AT KEW OBSERVATORY AND THEIR DISCUSSION. 



475 



two rates appears larger in Table XXX. than in Table XXIX. ; this is mainly due, 

 however, to experiment No. 59, and very probably arises from the fact that the time 

 interval between this and the previous experiment was shorter than the average. 



Taking the figures as they stand, we should conclude that the mean reading at 

 21 inches was lowered by only '02 of an inch when the time of reducing the pressure 

 was increased from 45 to 90 minutes. A.t first sight, the smallness of this difference 

 seems truly remarkable, because after lowering the pressure to 21 inches at the 

 normal rate we should, in accordance with Tables IX. and X., have observed a mean 

 fall of '02 of an inch in some 4 minutes. 



The influence of a still slower rate of pressure was tried in a very recent group of 

 experiments, Nos. 71, 72, 73, 74, and 75, carried out on March 16, 18, 22-23, 29-30, 

 and April 1, 1898. Of these, Nos. 71, 72, and 75 may be regarded as of the normal 

 type, except that the stoppage at the lowest pressure, 21 inches, lasted 2 hours. 

 Nos. 73 and 74 differed from these only in that all the pressure intervals were nine 

 times longer. These last experiments lasted of course nearly two days, the length of 

 stoppage at the lowest point being chosen so as to make this feasible without night 

 work. In reality, it was found convenient to reduce the stoppage in experiment 

 No. 74 by 45 minutes, and the corresponding curtailment, 5 minutes, was made in 

 the case of No. 75. I have disregarded this trifling difference, treating Nos. 71, 72, 

 and 75 as identical, and Nos. 73 and 74 as identical. From these experiments I have 

 calculated, and give in Table XXXL, the differences between the mean corrected 

 errors throughout the range for the slower rate (1 inch in 45 minutes) and the faster 

 rate (1 inch in 5 minutes). The results are given to the nearest '01 inch. 



TABLE XXXI. Mean Corrected Error Slower Rate, less Mean Corrected Error 



Faster Rate. 



Here, as in Tables XXIX. and XXX., there is, at least at the lower parts of the 

 range, a lowering of the reading accompanying reduction in the rate of fall of pressure. 

 Also the mean lowering of the reading at the lowest point, '05 of an inch, is notably 



3 P 2 



