402 Experiments on Aneroid Barometers and their Discussion. 



The second group of data are the results of special experiments 

 made at Kew Observatory during the last three years. These were 

 intended to link together the phenomena exhibited in the Kew 

 verifications, and to further investigate various points bearing on the 

 usefulness of the certificate hitherto issued to aneroids. 



The aneroid is an instrument exhibiting elastic after-effect, fre- 

 quently in a conspicuous way. When pressure is lowered and then 

 maintained constant, the aneroid's reading continues to fall, and when 

 pressure is restored to its original value, the aneroid reads at first 

 lower than it did originally, but exhibits a gradual tendency to recover. 

 These general facts have of course been long known. The most 

 characteristic features were in fact discussed 30 years ago by 

 Dr. Balfour Stewart, then superintendent of Kew Observatory, who 

 dealt with a considerable mass of experimental material. They have 

 also been the subject of a comparatively recent pamphlet by Mr. 

 Edward Wbymper, who gives the results of a number of interesting 

 long period experiments. 



The present paper is partly experimental and partly theoretical. 

 It treats of how the differences between the readings with pressure 

 descending and ascending in a normal pressure cycle, such as the 

 Kew test, vary throughout the range, and how the sum of these 

 differences varies from one range to another. It investigates how 

 the error, as pressure is reduced, varies with the rate of fall of pressure 

 (when uniform), how the fall of reading at a low stationary pressure 

 increases with the time, depends on the pressure, and varies with the 

 rate of the previous fall of pressure, and how the recovery after a 

 pressure cycle progresses with the time, and is modified by the nature 

 of the previous pressure changes. The influence of subsidiary 

 stoppages during the fall or rise of pressure is investigated, and 

 experiments are discussed showing the influence of temperature on the 

 various phenomena. 



Some of the aneroids employed for the special experiments having 

 been under observation for nearly three years, the opportunity is 

 taken of considering the secular change of zero, and also changes in 

 the elastic and the after-effect properties. 



Algebraic and exponential formulae are obtained by trial for such 

 phenomena as the variation of the differences of the descending and 

 ascending readings throughout a pressure cycle, the dependence of 

 the sum of such differences on the range, the fall of reading at the 

 lowest pressure and the final recovery. A theory, to some extent 

 empirical, is built up, leading to mathematical results, depending on 

 only three arbitrary constants, for the behaviour of an aneroid in the 

 ordinary Kew test over any range. One of these constants varies 

 with the aneroid, but is determined by the observed value of such a 

 quantity as the sum of the differences of the descending and ascending 



