Lovering on Magnetic Observations at Cambridge. 145 
part of the whole; that is, we have to measure effects, such as 
would be produced by shifting the centre of gravity through the 
one-millionth of an inch. It will be easily understood, from this 
statement, how great must be the effect of a minute disturbance 
of the relative parts of the instrument, or of inequalities in the 
bearing points of the axle; and experience has accordingly shown 
that it is altogether unavailable for the determination of changes 
of long period. 
“The same difficulties, and from the same source, have been 
found to attach to the usual method of observing the magnetic 
inclination, and its changes, however refined the construction of 
the instrument. The sources of error seem, in faci, to be inhe- 
rent in every direct process of determining the third element; and 
it is only by an indirect method that we can hope to evade 
them.* Of this character is the method now proposed. 
“If a soft iron bar, perfectly devoid of magnetic polarity, be held 
in a vertical position, it immediately becomes a temporary magnet 
under the inducing action of the earth’s magnetic force, the lower 
extremity becoming a north pole, and the upper a south pole. 
Accordingly, if a freely suspended horizontal magnet, whose di- 
mensions are small in comparison with those of the bar, be situ- 
ated near, in a plane passing through one of these poles, it will 
be deflected from the magnetic meridian. The deflecting force 
is the induced force of the bar, which may be regarded as 
* Two such indirect methods of determining the inclination have been pro- 
posed in Germany, one by Professors Gauss and Weber, the other by Dr. 
Sartorius von Walterhausen. That now suggested bears a close analogy, 
in principle, to the former of these; it differs from it, however, not only 
in the means employed, but also in the end in view, —the main object of 
the present method being the determination of the inclination-changes. 
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