246 
MAGNETIC SURVEY IN NORTH AMERICA. 
The mean of this series of the Force is 3*5330, and of the Inclination 7 5° 15'*65 ; 
hence we have the total Force 1 3*887* 
The three determinations are as follows : — 
By the observatory magnets in the twelve months of 1845 . . . 13*904 
By the three survey magnets in September 1845 13*897 
By two of the three survey magnets in April 1846 13*887 
Whence we have the total Force in absolute measure at Toronto =13*896. 
For the value of the total Force in the arbitrary scale, we have, — 
1. A determination of the absolute Horizontal Force at Woolwich in June 1846, 
made by Captain Lefroy with the survey magnets Nos. 30 and 31, as shown in the 
following Table : — 
Table IV. — Observations of the absolute Horizontal Force, made at Woolwich with 
the Survey bars Nos. 30 and 31. The corresponding readings of the Bifilar at 
Greenwich, kindly communicated by Mr. Airy, are inserted for reference. The 
times of vibration are entered as reduced to the common temperature of 80°, and 
corrected for torsion and rate : the angles of deflection are entered as observed. 
Date. 
Bar. 
Distance. 
Deflection. 
Vibration. 
Values 
of m. 
Values of X. 
Angles. 
Therm. 
Bifilar at 
Greenwich. 
Times. 
Therm. 
Bifilar at 
Greenwich. 
1846. 
feet. 
s 
June 8. 
30 
1-026 
10 43-5 
75-7 
4-7621 
80 
0-01897 
0-375 
3-725 "'1 
8. 
30 
1-126 
8 6-0 
76-0 
0-01938 
80 
0-375 
3-725 
8. 
30 
1-226 
6 15-8 
79-3 
3-9287 
80 
0-01973 
0-375 
3-726 
11. 
31 
1-026 
9 47-9 
85-2 
4-9944 
80 
0-01872 
0-344 
3-730 
11. 
31 
1-126 
7 23-7 
85-8 
0-01977 
80 
0-344 
3-732 
/ /Co^x 
11. 
31 
1-226 
5 43-0 
84-5 
4-1204 
80 
0-02112 
0-344 
3-735 
12. 
31 
1-026 
9 45-6 
89-0 
0-01888 
4-1240 
80 
0-01826 
0-344 
3-727 
12. 
31 
1-126 
7 22-4 
91-0 
4-9975 
80 
0-02221 
0-344 
3-727 J 
The mean is 3*7284 ; and the mean of eight observations of the Inclination, made 
also between the 8th and 12th of June, with the same needles of Gambey’s Inclino- 
meter which had been employed to give the Inclination at Toronto, was 68° 57'*9 ; 
hence we have the total force in absolute measure at Woolwich =10*388. 
We have therefore the total magnetic Force at Toronto to that at Woolwich as 
13*896 to 10*388; or as T338 : 1*. and taking 1*372 as the Force at Woolwich ex- 
pressed in the arbitrary scale, we have the corresponding value at Toronto 1*835. 
2 C . A statical determination of the ratio of the Force at Toronto and Woolwich by 
Mr. Fox’s apparatus, conveyed by Lieut. Lefroy in 1842 from Woolwich to Toronto; 
the particulars will be found under their proper dates in the general detail of the 
observations in § 11 ; the result was 1*340 at Toronto to unity at Woolwich; or 
1*838 to 1 372. 
3°. A second statical determination with the same apparatus, which Captain 
