MAGNETIC INCLINATION AT GOTTINGEN. 637 
Inner diameter of ditto. 75°525 millimetres. 
Outer diameter . . . 79°767 ize 
Hence, taking grammes and millimetres as units 
The moment of inertia of the ring . 29019 
ae epee theee of the needle* 52662 
From these numbers, combined with the times of vibration of 
the 23rd of September, given in Art. 9, and the length of the sim- 
ple pendulum vibrating seconds in Gottingen, taken at 994:126 
millimetres, we obtain by following the known method— 
Horizontal magnetic directive force, 
before the reversal 1°5556 
after the reversal 1°5352 
Strictly speaking, these numbers apply directly only to the 
place where the vibrations were observed, and include there- 
fore whatever local influences may have existed there; for the 
present object this influence, which in any case can only be very 
small, does not come into consideration. 
With an inclination of 67° 40! 54", it further follows that 
the whole magnetic directive force 
before the reversal of the poles is 4°0965 
Mech, iis cage phe Goides 4°0429 
The vertical magnetic directive force 
before the reversal of the poles is 3°7897 
Stevi «lds iovtieh tuisa) oe 3'7401 
Neglecting the small modification which the directive magnetic 
force of the needle suffers by reason of the excentricity of the 
centre of gravity, these four numbers may be regarded as the 
values of 6, according as the observations are made in the mag- 
netic meridian, or in the plane perpendicular to it. As 6 and y 
are always so small that we may take these quantities themselves 
instead of their sines, 
say, 
and we obtain, according to whether we take for our base the 
force of magnetism in the needle as it was before, or as it was 
after, the change of poles: 
for observations in the magnetic meridian — 
B = 1°1882 ¥, or B = 1:2039 y: 
* Strictly speaking, it is the sum of the moments of inertia of the needle and 
of the stirrup; it would be both impracticable and superfluous to attempt to 
separate the two, as none but horizontal vibrations are taken with this stirrup. 
