AT KEW OBSERVATORY AND THEIR DISCUSSION. 473 



TABLE XXVIII. Sums of Differences, Descending less Ascending Readings. 



The blanks are due to some uncertainty in the readings of No. 3 on one occasion, 

 and to the later experiments with No. 1 not being comparable with the earlier. 

 Between experiments No. 6 and No. 36 there intervened fully a year, and between 

 No. 36 and No. 56 nearly a second year. 



The table does not, I think, afford any clear proof of a change in the sum of the 

 differences. If any change occurred, the evidence is in favour of its being a 

 decrease. 



In all the experiments utilised in Table XXVIII., the aneroids had been exposed 

 to atmospheric pressure for at least two days previously. This introduces a difference 

 between these experiments and five of the ordinary type, Nos. 52, 53, 54, 55. and 59, 

 which were preceded by experiments over the same range, 30-21 inches, on the pre- 

 vious day. The difference in this respect is probably the chief reason for the relative 

 smallness of the mean sums of the differences of the descending and ascending readings 

 given by these five experiments, viz., '96 for No. 2, 1'43 for No. 3 and '66 for No. 4. 



Influence, of Rate of Change of Pressure. 



30. In ordinary mountain ascents the rate of change of pressure is much slower 

 than 1 inch in 5 minutes ; it is thus important to know how the various phenomena 

 are modified when the rate of change of pressure is reduced. A variety of experi- 

 ments were devoted to this investigation. In eight of these, Nos. 25, 26, 27, 28, 



37, 38, 57, and 58, pressure was altered at the rate of 1 inch in 10 minutes, or 

 half the normal rate, with a stoppage of the normal length, 10 minutes, at the lowest 

 point. In experiments 27 and 28 the pressure range was 30-15 inches, in the other 

 six instances it was 30-21 inches. 



The two pairs of experiments, 37, 38 and 57, 58, were each preceded and followed 

 by an experiment of the normal type over the same range. The experiments 36, 37, 



38, 39 were taken on consecutive days, July 6-9, 1896 ; the experiments 56, 57, 

 VOL. CXCI. A. 3 P 



