ON THE MAGNETIC INTENSITY OF THE EARTH. - 63 
Section IJ].—Gerenerat Conciusions. 
In considering the comparative fitness of the three kinds of. 
magnetic lines, those of equal variation, equal dip, and equal 
intensity, to promote a knowledge of the system of terrestrial 
magnetism, the lines of equal intensity have in one leading re- 
spect an advantage over the other two. Viewed under the most 
favourable circumstances and in its simplest aspect, the magnet- 
ism of the earth is still, it must be acknowledged, a highly com- 
plicated subject ; and needs not the additional complication of 
its phenomena being involved with considerations foreign to 
itself. Now the lines of equal dip and equal variation do 
not express simple magnetic relations. The lines of equal dip, 
for example, connect those stations on the earth’s surface where 
the direction of the magnetic attraction forms a certain angle 
with the horizontal plane at the station. But every station has 
its own horizontal plane depending on the direction of gravity, 
which has no known or necessary connexion with magnetism. 
The zero planes thus differing, the equality of dip does not ex- 
press, or necessarily imply, a simple magnetic relation, but has 
reference to the attraction of gravitation as well as to that of 
magnetism. ‘The lines of equal variation express a complex re- 
lation of a similar character. Here also the zero planes change 
with the station; and, the variation being the same at two sta- 
tions, by no means implies parallelism in the direction of the 
needle at them, or any other specific relation whatsoever inde- 
_ pendent of the geographical pole, which pole has no known or 
necessary connexion with magnetism. It is not the same with 
the lines of equal intensity. Whatever may be the sources of 
agnetic attraction, and wherever their situation in space,— 
rhether superficial as regards the earth,—or above or beneath 
its surface—the line of equal intensity expresses the equality of 
their resultant at all those points of the earth’s surface through 
_ which it is drawn, unmixed with any considerations foreign to 
‘magnetism. ‘They are pure magnetical isodynamic lines at the 
urface of the globe; and express a common relationship to the 
purces of magnetical attraction. ‘The instruction they convey 
s therefore more simple, direct and unequivocal than in the 
e of the other two. The eye of the mathematician may dis- 
n the pure magnetic indication through the complex signi- 
ation of the lines of equal variation and dip; but the lines 
of intensity are better suited to convey the system of magnetism 
s indicated by the phenomena to the general apprehension. 
