272 
MAGNETIC SURVEY IN NORTH AMERICA. 
on the 30th of April, L. A. was in constant and L. B. in occasional use. This con- 
tinued until the 20th of June at Rat Portage, when the circle was accidentally thrown 
down, and the axle of L. A., which was mounted at the time for observation, was 
bent. The injury which the circle had received was repaired a few days afterwards 
at the Red River settlement ; an endeavour was made to straighten the axle of the 
needle, and with so much success, that observations were occasionally made with it 
at subsequent stations ; the same weight was used as before, but the angles of deflec- 
tion were of course not comparable with the preceding ones. L. B. was now taken 
into daily use; and as in this needle the weight was the same as at Toronto and in 
the same position, the observations continued for some time forward to be directly 
referable to the base observations at Toronto. On arrival at York Factory the angles 
of deflection were again found to have become inconveniently large : a new weight 
was substituted, and continued in use until the 8th of August at Norway House, 
when the position of the weight was again changed, the angles of deflection in both 
positions being however observed : Norway House is thus a station common to three 
of the four series with this needle, and is itself directly connected with Toronto by 
one of the series. Observations with L. B. were continued to the close of the naviga- 
tion in 1843, but were not resumed in 1844 with either of the Lloyd’s needles. 
Under the circumstances which have been narrated, the course which has appeared 
to be best suited for the deduction of the variations of the magnetic Force resulting 
from the observations with L. A. and L. B., has been to ascertain, in the first instance, 
with as much precision as possible, the ratio of the magnetic force at Norway House 
to that at Toronto, and to regard Norway House as a base station for those series 
with L. A. and L. B., which are directly connected with it. By this means the only 
remaining unconnected series with Lloyd’s needles is that with L. A. between St. 
Helen’s and Fort William ; and this series has been connected with the others, by 
obtaining in a similar manner the value of the Force at Fort William as a base sta- 
tion from the observations with all the other needles. 
For the increase therefore of the total Force between Toronto and Norway House 
we have the following determinations by three independent methods : viz. — 
1 . By Mr. Fox’s Method and Apparatus. 
Needle F. A. — The first and apparently the best comparison with this needle is 
furnished by the angles of deflection observed with weights from two to four grains 
in natural temperatures at Toronto, on different days in January, March and April 
1843, before Lieut. Lefroy’s departure to join the Hudson’s Bay Company’s boats, 
and the angles of deflection observed with the same weights at Norway House on the 
13th July following ; the observations will be found in Table XIV. From this com- 
parison we have the force at Norway House = 1 *0196 to 1 at Toronto. 
For a second comparison with this needle we may unite the whole of the deflee- 
