460 DR. C. CHREB, EXPERIMENTS ON ANEROID BAROMETERS 



TABLE XIV. Law of Fall of Reading under Constant Pressure. 



After the first 6 minutes the agreement between the calculated and observed 

 values could hardly be better. The observed values at 2 and 4 minutes are con- 

 spicuously lower than the calculated, but this discrepancy is least in the case of the 

 24 experiments, which, on account of their much greater number, should give a more 

 reliable mean than the 3 special experiments Taking either set of figures, however, 

 one would most naturally draw the conclusion that the formula (8) does not apply 

 exactly when t is small. Another explanation is, however, possible. On an average, 

 an aneroid's reading at two successive intervals would be taken as unchanged if the 

 true alteration were less than - 005 of an inch, and this unquestionably was likely to 

 happen frequently in the case of the readings at t = and t = 2. That the falls 

 observed, during at least this interval, suffered from this or some similar cause, can 

 hardly be doubted, for it is most improbable, as would appear from the 3 special 

 experiments, that the fall in the first 2 minutes should be less than the fall in the 

 second 2. Again, as all the operations, such as the reading of the aneroids and 

 gauge, occupied some time, it is possible that the method of procedure adopted really 

 tended to curtail the first 2-minute interval. It is certainly a little suggestive that 

 the values calculated for t = 2, 4, 6, are so nearly identical with those observed in 

 the first 24 experiments at t = 4, 6, 8, respectively. 



19. In connection with 16 to 18 several considerations should be borne in 

 mind. The change of pressure prior to the stationary stage is gradual. The 

 circumstances are not analogous to that of a wire suddenly loaded. Thus, the fact 

 that the fall of reading in a given time is proportional to the pressure range can 

 hardly be regarded as merely a special case of the ordinary after-effect law. As we 

 shall see very clearly later, the phenomenon is largely dependent on the way in 

 which the pressure is reduced. A preliminary stoppage on the way down, or a 

 change in the rate of reduction of pressure, each exert an important influence ; the 

 one affecting the magnitude of the fall, the other its law of variation with the time. 



