which had a tendency to read too low. The 

 idea was that the plus errors of some might or would balance 

 the minus errors of others. 



This hope was speedily dissipated. I found that my aneroids 

 did not read against the mercurial barometer when used in the 



^ I do not ignore Mr. Glaisher's comparisons in balloon. See Appendix A, 



§ 18. 



