444 



DR. C. CHREE, EXPERIMENTS ON ANEROID BAROMETERS 



board was tapped before each reading, this being the invariable practice in the 

 ordinary Observatory test. Though not free from objections, this seems to reproduce 

 most closely natural conditions. 



Differences of Descending and Ascending Readings, from Kew Verifications. 



4. It being most convenient to extract data from books not in current use at the 

 Observatory, I have employed, in the following discussion, data from aneroids tested 

 between 1885 and 1891. I have, however, examined a sufficient number of the more 

 recent data to assure myself that the ordinary aneroid has since then undergone no 

 important modification. On the whole, there is, perhaps, a slight reduction in the 

 average size of the after-effect phenomena. 



Table I. shows the mean excess, in inches, of the descending over the ascending 

 reading at each inch of pressure for the several ranges specified, also the sum of the 

 differences between the descending and ascending readings, and the mean difference 

 for each range. It also mentions the number of instruments on which each set of 

 results is based. The results, with the exception of those from the second group of 

 30 aneroids taken over the range 30-24 inches, are shown graphically in the several 

 curves of 'fig. 1. Abscissae represent air pressures, ordinates the excess of descending 

 over ascending readings. 



Fi. 1. 



! 



7 



7 



15 



16 



18 



ZO Zl ZB 23 24 25 Z6 



Pressures in inches of mercury. 



7 



^9 JO 



