292 
MAGNETIC SURVEY IN NORTH AMERICA. 
Table XXXIII. 
Bar. 
Length. 
r. 
r 
r". 
r" f . 
in. 
ft. 
ft. 
ft. 
ft. 
30 
3-0 
1-0257 
1-3257 
31 
3-0 
1-0257 
1-3257 
17 
3-6 
1-2514 
20 
3-6 hollow. 
1-2530 
33 
2-45 
1-1019 
1-4019 
29 
2-0 
0-9328 
1-0828 
1-3828 
Experiments at Toronto. 
30 
3-0 
1-0257 
M757 
1-3257 
31 
3-0 
1-0257 
1-1257 
1-2257 
1-3257 
17 
3-6 
1-1514 
1-4514 
“ Experiments of Vibration . — The bars were vibrated in the series of 1843 in a stirrup 
weighing 322 grains, and similar to the one employed in the observations with the 
German magnetometer : the moment of inertia was ascertained by vibration with 
and without cylindrical weights ; it has since been re-determined in the method 
proposed by Dr. Lamont, employing accurately turned brass rings instead of weights, 
and the values obtained by the latter method have been applied in preference. The 
bars were vibrated throughout 1844, and at some of the stations of 1843, without 
any appendage, the suspension thread being attached by a very light wooden pin, 
set into the hole which is made in the centre of each for the purpose of fixing it in 
the stirrup : this method had the advantage of diminishing the time of vibration, 
and allowing the use of a suspension silk of almost insignificant torsion force; it 
was found to give greater regularity to the results. 
“ The moment of inertia of each bar, when vibrated without any appendage, is given 
eft b^" 
by the formula k= - - M 
it has also been determined by vibration with the brass 
rings, the results agreeing very nearly with those determined by measurement. 
“ The dimensions and weights of the different magnets were as follows : — 
Table XXXIV. 
Length. 
Diameter. 
Weight. 
Breadth. 
Thickness. 
in. 
in. 
gr- 
in. 
in. 
Bar 30 
3-017 
0-292 
415-3 
31 
3-017 
0-300 
416-7 
17 
3-635 
0-314 
572-5 
20 hollow. 
3-672 
0-320 
252-3 
23 
2-446 
0-300 
337-0 
29 
1-977 
0-290 
271-2 
Ring No. 1 
.... 
2-986 a 
289-19 
0-195 
0-078 
3 
.... 
3-644 a 
358-78 
0-195 
0-078 
Stirrup. 
2-970 
Between the points of suspension. 
Weight 5 
1-199 
0-315 
| 419-9 
6 
1-197 
0-315 
1 
a Outer diameter. 
