288 
MAGNETIC SURVEY IN NORTH AMERICA. 
Table XXIX. 
1842 
and 
1843. 
Station. 
Bar No. IX. 
Bar No. 13. 
Time of vibration. 
a^K. 
Time of vibration. 
T 2 K . 
Without weights. 
With weights. 
Without weights. 
With weights. 
September 
Quebec. . . . 
6-929 
13-616 
11-024 
7-137 
s 
13-420 
12-465 
September 
New York . 
Not observed. 
6-286 
11-862 
12-314 
December 
Toronto . . 
6-580 
12-965 
10-942 
6-670 
11-615 
12-239 
January . . 
Toronto . . 
6-617 
13-049 
11-193 
6-728 
12-689 
12-333 
Means . . 
log 2-04348 
11-043 
log 2-09124 
12-338 
“ Subsequent experience has shown the insufficiency of so small a number of obser- 
vations as the above, to determine the constant (r 2 K) with the requisite degree of 
precision; but as an improved instrument was substituted in March 1843 for the 
original transportable magnetometer, and the latter was returned to England at the 
same time, no opportunity has been afforded for repeating the observations. 
“ Experiments of Defection ; — The observed angles of deflection, multiplied by the 
ratio (l + are given for each station, without any further correction. 
“ Experiments of Vibration. — The observed times of vibration have been corrected 
for the arc, and for torsion. A note was made of the apparent chord of the arc 
of vibration as seen through the theodolite, at the beginning and ending of each 
series, with a few omissions. A movement of the end of the bar through a space 
equal to its diameter, corresponded to an arc of 12°. When the arcs were not re- 
corded, an approximate value has been taken, viz. for the initial arc 10°, and for the 
H 
terminal arc 4°. The ratio of the torsion force was observed at one station only, 
viz. Philadelphia. The same value has been applied at other stations slightly modi- 
fied for differences of intensity. 
<c Temperature. — The value of the coefficient for changes of temperature was not 
ascertained. The experiments of vibration and deflection were usually made at short 
intervals apart, and when both bars were employed, as was generally the case, one 
was vibrated during the time that the other was in use as a deflector ; consequently 
the correction will generally have been unimportant or counterbalanced on the mean 
of the two results. 
“ Induced Magnetism . — No correction has been applied for changes in the magnetic 
moment of the bars, due to their different positions with respect to the earth’s in- 
ducing force during the two parts of the experiment. This correction was not 
determined for the bars here employed. 
“ Gauss’s formula for three or more distances* has been employed for the calcula- 
* Scientific Memoirs, Part V. Art. II. 
