150 
FIFTH REPORT- 1835. 
III. Isodynamic and Isoclinal Lines. 
On a review of the preceding results of observation, it will be 
seen that they exhibit much irregularity. The errors of obser¬ 
vation (in which we are to include the effects of the unsteadi¬ 
ness of the magnetic state of the needles employed, as well as 
the various other uncertainties arising from the imperfections 
of our methods of observing,) have, of course, their share in 
these discrepancies ; but they are by no means sufficient to ex¬ 
plain the whole. The action of the earth on the magnetic needle 
is itself subject to irregularities, temporary as well as local: and 
it is to these that the observed anomalies must, in great part, be 
ascribed. Of the variations of the former kind we have already 
spoken. The direction and intensity of the terrestrial magnetic 
force, at a given place, are subject to fluctuations, or irregular 
oscillations round their mean state, the cause of which is as yet 
little understood; and it is only by means of simultaneous ob¬ 
servations, made at some fixed station within the limits of the 
district through which these effects take place, that we can hope 
to ascertain their amount, and to correct for them. Of the local 
disturbing causes some are sufficiently obvious. Thus the needle 
is in general affected by the vicinity of basaltic rocks, owing to 
the quantity of iron they contain ; and instances have been ob¬ 
served in which these rocks were even found to possess magnetic 
polarity # . But there seem to be grounds for believing that dis¬ 
turbing actions of a local nature are exerted on a much larger 
scale. Whether the earth’s magnetic force be an inherent pro¬ 
perty, and the resultant of the forces of all its parts, or whether 
it be simply the effect of thermo-electric currents produced by 
the heating action of the sun, the result must in either case be 
greatly modified by the configuration of a country, and by the 
nature of its superficial strata. If this view be just, the greatest 
irregularities should prevail in those parts of the earth in which 
the uniformity of surface is broken by hill and valley, and where 
the strata have been rent and contorted by the uplifting of moun¬ 
tain chains. In Ireland, according^, we should expect to find 
much greater anomalies in the direction and intensity of the 
magnetic force than in the plains of central Europe; and it 
must be, consequently, in the same degree more difficult to 
arrive at general results. 
* A remarkable case of this kind has been noticed at Fair Head, on the 
north coast of Ireland. The magnetic polarity of one of the columns which 
compose this wonderful fafade is said to be so strong as to invert the position 
of the compass needle, when the poles of the same name are made to approach. 
