488 DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



During the stoppage for a constant interval termed 10 minutes for brevity at 

 the lowest pressure, the creep augments by kmp, where m is independent of the 

 aneroid. 



During the rise of pressure, matters are more complicated. As pressure rises from 

 30 p -j- r 1 to 30 p + r inches, it is assumed that in virtue of the rise of 

 pressure above the lowest point the creep will diminish to the extent U (r I + r)/2, 

 or Td (2r l)/2, where I is constant in the same sense that m is. On the other hand, 

 the creep tends to increase in virtue of the pressure being still below 30 inches. 

 During the 5 minutes ascribe to the pressure its mean value 30 p + r \. The 

 length of time elapsed from the beginning of this 5 minutes to the beginning of the 

 corresponding 5 minutes during the descent is 5r + 10 + 5 (r 1), or 5 (2r -j- 1) 

 minutes. Then what I assume is that the creep downwards in this 5 minutes bears 

 to the creep in the corresponding 5 minutes in the descent the same ratio that the 

 creep in the interval from 5 (2r + 1) to 5 (2r 4- 2) minutes after pressure becomes 

 steady bears to the creep in the first 5 minutes after pressure becomes steady. This 

 does not absolutely assume the creep to be the same in magnitude when pressure is 

 changing as when it is steady ; but it is, of course, considerably speculative. 



In the ordinary Kew test, the time occupied by the experiment is not in excess of 

 the duration of the steady pressure during the special experiments Nos. 33, 34, 35, 

 used in constructing Table XIV. In the following calculations I have employed, not 

 the direct results of these experiments, but the values supplied by the formula based 

 on them. 



43. An example of the calculations will serve, I hope, to illustrate the process. 

 Take the cycle 30-24-30 inches, supposed of the normal type. 



As pressure falls the creep augments by '5k between 30 and 29 inches, by 

 (1 + 2) k/2 or l~5k between 29 and 28 inches, and so on. Thus the accumulated 

 creep amounts to '5k at 29, 2k at 28, 4'5& at 27, and so on. 



During the stoppage at 24 inches, the creep augments by Gmk. As pressure rises 

 from 24 to 25 inches we regard it as 24'5, or 5 '5 inches below 30. Also the 5 minutes 

 occupied by the rise represents the interval 15 to 20 minutes elapsed since pressure 

 in its descent entered the stage 25 to 24 inches. 



Now, by Table XIV., 



fall in interval 15-20 minutes _ 10 _ 

 fall in interval 0-5 minutes 57 ~ 



Thus, for the increase of creep during this 5 minutes, we take '18 X 5'5k, or I'Ok, 

 as against a decrease of '5^. 



The following scheme shows the magnitude of the creep as the pressure attains 

 the values specified in the first column. 



The common factor k is to be understood. 



