244 
MAGNETIC SURVEY IN NORTH AMERICA. 
common base, it becomes an object to investigate with more than ordinary care the 
value of the total magnetic Force at that observatory, both in the arbitrary scale and 
in absolute measure. 
In the present state of experimental methods and apparatus we must obtain the 
absolute Force from the observations of its horizontal component, combined with 
observations of the Inclination. At Toronto we have for this purpose the values of 
the horizontal component resulting from the regular monthly series, made in the 
observatory on the 16th, 17th and 18th of each month in the year 1845, with a sus- 
pended magnet of three inches, and a deflecting magnet of 3'67 inches, the same 
magnets being used throughout. The details of these observations will be printed 
in the volume of the proceedings at the Toronto Observatory for the year 1845 ; the 
results are as follows : — 
January . 3’5377* April. . 3*5351. July .... 3*5413. October. . 3*5373. 
February . 3*5376. May . . 3*5388. August. . . 3*5383. November. 3*5360. 
March . . 3*5373. June. , 3*5421. September. 3*5373. December. 3*5379. 
The mean of the twelve months is 3*5380 ; and the mean of the Inclinations observed 
on the days in each month, nearest to those on which the Force was observed, is 
75° 15'*5 : hence we have for the total force in absolute measure, 13*904. 
The magnets employed in the preceding determination were those of the Observatory 
unifilar. In September of the same year (1845), a very careful series of observations 
were made with the magnets Nos. 30, 31 and 17, of the unifilar magnetometer which 
had been employed on the survey ; the particulars are given in the following Table 
Table I. — Observations on the absolute Horizontal Force made at Toronto with 
the Survey bars. Nos. 30, 31 and 17, in September 1845, reduced to the mean 
reading of the Bifilar Magnetometer for the day of observation. 
Date. 
Bars. 
Distances. 
Bifi 
Mean of the day. 
ar. 
At the observation. 
Deflections. 
Vibrations. 
Corresponding 
bifilar. 
Suspension. 
Value 
of m. 
Valne of 
X. 
Angles. 
Therm. 
Times. 
Therm. 
Sc. div. 
Therm. 
Sc. div. 
Therm. 
Sc. div. 
Therm. 
feet. 
o 
11 26 26 
s. 
Sept . 19. 
30 
1 0257 
5 G 8*9 
64*0 
564*2 
65*0 
61*5 
4*8666 
61*6 
558*2 
64*4 
$. 
•3806 
3*5311 
19. 
30 
1 0257 
569*7 
64*9 
8 37 49 
61*8 
4*0136 
62*7 
565*2 
65*9 
*3805 
3*5315 
19. 
30 
1*2257 
572*0 
64*7 
6 40 47 
61*9 
*3806 
3*5311 
19. 
30 
1*3257 
571 7 
65*2 
5 16 35 
61*5 
•3806 
3*5313 
20. 
31 
1*0257 
575*5 
60*2 
569*9 
62 1 
9 54 19 
61*3 
4*3292 
60*6 
558*3 
61*9 
•3305 
3*5353 
20. 
31 
1*1257 
571*2 
62*1 
7 29 4 
61*3 
5*2409 
62*0 
575*9 
62*4 
S. 
•3306 
3*5347 
20. 
31 
1*2257 
569*6 
62*1 
5 47 35 
61*5 
•3305 
3*5354 
20. 
31 
1*3257 
569*3 
62*0 
4 34 53 
61*7 
*3306 
3*5340 
22. 
17 
1*1517 
589*2 
56*3 
592*6 
57*2 
13 26 57 
55*8 
4*4196 
57*6 
592*8 
57*9 
•6291 
3*5390 
22. 
17 
1*4517 
591*6 
57*1 
6 39 38 
56*2 
4*4149 
56*0 
584*7 
55*8 
. • . 
•6290 
3*5396 
24. 
17 
1*1517 
573*1 
57*4 
586*8 
57*0 
13 26 2 
57*5 
4*4138 
56*5 
581*1 
56*7 
*6287 
3*5362 
24. 
17 
1*4517 
587*0 
57*0 
6 39 11 
57*5 
*6279 
3*5404 
3*5350 
The 2nd observation of vibration of bar 1 7 entered on the 22nd was made on the 23rd, but is reduced to the 
mean bifilar reading of the 22nd. When s. is inserted in the column entitled “ suspension,” it implies that the 
magnet was suspended in a stirrup ; otherwise the magnet was suspended without a stirrup. The value of w 2 K 
for bar 30 is 21*580 without the stirrup, and 31*745 with the stirrup ; for bar 31, 21*853 without the stirrup, 
and 32*004 with the stirrup. 
