2 
MR. GEORGE W. WALKER ON THE MAGNETIC RE-SURVEY OF THE 
re-survey until the time of his death in 1915, yet he was not equal to the exertion 
of writing to me about it. It is thus seemly that I should record the loss to the 
re-survey, and my own sense of personal grief, occasioned by the death of my revered 
teacher. 
The first step taken in 1913 with regard to the work was to procure a suitable 
set of instruments. The instruments used by Rucker and Thorpe were not 
available, and several belonging to the Royal Society could not without great 
inconvenience be obtained. It was only after much delay and trouble that Unifilar 
Elliott, No. 66, and Dip Circle, Dover, No. 86, belonging to the Society, and hitherto 
in use at Falmouth Observatory, were placed at my disposal. I draw attention 
to this in order that like trouble may not occur again. In my opinion, the present 
apparatus, when returned to the Royal Society, should be preserved solely for, or for 
comparison with instruments used in, future magnetic surveys of the British Isles. 
The Unifilar required some overhauling and, as the certificate with it was rather 
old, I decided to have the more important constants re-determined. The inertia bar 
and deflexion bar were sent to the National Physical Laboratory for measure¬ 
ment. I made a series of observations to determine the moment of inertia of the 
Magnet 66a, and special experiments were made to determine the temperature 
coelficient of its magnetic moment. These latter were made in the Cavendish 
Laboratory by a compensation method which I had devised at Eskdalemuir 
Observatory, and I am greatly indebted to Sir Joseph Thomson for giving me the 
special facilities required. 
Concurrently with this a point was selected in a paddock belonging to Cambridge 
Observatory, and a concrete base about 1 metre square and metre deep was 
prepared in the ground. The exact centre is marked permanently on the surface of 
the cement, and the azimuths of two suitable reference objects have been determined 
(l) by myself using the Unifilar 66 for Sun observations; (2) by Mr. Hartley, chief 
assistant at the observatory, from observation of the Pole Star; (3) by the Ordnance 
Survey Department from the Trigonometrical Survey of Great Britain. The agree¬ 
ment was as close as could be desired. Magnetic observations were made on this 
concrete base and compared with values supplied by Greenwich Observatory. The 
results are given in detail later, and while the differences are perhaps not as constant 
as one could wish, the divergences are within the limits of experimental error. 
Accordingly, I have used corrections supplied from the variometers at Greenwich to 
correct my experiments at Cambridge in obtaining the constants of the apparatus. 
But I admit it would have been better if neighbouring variometers had been available. 
This however was not possible. 
It may be recalled that in determining Horizontal Force Rucker and Thorpe used 
two distances, 30 cm. and 40 cm., in the deflexion experiment in the field, and deduced 
the distribution constant P for their magnets by means of tlie field observations. The 
determination of distribution constants requires specially careful experiments and I 
