INTRODUCTION AND RESULTS. 
255 
sions “ line of no Inclination ” and “ position or point where the Inclination is 90°, ’’ 
to the more technical designations of “ magnetic equator ” and “ magnetic pole,” be- 
cause I have noticed that, since it has been known that the locality of the dip of 90° 
is not that of the maximum of Force, and that the line of no Inclination is not that of 
least intensity, an ambiguity has prevailed in the use of the technical expressions, 
which has frequently been prejudicial to a clear understanding of the passages in 
which those terms are employed. It would not be difficult to show, in the recent 
writings even of persons actively engaged in the advancement of magnetical science, 
instances in which the term “magnetic pole” is used, where it can have no other 
meaning than that of a point of greatest intensity ; and other instances in which the 
meaning is doubtful in itself, and is not cleared by the context. The line of no dip 
is also frequently confounded with that of least intensity, although they represent 
different phenomena, and occupy different positions on the globe. By some authors 
the terms “ pole of intensity ” and “ pole of verticity ” have been introduced, by which 
ambiguity is indeed avoided ; but, whilst the term “ pole ” is retained, it must be ad- 
mitted that in one of the two instances at least, it is used in a sense which (what- 
ever may have been its earlier signification) is inconsistent with modern definition 
from high authority (that of M. Gauss), and with that general understanding of its 
meaning, which has grown up under the hypothesis, imagined by some natural philo- 
sophers and now known to be erroneous, that the magnetism of the globe is analo- 
gous to that of a sphere of soft iron rendered magnetic by induction. The points of 
maximum of Force have also been called “ foci ” or “ centres of Force,” terms how- 
ever which have not been suffered to pass without objection. In papers strictly 
limited as these Contributions are to investigations into the facts of terrestrial mag- 
netism, I have thought it preferable to employ expressions, which although they may 
have the real disadvantage of appearing somewhat awkward, especially when they 
come to be frequently repeated, are subject to no ambiguity, and are obviously un- 
connected with any hypothesis. 
In advocating the consideration which, in a general view of the earth’s magnetism, 
appears to be due to the points in each hemisphere which form the centres of the 
systems of isodynamic lines, and are themselves points of greatest Force, — and to the 
circle or curve which encompasses the earth and unites the points in each geogra- 
phical meridian where the magnetic force is weakest, — I am far from desiring an 
undue importance to be attached to them, over the points where the needle is ver- 
tical (or its inclination is 90°,) and over the line where the direction of the needle 
is horizontal (or is parallel to the surface of the earth). No general view of terrestrial 
magnetism is just, or is agreeable to our present knowledge, of which the characteristic 
features of both phenomena, both of the Inclination and of the Force, do not form a 
part. In purely magnetical relations indeed, — and remembering that as yet there is 
no connection established between polar and gravitating forces, — the points of the 
earth’s surface, where the action of the magnetic forces is most intense, have, as it 
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