AT KEW OBSERVATORY AND THEIR DISCUSSION. 



487 



Speaking generally, the reading of the aneroids tended to fall or to rise during the 

 subsidiary stoppage according as the stationary pressure was below or above 

 26 inches ; but unless it was below 25 or above 27 inches such change of reading 

 was extremely small. 



The influence of the stoppage in these experiments on the readings during the 

 subsequent rise of pressure, can be traced with nicety, because the part of the cycle 

 preceding the subsidiary stoppage was strictly of the normal type, and so could be 

 utilised at once in conjunction with complete normal experiments to fix the standard. 

 In this way I calculated the percentage increment produced by the subsidiary 

 stoppage in the sum of the differences of the descending and ascending readings at 

 points further up the scale. The results appearing in Table XLI. are the means for 

 aneroids Nos. 1, 2, 3, and 4 ; + signifies an increase, a diminution. The subsidiaiy 

 stoppage lasted 24 hours when it occurred at 24 or 26 inches ; in the other cases it 

 lasted only one hour. 



TABLE XLT. Influence of Stoppage during Ascent of Pressure. Percentage Change 

 of Sum of Differences (Descending less Ascending Readings) at Higher Pressures. 



A slight rise of reading during a prolonged stoppage is not incompatible with an 

 increase in the sum of the differences of the descending and ascending readings at 

 higher pressures. For, as we have already seen, there is reason to believe that a 

 stoppage tends to make subsequent recovery slower. 



Theoretical Deductions. 



42. Reasoning from the experimental data I have built up a theory, of a some- 

 what empirical kind it is true, which reproduces satisfactorily the phenomena 

 presented by the normal type of test at Kew Observatory. 



A lowering of pressure is supposed to produce two independent lowerings of an 

 aneroid's reading. One is a perfectly reversible or wholly elastic phenomenon, the 

 other is the source of the after-effect and is termed the creep. It is supposed that 

 during the constant interval called 5 minutes for brevity occupied in the fall of 

 pressure from 30 n to 30 n + 1 inches the magnitude of the creep is 

 k (n + n + l)/2. Here k is constant, for a particular aneroid, so long at least as the 

 temperature is unchanged. During the fall of pressure to. say, 30 p inches, the 

 accumulated creep thus amounts to 



