260 
MR. A. W. RUCKER AND DR. T. E. THORPE ON" A MAGNETIC 
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Four most northerly stations.14‘4 W. 
Intermediate group of eight stations . . 6’0 E, 
Six most southerly stations.3'0 W. 
A glance at the isogonals (Plate II.) in this part of the kingdom, is sufficient to show 
thar ic is impossible to believe that the real terrestrial curve not only crosses and 
recro.sses that which we have drawn between Plymouth and Holyhead (which is quite 
possible), but that the amplitude of the oscillation amounts to more than 20' of 
Declination, or to about 45' of longitude. If this were so, the terrestrial would be 
nearly as sinuous as the true curves [cf. Plate V.). 
It is also to be observed, that even if there is a slight tendency of the kind supposed, 
it will be partly corrected by the first method. The objection to that is, that too 
much stress may be laid on the peculiarities of individual stations—to this, that 
too great weight may be given to the characteristics of districts. 
If the two methods point to the same conclusion the two criticisms are mutually 
destructive. 
A more serious objection is, that the indications are, at times, somewhat ambiguous. 
An increase in the Declination may be due to a small force acting at right angles to 
the magnetic meridian, or to a large one acting nearly parallel to it. The value of 
the disturbance of the Horizontal Force will often decide to which of these the effect 
is due, but nevertheless, it is not easy to deduce definite conclusions from a mere 
inspection of the maps. This consideration has, therefore, led us to supplement this 
method by another. 
(3.) In this third method, we calculate the magnitude and direction of the 
disturbing force at each station. If 8 be the observed, and the calculated value of 
the Declination, if a similar notation be used for the other elements, and if N, W, and 
Z be the northerly, westerly, and vertical components of the disturbing force, we have 
N = H(; cos 8^ — H cos 8, 
W = He sin 8e — H sin 8, 
Z = He tan Oc — H tan 0, 
whence the magnitude and direction of the disturbing force are known. Obviously the 
danger to be feared, in this case, is that the differences with which we deal are too 
small to give trustworthy results. The calculation must, however, lead to conclusions 
compatible with those to which the study of the disturbances points, and even if the 
results are only approximate, they are valuable as indicating, in a way which is more 
easily interpreted, the direction and magnitude of the forces under investigation. 
Having thus described the general course which we propose to adopt, we shall now 
consider the methods more in detail. After that we shall apply them to certain dis¬ 
tricts, of which we have made a special study, and then to the whole area of the survey. 
In attempting to discover the order which underlies the apparent irregularity of 
the disturbances, it is necessary to proceed on a rvorking hypothesis, and we shall 
