470 DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



discontinuities is not known. In some instances they may have been brought 

 about by a somewhat too vigorous tapping preparatory to taking a reading. Some- 

 times they occurred during exposure of the aneroids to atmospheric pressure. Unless 

 a discontinuous change were of a size considerably larger than '01 of an inch it 

 would be difficult to demonstrate its existence. It is quite possible, in fact, that all 

 the changes in the experimental aneroids were discontinuous. There is certainly 

 no clear indication of a general tendency in the zeros of these aneroids to alter in a 

 definite direction with age. The fact that discontinuous changes may not unlikely 

 arise from time to time in the zero of. an aneroid, shows the absolute necessity of 

 periodic comparisons with a mercury barometer. 



27. Apart from shift of zero, it is conceivable that secular change might occur in 

 at least two ways. There might exist something equivalent to gradual alteration 

 of elastic moduli, whereby changes of reading answering to definite pressure changes 

 would either increase or diminish. Quite independently, there might be change in 

 the qualities whereon depend the elastic after-effect, manifesting itself in the 

 alteration of such quantities as the sum of the differences of the descending and 

 ascending readings in a given pressure cycle. 



To examine into the first possibility I took note of the size of the corrected errors 

 (see 22) in the descending readings of a number of the experiments. The earliest 

 24 experiments were dealt with first. Aneroids Nos. 2 and 3 were used in 16 of 

 these experiments. In the earlier 8 the pressure 24 inches was reached 5 times, 

 the pressure 21 inches only twice ; the later 8 experiments differed only in that the 

 pressure 24 inches was reached no more than 4 times. Subtracting the mean 

 corrected error at each inch of pressure in the earlier group from the corresponding 

 mean error in the later group, we obtain the algebraical increase in the error (or 

 rise in the reading) during the interval. 



Aneroid No. 4 was exposed to the whole 24 experiments. In the earlier 12 the 

 pressure 24 inches was reached 9 times, 21 inches 6 times, 18 inches 4 times, and 

 15 inches twice ; the later 12 experiments differed only in that the pressure 24 

 inches was reached but 8 times. The mean corrected errors in the two groups 

 of experiments were dealt with exactly like the corresponding errors in aneroids 

 Nos. 2 and 3. Aneroid No. 1, being so much more erratic than the others, is not 

 discussed. The data obtained from the other three aneroids are recorded in 

 Table XXV. 



