522 
UR. A. W. RtTCKER AND DR. T. E. THORPE OX A MAGNETIC 
Local and Regional Distdebances deduced from the 1891 Survey. 
Having thus found the terrestrial isomagnetics for January 1, 1891, from the 
observations made in 1890 and 1891, we are able to calculate the Disturbino; Forces at 
the places at which observations were made in those years. This was done exactly 
as described in our previous Memoir (p. 260), and the results aided us in our choice of 
stations for the summer of 1892. The Disturbino’ Forces were then calculated at these 
places also. The results are entered in the first three columns of Table XXV., p. 590, 
together with the corresponding numbers obtained from the first survey exactly as 
they are given in our 1886 Memoir. The Horizontal Component of the Disturbing 
Force is indicated by F^, the angle which it makes with the geographical meridian 
measured positive from north to west is (f>i, and the Vertical Component of the 
Disturbing Force is These furnish the data for a map of the magnetic disturb¬ 
ances @f the British Isles, obtained in the same manner as in our earlier survey, but 
based upon observations made at a much larger number of stations, viz., 677 as 
against 205, of which, too, nearly all were different from those at which we had 
previously observed. It is, therefore, important to determine whether the general 
result of this new and independent investigation confirms the conclusions drawn from 
our earlier work. 
If the comparison is to be made without introducing the coefficient of secular 
change, it can best be carried out by determining whether the two surveys are in 
agreement as to the loci of attraction on the north pole of the needle. 
In Plate 14 of our “ 1886 Memoir,” the districts within which we thought that the 
main loci of attraction probably lie were indicated by shading. Eight such regions 
Avere shoAvn, Avhich may be described as those of— 
(1) The Caledonian Canal; (2) Skye and Mull; (3) the Scotch Coal Field; (4) 
Antrim; (5) Connemara; (6) North Wales; (7) the Palaeozoic Ridge from London 
to AVexford, with a brancli from Reading to Chichester; (8) a line through Lincoln¬ 
shire and Yorkshire from the AAMsh to Cumberland. 
In addition to these, turn peaks or centres of attraction are indicated (a) at Kells in 
Ireland and {,8) near Alelton AIoAA’bray. 
AVe have, therefore, prepared Map No. 4, Avhich proves conclusively that the two 
surveys agree exactly as to the main loci of disturbance. All the results represented 
in the map depend on the second (1891) sinwey only. A fcAv of the stations are the 
same as those at which Ave obserAmd in the 1886 surA-ey, but this is because (for 
reasons given beloAv) the observations have been repeated. The figures give the 
Vertical Disturbing Forces, i.e., the excess of the observed over the calculated Anlues, in 
terms of 0 0001 metric or 0'00001 C.G.S. units. Positive forces, i.e., downward 
attractions on the north pole, are indicated by red, negative forces by blue, figures. 
The arrows point in the directions of the Horizontal Components of the Disturbing 
Forces. 
