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order. Thus eight separate observations are obtained, of 
which the average gives the deflection due to any given 
distance. 
The only further observation required for finding the 
earth’s horizontal intensity is that of the number of vibra- 
tions of the bars under the influence of the earth’s magnetism. 
It is clear that when a bar is vibrating about the magnetic 
meridian its magnetism is slightly increased by the inductive 
influence of the earth, and that thus the number of vibrations 
is increased. The use of two bars enables me to get rid of 
this difficulty in a very simple manner. I find the distance 
at which one bar must be held below the other so as to 
counteract the earth’s magnetism ; then I suspend the two 
bars, one beneath the other, at that interval, and vibrate 
both together. The result gives the number of vibrations 
due to the magnetism of the bars as uninfluenced by 
induction. 
The method of suspension I have found most convenient 
is as follows : — A fine wire is twisted round the centre 
of one of the bars, forming a loop which is caught by a small 
hook fastened to the extremity of the suspending fibre. 
Then a filament of silk of the proper length, tied at the 
ends so as to form a long loop, is thrown along the length 
of the suspended bar, and the ends of the other bar are 
placed in the ends of the loop. There is softie slight difficulty 
in making the bars hang quite horizontally, but this may 
be overcome by a little practice. The proof of accurate 
horizonal position is made by looking at the bar and a spirit 
level simultaneously from a distance with a telescope. 
The bars being arranged as above in one system, and 
having nearly the same amount of magnetism in each, vibrate 
with perfect regularity. To ascertain the time occupied by 
the vibrations I look at the system through a telescope 
placed at a distance of two or three yards, a clock beating 
seconds being within hearing. Then I have a slate divided 
