246 
MR, A. W. RUCKER AND DR. T. E. THORPE ON A MAGNETIC 
The DijDS given l)y this formula for tlie central stations in each district are, in the 
following Table compared with those given in Table VII., p. 245. 
District. 
Inclination at Central Station. 
Difference. 
Mean of Values 
observed in District. 
Calculated. 
I. 
71 8-0 
71 10-3 
-2-3 
11. 
70 19-6 
70 21-2 
-1-6 
III. 
69 3-0 
69 1-8 
+ 1-2 
IV. 
67 46-6 
67 44-5 
+ 0-9 
V. 
.69 59-8 
69 59-3 
+ 0-5 
VI. 
69 24-8 
69 2.5-3 
-0-5 
VII. 
67 41-2 
67 41-2 
0-0 
VIII. 
69 24-3 
69 25-6 
-T3 
IX. 
68 18-2 
68 19-7 
-1-5 
In Plate III. we show* the broken curves obtained by the district lines and also 
the terrestrial isoclinals as represented by the formula. 
The agreement is on the whole satisfactory, but it is possible that a closer approxi¬ 
mation to the true terrestrial lines might have been obtained had we made the 
inclination to the geographical meridian increase rather more rapidly in the west. 
This would have diminished the discrepancy in the south of Ireland. On the other 
hand it would have considerably increased it in the north of Ireland and the central 
districts of Scotland, in which large regional disturbances undoubtedly exist. It does 
not, however, appear to be safe to depart from the rule that the terrestrial lines 
deduced from any survey should give, as nearly as possible, the mean directions of the 
true lines in the district under investigation. This end is better attained by our 
formulae than if the agreement were closer in the south of Ireland. 
Our 67° isoclinal agrees almost exactly with that of M. Moureaux in the more 
easterly parts of the English Channel (see fig. 2, p. 243). Both just cut Dungeness 
and Beachy Head. For the longitude of Falmouth, however, our line is 10 or 12 miles 
to the south of that of M. Moureaux. 
As it is at this point much nearer to the English than the French coast, our result 
is probably the more trustworthy, and this opinion is confirmed by a study of the map 
of the French Survey. The isoclinals drawn by M. Moureaux upon a map on 
Mercator’s projection are curved in the south of France, the convexity being towards 
the north. The curvature becomes less as the Dip increases, and the 67° line is repre¬ 
sented as quite straight. M. Moureaux cannot have much to guide him in drawing 
this line, and a divergence such as that which exists between his line and ours could 
easily be introduced by an apparently trifling error in the estimation of the rate of 
disappearance of the curvature of the lines. Our lines would be slightly concave to 
the north if drawn upon a map on Mercator’s projection. 
