BRITISH ISLES FOR THE EPOCH JANUARY 1, 1915. 
5 
observe 13 points in the Hebrides, 2 in the Isle of Man, and 6 in the Channel Isles. 
In any case those points are somewhat Inaccessible, and in war time cannot be 
completed without serious trouble which can hardly be justified. Accordingly it 
has been decided to reduce the survey with the results so far obtained. 
Having now given a brief sketch of the general field work of the survey we 
proceed to detailed consideration of the observations. 
It is first desirable to say that the survey has been carried out under the auspices 
of a Committee representing the Royal Society, the Ordnance Survey Office, and the 
British Association, as these bodies have financed the work. 
My personal thanks are due to the Committee for providing a motor car for tiie 
work. This has greatly facilitated rapid progress, and the instruments were thus 
exposed to a minimum of disturbance in travelling from place to place. Acknowledg¬ 
ment is due to— 
(1) H.M. King George V. for permission to observe at Windsor Castle; the 
Cambridge Observatory for a base station, and proprietors throughout the 
country for access to private grounds ; 
(2) The Admiralty for the use of marine chronometers ; 
(3) The Astronomer Royal for the vast amount of magnetic data required for 
reducing the field results to epoch ; 
(4) The police authorities for helpful protection since the war began; 
(5) The Postmaster-General for permission to check chronometers at post-offices 
where the Greenwich 10 a.m. signal was received. 
The normal procedure at each station was to set up the tripod and centre the 
Unifilar exactly over the brass stud imbedded in cement which marks the station. 
The bearing of the reference object was then determined ])y setting the vertical 
cross-wire of the telescope on the R.O. and taking the circle reading. 
The magnetic meridian was then found by the readings of the circle when the 
vertical cross-wire of the telescope was on the centre of the scale of the collimator, 
magnet. Two readings only were made: (l) magnet scale erect; (2) magnet scale 
inverted. 
It is very gratifying in this connexion to be able to say that the silk suspension 
with which I started from Cambridge in March, 1914, has been carried without a 
break throughout the whole survey. It has thus travelled safely over 11,000 miles. 
The Horizontal Force was next determined by making (l) a vibration experiment 
in the manner used at Kew; (2) a deflexion experiment using one deflexion distance 
25 cm. in four positions. The setting was not made by eye, but by placing the 
magnet carriage against fixed geometrical stops on the bar. Here also the same silk 
suspension in the deflexion experiment was used throughout the survey. 
In the determination of Inclination I did not consider it any advantage to make 
32 readings of the azimuth circle to get the magnetic meridian. I accepted the 
