618 
MR. A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
and this centre dominates the district as far south as Kinloch Aline on the Sound 
of Mull. 
The dh’ections of the Horizontal Disturbing Force at Salen and Loch Nevis suggest 
a westerly extension of this region. However this may be, the ridge line, as deduced 
from the Vertical Forces, passes through Kinloch Aline, and thence, through Scarnish, 
westwards.'^ It would be impossible to trace this lilie in detail across the basalts of 
the north of Mull, and it is probable that we know relatively little of the magnetic 
characteristics of the district (largely composed of sea) which lies between, and to 
the west of, the south of Skye and the south of Mull. There is, however, very strong 
evidence in favour of the existence of an important peak under the sea to the south 
of the Hebrides. 
We pointed out that this was probable in our previous paper (“ 1890 Memoir,” 
pp. 300 and 302). 
Loch* Boisdale was at that time our most southerly station in the Hebrides, and the 
Vertical Force at that place was very low. If our surmise were correct, the Vertical 
Forces ought to increase, and the Horizontal Disturbing Forces to point more directly 
south as we pass southwards from Loch Boisdale towards the assumed centre of 
disturbance. Dr. Thorpe, therefore, made observations at Castle Bay, Barra, and on 
Bernera, the most southerly of the Hebridean group. Our forecast was completely 
justified. The Vertical Disturbing Force increased (algebraically) from — 0'0688 
metric unit at Loch Boisdale to — 0‘0209 at Barra and + 0’0376 at Bernera, while, at 
the latter place, the direction of the Horizontal Disturbing Force is due south. A brief 
account of this result has already been given to the Boyal Society (‘Proc. Boy. Soc.,’ 
vol. 47, 1890, p. 443). The Horizontal Disturbing Forces at four stations on Islay 
and Jura point to tlie west, and it may well be that this is in fact due to the influence 
of the attractive centre soutli of Bernera, That there is also a more local centre is 
now however proved by the fact that the Horizontal Disturbing Force in the neigh¬ 
bouring island of Oronsay acts in a southerly direction. 
Loch Gair and Loch Melfort are stations of minimum Vertical Disturbing Force, and 
the line which joins them partially separates the regions dominated by the Mull ridge 
line and by the Islay centre respectively. As so much of the intervening space is covered 
by the sea, it is difficult to settle the exact relations between these, but it is noticeable 
that the peak, which must be somewhere between Islay and Oronsay, is almost in the 
prolongation of the line of the Great Glen. 
Districts 2 and 3. —Cape Wrath and Loch Broome. 
On the north-west of the Great Glen District there are two sub-districts which 
may be named after Cape Wrath and Loch Broome respectively. 
In the first of these there are two stations of high Vertical Force at Loch Eriboll 
We neglect the observation at Ganna as that island is the seat of violent local disturbances. 
