AT KEW OBSERVATORY AND THEIR DISCUSSION. 



477 



90 minutes in the other, though naturally showing conspicuous irregularities, points 

 to the same general conclusion. It is clear that the creep, or fall of reading, at the 

 lowest point is largely dependent on the rate of the previous change of pressure. A 

 slow rate of change affords opportunity for simultaneous creep, but at the same time 

 it reduces the tendency to creep at lower pressures. 



If we may judge from Table XXXII., supposing the time taken to produce a given 

 fall of pressure to be increased in a given proportion, then the time required for the 

 subsequent creep to attain a certain value is likewise increased in the same or nearly 

 the same proportion. If the proportion were exactly the same in the two cases one 

 would hardly, I think, expect to meet with the differences in the corrected errors 

 shown by Tables XXIX., XXX... and XXXI. 



32. In treating of the influence of the rate of change of pressure on the differ- 

 ences between the descending and ascending readings, I shall consider first the final 

 group of experiments Nos. 71 to 75. The following Table XXXIII. shows the ratio 

 of the mean difference of the descending and ascending readings at each point of 

 the range from the two experiments at the slower rate (1 inch in 45 minutes) to 

 the corresponding mean from the three experiments at the faster rate (1 inch in 

 5 minutes). 



TABLE XXXIII. Ratios Differences Descending less Ascending Headings, 



slower : faster. 



The mean of the means is I "04. 



We can hardly avoid drawing the conclusion that, provided all the time intervals 

 in the cycle be altered in the same proportion, the sum of the differences between the 

 descending and ascending readings, and the law of variation of the differences 

 throughout the range, are either absolutely unaffected or very nearly so. The 

 departure of the mean ratio in Table XXXIII. from unity is mainly due to experiment 

 No. 72. The interval allowed for recovery after the immediately preceding 

 experiment was possibly insufficient. The agreement between experiments 71 and 

 73 and between experiments 74 and 75 could hardly have been improved. In the 



