494 DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



One experiment, at least, was, however, made to ascertain whether the rate of 

 reduction of pressure was of any consequence. Two aneroids were tried, a comparison 

 being made of their readings when pressure was reduced at the two different rates, 

 1 inch in 1.0 and 1 inch in 30 minutes. In one of the aneroids the readings were 

 sensibly lower all through the range at the slower rate ; in the other the change of 

 rate had no certain influence. 



49. No. 3, which has been already referred to, is divided into parts. Part 1, 

 treating of " Comparisons in the Field," enumerates changes of zero and of reading 

 observed in a number of aneroids used at lofty stations, but hardly bears on the 

 present paper. Part 2, " Experiments in the Workshop," gives numerous details as 

 to the creep during several weeks' exposure to low pressures. It also records a 

 considerable number of observations showing the recovery of aneroids on return to 

 atmospheric pressure after a week's exposure to a lower pressure. 



Mr. WHYMPER was apparently unaware of Dr. BALFOUR STEWART'S work, and 

 regarded the creep at low pressures as a new phenomenon. He noticed that the 

 rate of creep diminished as the time increased since the low pressure was reached, and 

 that the recovery on return to atmospheric pressure was most rapid at first. He 

 seems, however, to have made no serious attempt to study more exactly the laws of 

 the phenomena. Judging from some remarks on his p. 25, he was discouraged from 

 doing so by the belief that different aneroids varied very much in the laws they 

 followed. The fall of reading during the first day's exposure to a low pressure might 

 vary, he says, from about one-third"" to more than three-fourths of the fall in the first 

 week. 



For evidence of this he refers to a table on his p. 26, giving particulars of the creep 

 in 29 aneroids during the first day and the first week. The evidence seems to me 

 somewhat inconclusive. In the first place, there seems no information as to whether 

 the original lowering of pressure was carried out at an invariable rate. Thus it is 

 not clear whether an exact comparison is possible between the aneroids which were 

 exposed to different pressures. Those exposed to the same pressure were presumably 

 exposed to the same conditions, and in their case there are no variations at all 

 approaching those mentioned by Mr. WHYMPER. In fact, I notice only two instances 

 in the whole table in which the creep in the first day was less than the half of that 

 in the first week ; and in both these cases there was no companion aneroid tried at 

 the same pressure. In the first instance a 3-inch Watkin aneroid, exposed to a 

 pressure of 24 inches, is credited with a creep of '111 inch in a day and '273 inch in 

 a week ; in the second instance, that of a 4|--inch Watkin aneroid, the creeps were 

 052 inch in a day and '144 inch in a week. The presumption is that the aneroids 

 were only read to '01, or at most to '005, of an inch, the third decimal coming from 

 the reading of the mercury barometer ; in the second instance mentioned above, 

 a trifling error in the reading would make a large difference in the result. 

 * A footnote says " less than a fifth. " in some exceptional cases not quoted. 



