MR. CHRISTIE ON THE MAGNETICAL OBSERVATIONS 
382 
Fort Reliance . “ These observations were made in my tent, which had a leather 
circular lodge or wigwam attached to it, in which was a small fire. The ground on 
which the stand was placed was gravel, about one hundred yards from the lake.” 
Rock Rapid. “These observations were made as before in the tent, the theodolite 
stand being placed on shingle, at the foot of a high gneiss rock.” 
Point Beaufort. “ The observations made as usual ; the stand on shingle at the 
base of a gneiss rock three to four hundred feet high.” 
Montreal Island. “ The stand was placed on firm sand about sixty yards from some 
low rocks, and a sail was put over the tent to afford more shade.” 
Point Ogle. “ The stand was firmly fixed in sand and shells, of which the beach 
was composed, thirty paces from the beach, which was packed with ice.” 
Independently of the accuracy with which the observations were made, and of the 
precautions taken to exclude the effects of any particular local influence upon the 
needle, it is evident that the correctness of the results deduced from them in the 
foregoing Table must depend upon the permanence of the angle y, and upon a correct 
value having been assigned to that angle. It is therefore necessary to inquire whether 
any tests can be applied to these observations which may indicate the extent of the 
errors by which these results may be affected. 
Values being assigned to y and £ in the equations (1.) and (2.) or (3.) and (4.), 
these equations furnish measures of the terrestrial magnetic intensity, viz. 
cos ('9 — y) 
M = 
sin ('9 — 8) ’ 
cos S cos (y 
D) 
cos D ' sin (S — 8) ’ 
or M = 
cos (fi + y) 
sin (fi — 8) 
or M = 
sin S sin (y — D) 
sin D ' cos (S — 8) 
( 6 -); 
( 7 .) 
In order, however, that the values of M, thus determined, should be as little as 
possible affected by errors of adjustment of the instrument or needle, it is necessary 
that the centre of gravity of the needle should be distant from the axis of motion, and 
that the angle y should be large*. The needle with which these observations were 
made not having been constructed for this purpose, but with the view of determining 
the variations in the terrestrial intensity by means of its times of vibration, its centre 
of gravity was made to coincide as nearly as possible with its axis of vibration. In 
determining therefore the values of M from the equations (6.) by means of it, we are 
not to expect very close approximations to the values derived from other principles. 
Admitting this, however, the results ought not in any case to be quite discordant. 
In the followingTable I have given the values of M deduced from the equations (6.), 
assuming the value of y to be 16° 29' 19" ; and in the following column, for the pur- 
pose of comparison, the relative values of M deduced from the times of vibration of 
the needle No. II. in the plane of the meridian'f'. 
* Philosophical Transactions, 1833, p. 348. 
+ The observations from which these are deduced are given in a subsequent part of this paper. 
