78 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
allowing him special facilities and enabling him to obtain the requisite time for 
carrying out his part of the research. 
PART I.—THE MAGNETIC OBSERVATIONS. 
By G. W. Walker, F.R.S. 
The observational part of the detailed magnetic survey recommended by the Iron- 
ore Committee was carried out in August and September, 1917. Observations were 
obtained at five points round Irthlingborough, and at ten points within the triangle 
of which Melton Mowbray, Loughborough, and Nottingham form the corners ; while 
at Melton and Loughborough (which were stations in the General Magnetic Survey 
of 1914-15) observations were repeated as a control. 
We are under great obligations to the Director-General of the Ordnance Survey 
for sending his chief observer to mark and fix the stations, and to the Astronomer 
Royal for control magnetic data from Greenwich. 
The field data are entered in the observation books of the General Magnetic 
Survey, wliich are preserved by the Royal Society for future reference. The 
procedure of reduction has been exactly the same as in the General Survey, and the 
disturbing forces were obtained by deducting from the observed values, the magnetic 
forces which characterise the British Isles as a whole. The latter in Leicestershire 
are approximately. North = 17,200y, West = 4,850y, Vertical = 44,000y. 
The unit of measurement is denoted by “ y,” which is O’OOOOl (or 10"®) of a Gauss 
or dyne. It should be kept in mind that in field work (even under the favourable 
conditions afforded by a limited area) we cannot depend on the horizontal component 
being correct to nearer than lOy or the vertical component to nearer than 30y. I 
quite believe the errors are often less, but we cannot be sure by this method of 
observation. 
The datum plane from which distimbances are reckoned is by no means precisely 
known, and in any case the disturbances may contain “ district ” effect, by which I 
mean effect common to an area comprising perhaps several counties as distinct from 
effects only parochial in extent. For exploring purposes it is thus convenient to 
consider “relative ” disturbances of stations, and these are obtained by deducting the 
mean effect for the stations concerned. I have calculated these and charted them, 
but I wish to make clear that this is merely an artifice intended to give a clue to the 
cause of the disturbances. 
The disturbing forces are tabulated in two sets, viz. : (l) Irthlingborough series, 
and (2) Melton series. 
(1) The Irthlinghorough Series .—The points (A to E) were chosen to secure as 
much variation as seemed possible, and Mr. Falcon, manager at Irthlingborough, 
gave me great help in choosing them. E was to be near the edge of the bed, A and 
