SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY I, 1891. 615 
Forces. The valley lines were then transferred without change to Map 13, on which 
the directions of the Horizontal Disturbing Forces are shown. The ridge lines were 
also transferred to Map 13, but were there drawn shifted from the stations through 
which the Vertical Force Ridge passed towards the side to which the Horizontal 
Disturbing Force at each station pointed. 
The Horizontal confirm the Vertical Disturbances if, at the next station on the side 
towards which the ridge line is shifted, the arrow points in the opposite direction to 
that at the station from which it is shifted, a centre of attraction being thus indicated 
between them. 
These ridge lines are drawn freehand in Map 13, and we only profess to have fixed 
their positions as somewhere between the nearest stations on either side. 
The arroAvs show the directions of the Horizontal Disturbing Forces. They are 
coloured red or blue according as the Vertical Force Disturbance is positive or 
neo-ative. When the Horizontal Disturbino- Force is below the limit of the accurate 
determination of direction, its magnitude is given on the map in terms of O’OOOl 
metric unit. 
Magnetic Districts of the United Kingdom. 
1, The Great Glen, or Caledonian Canal District. 
The neighbourhood of the geological fault marked by the great Glen, along which 
runs the Caledonian Canal, is distinguished by high Vertical Forces. That this 
ridge line is prolonged to the north of Inverness, nearly in the line of the Glen, is 
shown by the high Vertical Forces at Wick and at Helmsdale, and by the directions 
of the Horizontal Disturbing Forces at those places, and at Tain, Forres, Dingwall, 
and Inverness,' Both surveys and, we may add, our deductions from Mr. Welsh’s 
Survey of Scotland (1857-58), agree in placing the ridge lines near to, but a little to 
the west of the Great Glen near Ben Nevis (Banavie). All three surveys are 
also in accord as to the fact that further north the liclge line is to the east of the 
Glen {cf. “ 1890 Memoir,” Plate 13). Thus the directions of the Horizontal 
Disturbing Forces at Fort Augustus and Temple (Urquhart) point to the east. 
We believe that the most probable cause of this deviation from the line of the 
Glen is that the ridge is intersected by another which crosses it between Fort 
Augustus and Temple (Urquhart), and connects it on the one hand with the basalts 
of Skye, and on the other with a very definite ridge line, which we had previously 
discovered near Elgin. 
The Vertical Disturbing Forces (see Map 12) are unquestionably in accord with this 
viev/. 
The two lines of maximum Vertical Disturbance are remarkably clear. 
The evidence thus obtained is not weakened by the Horizontal Disturbing Forces, 
but, as we should expect, the ridge lines show a tendency to break up into peaks, and 
in some cases, the physical importance of the intervening valleys is doubtful. 
