1868.] 



on Aneroid Barometers. 



473 



No. of 



instru- 

 ment. 



Correction at 





72° F. 



78° F. 



88° F. 



100° F. 





-•105 



-•135 



— •140 



-•14s 



-•145 



3- 



-•055 



— •090 



-•095 



-•095 



— •100 



4- 



-•095 



-•095 



-•095 



-•080 



— •060 



5- 



— •106 



— •106 



— *iii 



— •III 



— •III 



6. 



— *IOI 



— •III 



— •III 



— •106 



— •106 



7- 



— •061 



-•061 



— •061 



-•061 



-•031 



These results are, on the whole, very satisfactory, and appear to show 

 that a well-made compensated instrument has its indications comparatively 

 little affected by a very considerable temperature change. 



It ought always to be borne in mind that an aneroid is not capable of 

 being read to the same accuracy as a standard barometer, and that the 

 of an inch is a very small quantity. These temperature experi- 

 ments were made at the ordinary atmospheric pressure, 



I am unable to say what effect a change of temperature would have at a 

 diminished pressure. 



(3) Sudden changes of pressure. — A preliminary investigation, made at 

 the request of Mr. De La Rue, into the behaviour of an aneroid belonging 

 to the Italian Government, seemed to show considerable error at low pres- 

 sures. For the purpose of investigating the influence of sudden changes 

 of pressure upon the indications of aneroids, I then applied to some of the 

 best known makers of these instruments, for the loan of several, and 

 through their courtesy in lending me a sufficient number, and for a suf- 

 ficiently long time, I have been enabled to investigate this influence at some 

 length. 



In the following experiments, the instruments were, to begin with, sus- 

 pended vertically, at the usual atmospheric pressure. They were tapped 

 before being read. The pressure was then lowered an inch, and the in- 

 strument allowed to remain ten minutes at this pressure before being read, 

 after having again been well tapped. 



The pressure was thus reduced an inch every time, being allowed to 

 remain ten minutes at each stage ; the instrument was always well tapped 

 before being read, by means of an arrangement contrived for this purpose 

 by Mr. R. Beckley. The exhaustion was carried downwards to 19 inches 

 in the case of those instruments in which the scale was sufficiently great, 

 and the instrument was allowed to remain an hour and a half at its 

 lowest pressure ; the air was then admitted an inch at a time, the previous 

 arrangement as to time and tapping being followed. 



