SURVEY OP THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 
57 
Dip Circle by Dover, No. 74. These instruments were also used by Professor Rucker 
in his portion of the surveys of England, Wales, and Ireland. The instruments 
employed by Dr. Thorpe in these countries are the property of the Science and Art 
Department. They are of the same patterns and by the same makers ; the Magneto¬ 
meter and Dip Circle are numbered 61 and 86 respectively. 
Arrangements have been made for placing this latter set of instruments in tlie 
Collection of Scientific Apparatus at South Kensington. They will be kept for 
surveying only, and will'be available for comparison with instruments which may be 
used by future observers. 
Base Station. 
Our base station is the Kew Observatory. Both sets of instruments were tested 
there before they came into our possession, and we have also made numerous com¬ 
parisons with the Kew instruments during the progress of the survey. We take this 
opportunity of expressing the great obligation we are under to Mr. G. M. Whipple, 
the Superintendent, and to Mr. T. Baker, the First Assistant in the Observatory, 
who, with the approval of tlie Kew Committee, rendered us every assistance in their 
power. Our frequent visits and requests for information as to diurnal variation and 
disturbance have imposed much' additional labour on these gentlemen. The help we 
requh’ed has, however, invariably been given with a readiness and heartiness which 
merit our grateful acknowledgments. 
We tested our instruments at Kew in 1884, 1886, and 1887. 
A very large magnetic disturbance set in on March 30, 1886, the 31st was also 
much disturbed and the effects were still to be distinguished on April 1. We have, 
therefore, neglected the observations made on that day, but, with this exception, the 
following tables contain all the determinations made by us in the Magnetic House. 
The first three columns give the date, instrument, and observer, and that headed S 
the value of the element obtained. 
The numbers given are not corrected either for diurnal variation or disturbance. 
In the column headed K is the corresponding value read off from the curves of the 
continuously recording instruments at Kew, standardised by means of the monthly 
observations wdiich are taken there. 
The differences are given in the next column. They are very nearly constant, but 
are rather larger than we should a juniori have expected. Our own instruments are 
evidently in accord. 
Subtracting the mean difference from the Kew values, we should, if both sets of 
observations were perfect, reproduce our own numbers, so that a comparison between 
columns S and K — /3 gives the error of experiment and comparison which is tabulated 
in the last column. 
MDCCCXC.—A. 
I 
