Vly 4 MR BROUN ON THE BALANCE MAGNETOMETER, 
tion. The following series of observations made during a magnetic disturbance, 
will, however, prove it more distinctly. 
TaBLeE IV. 
Observations for the Time of Vibration of the Balance Needle in a Vertical 
Plane, the position of rest varying. 

Gottingen Balance Magnetometer. Time of one Vibration. 
eoiaaerncuns 
i Reading. Thermometer. Observed. Sgt dati 
4 ey al Mic. Div. s s 
April 15 22 52 —148 47.7 8.84 9.01 
17 150 +101 52.5 9.21 9.02 
2 20 + 4 53.2 9.46 9.22 
315 + 25 54.2 9.46 9.14 
3 25 + 23 54.7 9.36 9.00 
5 15 — 18 56.4 9.62 9.13 
7 45 — 10 56.3 9.66 9.18 
8 45 — 90 56.0 9.68 9.22 
10 20 —190 55.2 9.42 9.02 
10 30 —176 55.2 9.62 9.22 
13 40 —310 54.3 9.23 8.90 
13 50 — 293 54.3 9.35 9.02 
22 15 —185 61.1 9.28 9.20 
22 25 —185 Ole 9.38 9.30 
18 22 30 —134 50.3 9.01 8.99 

The positive and negative signs indicate that the north pole of the needle 
was below or above the horizontal. It would have required a change of 50° Fahr. 
to have produced alone a difference of 400 micrometer divisions. Such a change 
of temperature, according to j 6, would have been equivalent to a change of 
3.8 in the time of vibration. The observed times differ only a few tenths, and the 
times corrected for temperature agree within the limit of the errors of observa- 
tion.* 
9. It results from these facts, that the time of vibration in a vertical plane 
cannot be used at present in the reduction of the observations, as theory takes 
no account of them. The theoretical corrections for differences of arc or the varia- 
tion of the moment of inertia due to temperature would, in the examples given, 
be inappreciable. 
10. I shall now consider —, the temperature correction for the position of 
the needle. 
The method which has been adopted for its determination is as follows :— 
The magnet, whose temperature correction is to be obtained, is placed at right 
angles to a magnet freely suspended, which is thus deflected by an angle uw from 
the magnetic meridian. If m be the magnetic moment of the deflecting magnet, 
* The time of vibration throughout the year varies from other causes. The law which regulates 
these variations I have not yet determined. 
