300 
MR. A. W. RtiCKER AND DR. T. E. THORPE ON A MAGNETIC 
distorted (Plate V.), and indicate a centre of attraction between the two. A.11 such 
points are marked D. 
Loops in the Vertical Force isomagnetics indicate maxima and minima of Vertical 
Force, which are marked V and v respectively. As no observations could be made to 
the west of the islands in the south-west of Scotland, it is impossible to prove formally 
that the Vertical Forces in this district are maxima. The values at Scarnish and 
Iona are, however, the same as those at Wick and Golspie respectively, which are 
nearly two degrees farther north. We have, therefore, felt justified in marking them 
as maxima, thereby indicating that they are very large. Ganna and Portree are so 
disturbed that but little reliance can be placed upon observations taken there, but 
they both give maximum values of the Vertical Force (Plate VIII). At Corpach and 
Glenmorven we find that Mr. Welsh’s observation gave maximum values of the Ver¬ 
tical Force. At two of the most northern stations the Horizontal Force is a minimum ; 
at three of the most southern it is a maximum (Plate VIII.). They are marked with h 
and H respectively. 
From this map we can form an opinion as to the magnetic constitution of the 
district. Points of high Vertical Force, and centres to which the needle is attracted, 
cluster thickly along a line which passes from Elgin to Inverness, thence along the 
line of the Caledoiiian Canal to Corpach, and so to Mull. 
Another similar line runs north through Skye, and indications of a region of low 
Vertical Force, which separates the two, are given in the minimum values found at 
Gairloch and Loch Inver. There is a subsidiary centre of attraction near Strachur. 
The view that a general attraction is exerted toward the centre of the district is 
supported by the occurrence of maximum and minimum values of the Horizontal 
Force in the south and north respectively. The minima at Loch Boisdale and Soa 
and the maximum at Loch Maddy (which necessarily follows) indicate a strong 
subsidiary centre of attraction to the south of these places. Balfour Stewart, 
arguing from Welsh’s results, placed such a centre to the south of Mull, but as we 
shall show, it is probably to the west of that island (‘ Brit. Assoc. Beport,’ 1859? 
p. 190). 
It must be remembered that all these conclusions follow from the mere inspection 
of the results of the observation with no more calculation than is necessary to reduce 
them to the same epoch. In the next map (fig. 25) we show the results of the calcu¬ 
lations carried as far as possible, i.e., to the point of deducing the disturbing forces. 
The shaded parts are the regions of positive [i.e., of great) Vertical Force. The 
boundaries are fixed by the very rough method of assuming that between neighbour¬ 
ing stations the rate of change of the disturbing force is uniform. The arrows repre¬ 
sent the Horizontal disturbing Forces in magnitude and direction. The dotted arrows 
represent the disturbing forces calculated from Mr. Welsh’s survey. 
Tliere are only two or three discrepancies, and they all occur, as we anticipated, near 
valley lines. The most remarkable is Stornoway. There can be no doubt as to the 
