AT EEW OBSERVATORY AND THEIR DISCUSSION. 



463 



where t represents as before the time elapsed in minutes since the return to atmos- 

 pheric pressure. 



TABLE XVII. Law of liecovery, Observed and Calculated Values of D/D . 



The agreement is certainly closer than I anticipated from the use of any formula 

 with only two arbitrary constants. Of course I was led to try '369 in the index 

 of (9) from having already found + '369 in the index of (8) ; but this does not affect 

 the significance of the fact that formulae with the same numerical index should so 

 closely reproduce the depression and recovery phenomena. 



The expectation that (9) would apply to all aneroids treated as the 4 were, could, 

 of course, only be justified by very wide experiment. As will be shown later, the law 

 of recovery certainly depends on the rate and type of the pressure changes. 



Effects of Temperature. 



22. Aneroids are usually " corrected for temperature," i.e., there is a compensating 

 arrangement to prevent the reading altering when the instrument is exposed to 

 varying temperature at ordinary atmospheric pressure. The experimental aneroids, 

 tried as usual at three temperatures at atmospheric pressure, appeared correctly 

 compensated. It would appear, however, from the special experiments, that this is 

 not incompatible with imperfect compensation at lower pressures. 



Temperature is likely to influence several parts of the mechanism, and that 

 possibly in more than one way. It presumably alters the elasticity of the vacuum 

 box and iron spring, and causes slight changes in the dimensions of the apparatus. 

 The compensation works by producing slight curvature in a lever, but as the position 

 of the lever varies with the pressure, a curvature that suffices at one pressure may 

 be insufficient at another. Again, temperature might influence the after-effect 

 phenomena, and so modify the reading in a variety of ways. 



I thus investigated the influence of temperature on the readings with pressure 

 descending, on the fall at the lowest pressure, and on the differences of the descending 

 and ascending readings. The experiments bearing most directly on the question 

 consisted of a group, Nos. 51 to 55, made on five consecutive days, May 10 to 14, 

 1897. No. 51 was preliminary, and was not utilised in the calculations. Of the 



