BIFILAR OR HorIzONTAL FORCE MAGNETOMETER. XXXill 
300 sc. div. being added to all the readings for 1845, 340 has been added to all 
those for 1846, and the same zero, 500, is applicable to both. 
44. The mean time of one vibration of the bifilar magnet, is between 26s and 27s : 
the natural are of vibration is generally very small, and when considerable, the time 
of vibration was found less than from large artificial vibrations.* 25% or 26s has been 
used in the observations for 1845 and 1846. 
45. The observations of the bifilar were made as follows: The point of the 
scale coinciding with the vertical wire of the fixed telescope is estimated to the 
tenth of a scale division at 25° before the minute of observation, at the minute, and 
25s after it; the three readings being a, b, and c, the mean is deduced from the 
a+2b+¢ 
ci 
formula The mean thus obtained is corrected to the temperature of 
26° Fahr.,a constant quantity has been added to all the corrected means. If N be 
the observed mean, and ¢ be the observed temperature of the bifilar bar, the cor- 
rected means ”, given in this volume, are obtained from the formule 
1845. n=N + 300-0 + (¢—26°) 1:90 
1846. »=N + 340-0 + (¢—26°) 1-975 
1:90 and 1-975 being the temperature coefficients in scale divisions in 1845 and 1846 
respectively. 
The means fin parts of the whole horizontal force given in the abstracts of re- 
sults, are obtained by the formule 
1845. f= (n — 500) 0-000140 
1846. f= (n — 500) 0-000135 
0-000140 and 0-000135 being the values of & for 1845 and 1846 respectively. 
No correction has been applied for the effect of the balance magnet, which is 
constant. 
BALANCE OR VERTICAL FoRcE MAGNETOMETER. 
46. The balance magnetometer was made by RoBinson of London ; it is com- 
posed of a magnetic needle cfc, 12 inches long, about # inch broad, and about 3. 
inch thick, with knife-edged axle 7, which rests upon agate planes ; brass rings c ¢ are 
attached to the extremities of the needle, each ring carrying a cross of spider threads. 
The needle is placed at right angles to the plane of the magnetic meridian, it is ac- 
curately adjusted to horizontality by a screw e which balances the needle, another 
screw d working vertically, regulates its sensibility. The apparatus, and a ther- 
mometer h which gives the temperature of the needle, is covered by a rectangular 
box k having glazed openings on both sides opposite the spider crosses ; those on one 
side allowing light to be thrown on the crosses from two small mirrors, (one of which 
* Introduction, 1841-2, pp. xxviii., xxix. 
MAG. AND MET. OBS. 1845 anpD 1846. a 
