8 
MR. A. W. RUCKER ARE DR. T. E. THORPE OK A MAGXETIC 
Base Station, 
As before, oiii’ base station is the Kew Observatory, and we must once more 
ex])ress our great obligations to the Kew Committee of the lioyal Society, to the late 
Mr. G. M. WiiirPLE, the Superintendent, and to Mr. T. Baker, the first Assistant 
in the Observatory, for their cordial co-operation in our work. As a greater number 
of observers and more instruments were employed than in our original survey, the 
demands we were compelled to make upon the time and attention of the officers of 
the Observatory were much larger than during the progress of our earlier work. 
The instruments used in the 1886 survey "were found at that time to be in close 
accord with each other, though the results differed to an appreciable extent from 
those given by the standard instruments at Kewc (“Mem.,” 90, pp. 58-59.) 
Our instruments were of recent construction and were themselves in agree¬ 
ment, so that the absolute values given by them were at least as trustworthy as 
those furnished by the older Kew standards. The observations made at the Obser¬ 
vatory were important, irot as comparisons with an absolute standard, but as tests of 
the constancy of the survey instruments during the period of the survey. 
As no change was detected, it was not considered necessary to apply to the 
observations the constant corrections, whicli would have been recjuired to reduce 
them to exact conformity with Kew. 
Thus on the average in our earlier survey ;— 
(1.) The Declinations were 2''5 less. 
(2.) The Horizontal Forces were 0'0029 metric or 0’00029 C.G.S. unit greater ; and 
(3.) The Dips were 2''7 less* than if they had been measured rvith the Kew 
instruments. 
We have, therefore, had to consider whether it was desirable to adhere to this 
Survey Standard. 
Evidence is accumulating tliat the differences between the results given by so-called 
absolute instruments for the measurement of the magnetic elements are considerable. 
Our own experience shows that instruments of the same pattern are not in exact 
accord. They differ from each other and they differ from Kew. 
Dr. Van PtUCKEVORSEL has recently compared the absolute instruments at several 
of the chief European magnetic observatories by carrying the same magnetometer 
from place to place. He found that the differences between them far exceeded mere 
errors of observation. 
An inquiry into the cause of these discrejiancies is desirable, but it was not 
essential for our immediate purpose, provided that our instruments were frequently 
compared. 
We have, therefore, been obliged to content ourselves with recording a few 
* In the case of the dip this apparent diti'erence has since been aceouuted for, see p. ‘23 beloNV. 
