446 DR. C. CHREB, EXPERIMENTS ON ANEROID BAROMETERS 



The second group of aneroids tested over the range 30-24 inches were all large 

 and of a special make, and so best treated apart. They showed such exceptionally 

 small after-effect that I multiplied the actually-observed mean differences by seven, 

 except in the last two columns, to make the figures similar in size to those given by 

 the first group of 13 aneroids tested over the same range. In this way one sees 

 more clearly that the general laws obeyed by the after-effect are to all appearance 

 independent of its absolute amount. 



In no case were the aneroids read to nearer than O'Ol of an inch, the third 

 significant figure in Table I. being introduced merely in taking arithmetical means. 

 Thus the law of variation throughout the range of the quantity tabulated is 

 difficult to ascertain with great exactness over the shortest ranges, especially when 

 the aneroids are of such exceptional quality as the second group tested between 30 

 and 24 inches. 



When results are derived from a number of aneroids, such disturbing factors as 

 local irregularities of graduation lose their influence ; whereas with one or a small 

 number of aneroids they may affect the smoothness of the results, however numerous 

 be the observations. 



5. A glance at fig. 1 shows a similarity of type in the curves ; but this is some- 

 what obscured by the variety in the pressure ranges and in the lengths of the 

 maximum ordinates. The method of bn'nging this similarity into clearer relief will 

 be most easily followed by taking a particular case, say, that of the range 30-21 

 inches. Here there are 10 differences of descending and ascending readings. Call 

 these, starting from 21 inches and proceeding upwards, d l} d 2 . . . d w . The mean 

 difference is 



and dj/d, d.,/d, &c., are the ratios of the several differences to the mean. Now draw 

 a curve whose ordinates represent the size of these ratios on any convenient scale, 

 while the abscissa represent the corresponding fractions, 0, 'i, % &c., of the range 

 measured from the lowest pressure. The curves obtained in this way for the four 

 longest ranges of Table I. are shown in fig. 2. In a short range the number of 

 points on the curve determined by the data is so small as to leave the shape a little 

 uncertain. Thus to have given together curves for all the data in Table I. would 

 have sacrificed the clearness of detail without securing adequate compensation. It 

 will be observed that the curves in fig. 2 are almost coincident near the central part 

 of the range. At the lower end of the range there is a slight but appreciable 

 difference, the ordinate tending to decrease as the range becomes longer. The 

 closeness of the curves, striking as it is, would, I think, be improved by altering the 

 cycle and the method of treating the results in the following three respects : 



(i.) Making the duration of the stoppage at the lowest point proportional to 

 the length of the range. 



