SURVEY OF THE BRITISH ISLES FOR THE EPOCH JANUARY 1, 1886. 319 
towards or away from the nearer and more elevated portions, according to the nature 
of their magnetisation. 
If the slope were less than the angle of Dip, tiie sides of the suh-terrestrial hill 
would be magnetised, so as to attract the north-seeking pole, and the empirical rule 
that the Horizontal Forces tend toward places of greatest Vertical Force would be 
amply and completely explained. 
Dr. Naumann’s argument loses a great part of its force if we regard a widespread 
disturbance of the isogonals around a mountain range as due not to the direct action 
of the mountains but to a far reaching mass of rocks of which they are the 
culminating point. 
Lastly, as to the statement that rocks containing iron only become magnetic when 
brought to the surface—without impugning the actual observations, we can only say 
that so important a generalisation requires a much more extended basis of fact than 
any that is provided for it. If the Malvern granites produce in their immediate 
neighbourhood a magnetic effect, it is evident that rocks, the susceptibility of which 
can only be detected by refined methods, may in lai’ge masses deflect the lines 
of force in the earth’s magnetic field to an appreciable extent. 
On the whole then, while fully admitting that there is room for much further 
investigation on this head, and that any view is more or less hypothetical, we do not 
think that any theory has hitherto been proposed which offers fewer difficulties than 
that of rock magnetism. 
If the cause of the magnetic disturbance is induced magnetism, it would vary with 
the direction and intensity of the earth’s magnetic field, but the change conld hardly 
be great relatively to the secular variations of the elements. It is therefore important 
to note that in the ‘Phil. Trans.’ for 1870 (vol. 160, p. 274), Sir E. Sabine gives the 
Declination at Greenwich and Kew as having been practically identical in 1842'5. 
His values are :— 
Greenwich.23° 13', 
Kew.23° 11'. 
As according to his isogonals the Declination at Kew should, at that epoch have 
been 9' greater than that at Greenwich, whereas, according to his table, it was 2' less, 
the difference due to disturbance was — 11' {i.e., the Kew value was too small), 
I-whereas in 1860 it was + 11', in 1865 -h 10', and is now 10' (p. 270). If then we 
accept Sir E. Sabine’s table as correct, we must assume that a disturbance difference 
amounting in all to 22'was established between 1842'5 and 1860, which has remained 
constant during the twenty-nine years which have elapsed since the latter epoch. 
We, therefore, wrote to Mr. Whipple asking him for information as to the 
observations given by Sabine, as having been made at Kew in 1842'5. It should be 
noted that the authority quoted by Sabine is “ Ob',” which indicates an official origin 
for the value given. 
