BALANCE OR VERTICAL ForcE MAGNETOMETER. xii 
horizontally, having a knife-edged axle which rests upon agate planes, the ten- 
dency of the earth’s magnetism to depress the one end being counterbalanced by 
an excess of weight tending to depress the other. 
The variations of the vertical component of the earth’s magnetic force will 
therefore cause changes in theposition of the magnet, and may be measured by 
these changes, as observed by a micrometer, the correction for temperature being 
applied. 
The following were the values of the instrumental coefficients at the be- 
ginning of 1847 :— 
Value of one scale division in parts of force = k = 0:0000100 (Introduction, 1845-46, p. xlii.) ; 
Temperature correction for 1° F. in parts of force = g = 0:000079 (Introduction,1845—46, p. xlviii.) ; 
and consequently the temperature correction in scale divisions 
CE eae eae 2 
SOS 7°90. 
On January 18, 1848, the balance magnet was changed from a plane perpen- 
dicular to the magnetic meridian into the plane of the meridian. 
After this change the following observations were made on Jan. 25, 1848, 
to determine the value of #, the scale coefficient, by the method of deflections. 
The deflections were made with Professor Forsss’ bar placed N. and S. of the 
balance magnet, and the following are the results :— 

Deflection of Deflection of Value of & from the 
Distance of Bar | Balance Magnet | Balance Magnet | formula k = aS Mi 
in Feet. in adjustment in | suspended hori- n tan dip. 
mic, divisions. zontally. (See Introduction, 
1845-6, p. xli.) 






(n) (u 
1:9 409:1 88er217 0:00002123 
2:0 41:5 fo, 30" ‘00002118 
9-2 245°6 Key OY -00002122 
2°4 182°6 89’ 28” -00002124 
2°6 140°5 30’ 18” ‘00002120 
3:0 88-7 194,567 -00002115 
Mean value of k=0'00002120 
129, 1, 0000790 . 5. 
hence q =i “p000018 72. 

1848, Aug. 284 5°.—The vertical-force box was removed for 20 minutes. 
» Nov. 3° 23".—The cover was placed on the balance magnet—off since 
the instrument was placed in the meridian. 
1850, July 29.—Time of vibration for small are = 7-36. 
In August 1857 observations were made by Mr WELSH to determine the value 
of k, the scale coefficient of the balance magnet, by the method of deflections. 
The deflections were made with a 3°65-inch magnet, in a manner precisely 
