468 DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



on return to atmospheric pressure was exceptionally small in the two high tempera- 

 ture experiments ; there was, however, no corresponding phenomenon in the case ol" 

 the experiments dealt with in Table XXIII. 



Secular Change in Aneroids. 



26. Variations in the reading of an aneroid at a standard pressure, say 30 inches, 

 may arise from two causes true secular change of zero, and elastic after-effect. In 

 practice it is by no means easy to separate the two causes with absolute accuracy ; it 

 is desirable, however, that the theoretical distinction should be clearly grasped. 

 When a low pressure is maintained for a long time, the interval required for the 

 after-effect to disappear on return to the standard pressure is, as Mr. WHYMPER has 

 found, correspondingly great. This increases, of course, the difficulty of tracing the 

 true secular change. In all but the latest experiments at Kew Observatory there 

 was little trace of after-effect when 24 hours had elapsed since the return to 

 atmospheric pressure. It is thus comparatively easy to trace the true secular change 

 in the experimental aneroids, with at least a close approach tp accuracy. This is 

 done in Table XXIV. The quantity tabulated for each aneroid is the error or 

 algebraic excess of its reading over the true pressure, as registered by the mercury 

 gauge, at the commencement of experiments preceded by at least 40 hours' exposure 

 to atmospheric pressure. Experiments during the same month were grouped 

 together, and the mean of the errors found. 



All the aneroids except No. 4 had a zero error to commence with. 



