RITRYEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1891. 643 
Subtracting the means from the individual values we get the following secondary 
disturbances. 
No. of station. 
71 , 
10. 
V . 
757 
+ 41 
+ .35 
+ 89 
756 
- 4 
+ 8 
+ 29 
755 
-2.3 
-20 
- 6 
758 
-12 
-.32 
-22 
759 
- 1 
+ 8 
-90 
There is thus a regular decrease in the Vertical Force, as the hill is crossed from 
south to north. This is the effect which would be produced if the opposite magnetiza¬ 
tions of the two vertical faces were the efficient causes of the phenomenon. 
The Horizontal Disturbing Forces are very small, and only exceed the limits of 
accurate determination of direction at two stations (757 and 758). At both of these 
they indicate an attraction. The results would probably be best explained by the 
hypothesis that the crystalline mass is more magnetic at some depth below the surface 
than in the exposed parts. 
This is quite possible. Elvanite is not a strongly magnetic rock, and though but 
little stress can be laid on results obtained with a single specimen, a portion brought 
away by Mr. Gray showed no magnetic susceptibility. If the rock a few hundred 
feet underground contains more magnetite, and if the surface of separation between 
the magnetic and non-magnetic parts is rounded or otherwise symmetrically arranged 
about the core, the observed facts could be very easily accounted for. 
We now proceed to investigate cases in which we have found no magnetic effects 
at moderate distances from large masses of trap of much greater thickness than those 
heretofore considered. 
The first locality that we choose for detailed discussion is the basaltic island of 
Ganna, near Skye. We investigated its magnetic properties during our earlier survey. 
The island is about 5 miles long by a mile and a half broad, and we felt certain 
that when the yacht in which we were making observations was 200 yards from the 
eastern shore, sailing almost magnetic south, the compasss was not deflected 1°'5. 
Hence, as at Canna, the normal Horizontal Force is about 0T5 C.G.S. unit, the 
Disturbing Horizontal Force perpendicular to our course cannot have exceeded 
'."'v X 0T5 = 0‘004 O.G.S. unit. 
a I ■: > 
Let us now enquire whether, if the island be regarded as a basaltic mass magnetized 
by the earth’s field, the Horizontal Disturbing Force falls bGow this limit. 
The greatest length is east and west, the highest point is 724 feet above the sea. 
We shall, therefore, certainly exaggerate the effect of the upper surface of the island 
4 /t 2 
