244 
MR. A. W. RI'CKRR AND DR. T. E. THORPE ON A MAGNETIC 
he assumes in the western parts of the English Channel is continued to the Straits of 
Dover, his 17° isogonal and oui own would be in practical accord. 
As regards the considerable deviation between the district and calculated terrestrial 
lines in the North of Scotland, we have no hesitation in adhering to the calculated 
lines as being probably the more correct. The masses of basalt and other igneous 
rocks which occur in the West of Scotland and North of Ireland produce a very 
marked effect on the magnetic elements. The general formula gives the Declination 
at the outlying station at Lerwick almost as accurately as the local district equations 
obtained from Scotland only. The three values are :— 
Observed Declination .... 
^ , 1,11 r Local formula . 
wtilcul&^tGCi by "A - rt 1 
LOeneral lormuJa 
20 29-7 
20 31-3 
20 33-7 
Regarding this station as giving a fixed point it is evident that in drawing the curves 
to it from the South of Scotland, great weight ought to be given to the general law 
which is found to be obeyed wdth accuracy in Great Britain from the English Channel 
to the Tay, and in the South and East of Ireland. The only way of eliminating the 
effect of regional disturbances of such magnitude as these which exist in Scotland, is 
by studying the shape of the terrestrial lines in adjacent districts. Observations to 
the north of Scotland would be very valuable for this purpose, but as it would be 
difficult to obtain them we think that the results of the general formula must be 
accepted as a close approximation to the truth. We shall return to this subject in the 
discussion of local disturbances. 
(2.) The Isoclinal Lines. 
Tlie isoclinals were obtained in a precisely similar manner to that above described. 
The districts were the same, but the positions of the central stations were in some 
cases slightly different, as at a few places Dips had been observed without Declinations 
and vice versa. 
In District VII. the inclusion of the Channel Isles led to rates of change with 
latitude and longitude so widely different from those obtained elsewhere that it was 
thouglit better to omit them. When this was done the coefficients assumed normal 
values. We give in the next Table the latitude and longitude of the central stations, 
and the corresponding values of the Dips, and the rates of change of Inclination per 
degree of latitude and longitude. 
