KEPORT OF THE KEW COMMITTEE. 



Iv 



The following is an example of a day's comparison after this method, ex- 

 hibiting an error which has thus been brought to light : — 



It ought to be remarked as necessary to the completeness of the chccli, 

 that the observer should first of all by means of his subsidiary ivory scale fill 

 in column B, and then (meanwhile concealing E from his view) fill in column 

 A from the ordinary tabulation sheets. The correctness of the column A— B 

 should be tested at the Central Observatory. 



Having by this means obtained correct tabulations, the next point is to 

 check the accuracy with which the residual correction has been obtained and 

 applied (see Report for 1867, page 4G). And first, with regard to the method 

 by which it is obtained, the latest practice has been to calculate it for each 

 day separately, making the day begin at 11 a.m. The advantage of this ar- 

 rangement is that each fresli paper, which is always put on between 10 and 

 11 A.M., will have its own residual correction*. The accuracy of calcu- 

 lation of this correction ought to be checked, and such a check may bo 

 devised out of the practice pursued at Kew, of taldng the mean monthly 

 difference between simultaneous readings of the Standard and liarograpli 

 readings corrected. If these differences are taken for each day apart, 

 beginning the day at 11 a.m. and giving each difierence its appropriate sign, 

 then the residual correction may be presumed to bo accurate, when for 

 that day there are as many tninus as j)lus differences. Also, when any 

 such difference exceeds, say, '005 of an inch, the accuracy with which the 



* A special arrangement regarding the residual correction has been made for Sundays 

 and those days on which there are few observations of the Standard Barometer. 



