124 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
second series of vibrations with two cylinders of equal weight and dimensions sus- 
pended across the end of the bar, in the manner described by M. Weber. The times 
of vibration are uncorrected for the torsion of the suspension thread, or for the 
changes of horizontal intensity occurring during the intervals of the experiments. 
“ Abstract of Observations of the absolute Horizontal Intensity. 
1842. 
o? 
d 
Experiments of Deflection. 
Times of vibration. 
Values of 
Distances in feet. 
Angles of Deflection. 
Log of 
m 
x' 
Without 
weights. 
With 
weights. 
Log of 
m X. 
X. 
m . 
Temp. 
of 
Magn. 
May 26. 
10 
1-4746 
1-8671 
7 56-8 
o / 
3 56-0 
O / 
9-34969 
S 
6*218 
S 
11*630 
0-49070 
3-7196 
0-832 
o 
62 
28. 
15 
1-4832 
1-8668 
7 20-5 
3 42-0 
9-32349 
6-433 
0-46439 
-2 192 
•783 
63 
28. 
13 
1-4831 
1-8669 
7 20-7 
3 41-9 
9-32296 
6-442 
0-46410 
■7202 
•783 
62 
30. 
13 
1-4831 
1-5164 
1-8669 
1-9086 
7 17-4 
6 48-9 
3 40-4 
3 26 0 
9-31982 
6-481 
12-078 
0-45885 
•7112 
•775 
67 
30. 
15 
1-4165 
1-4832 
1-9085 
1-9668 
8 23-0 
7 18-1 
3 27 0 
3 09-3 
9-32244 
6-433 
12-000 
0-46439 
•7237 
•782 
68 
From these experiments we may regard 3*72 as the approximate value of the 
horizontal force at Woolwich at the period referred to ; and 3 72 -f- e as the corrected 
value at the same station, corresponding to the period which shall hereafter be taken 
as the epoch of the magnetic maps of the globe, which these and similar contributions 
will combine to form , — e being a small quantity depending partly on the epoch, and 
partly on the possibly increased precision of determinations hereafter to be made 
with improved apparatus. In the mean time 3*72 has been adopted, and will continue 
to be used, as the provisional value of the horizontal force at Woolwich ; and the 
intensities at Sir Edward Belcher’s stations have been computed and are expressed 
accordingly. 
The general table of the determinations of the horizontal force (Table I.) is divided 
into three portions. Part I. contains a condensed abstract of the observations with 
Nos. 5, 6, 7 and 8, antecedent to March 1839, at several of the ports of the west 
coast of America. The column entitled “ corrected times,” shows the mean times 
of vibration corrected for the chronometer’s rate and for the arc of vibration, and 
reduced to a mean temperature of 75° ; employing for that purpose the coefficients 
found experimentally for each needle. The corrections for the arc of vibration have 
CL Cf! 
been made by multiplying the time of vibration by 1 — -^r, a and a! being the sines 
of the semiarcs at the commencement and conclusion of the observation. Previous 
to March 1839, the commencing semiarc was always 40°; subsequent to that period 
always 20°. The concluding arcs are specified in the Table. Panama is here em- 
ployed as a base station; and the means of the corrected times in March 1837, 
October 1838, and March 1839, have been taken, for each needle respectively, as the 
approximate times throughout the interval ; the values of the horizontal force are 
given in reference to the scale of absolute measure, the force at Panama being taken 
as = 7'743 (Part II. of Table I. of this memoir). 
