274 
MAGNETIC SURVEY IN NORTH AMERICA. 
I have regarded the mean, 1*0208, as the most satisfactory result which I am able to 
derive from the observations with this needle at Toronto and Norway House ; and 
therefore as the result by Dr. Lloyd’s method. 
3. By absolute measures of the Horizontal Force. 
The value of the horizontal force in absolute measure at Norway House is given 
by Lieut. Lefroy at 2*1742, derived from the experiments with two magnets in July 
1843, and with three magnets in September 1844, Table XXXVIII. 
The Inclination observed at Norway House in 1843 and 1844 is 81°09'*8. Hence 
the total magnetic force at Norway House =14*15 in absolute measure. 
At Toronto we have the horizontal force 3*535 by the experiments with the three 
survey magnets in January, February and March 1845, Table XXXVIII. The Incli- 
nation at this period, taken to the nearest minute from the observations at the obser- 
vatory, made twice in each week, is 75° 14'. Hence the total magnetic force at To- 
ronto at that period was 13*87. If therefore the total force at Toronto be taken as 
unity, the ratio at Norway House is ^^= 1*0205. It is right to notice that the 
13*87 
value of the total force at Norway House would be altered 0*027, and the ratio of 
the force at Toronto and Norway House *0019 by a correction of 1' only in the ob- 
served dip ; and that the uncertainty which must attach to a determination of the dip 
made with a single instrument, even under the most favourable circumstances, taking 
into account both the possibility of constant errors, and the probable observation error, 
can scarcely be deemed so small as 1'. 
Collecting then in one view* the results of the three methods, we have the ratio of 
the total Force at Norway House as follows : — 
By Mr. Fox’s method . . . 1*0184 to 1 at Toronto. 
By Dr. Lloyd’s method . . . 1*0208 to 1 at Toronto. 
By absolute measure . . . 1*0205 to 1 at Toronto. 
As there do not appear sufficient grounds to assign a decided preference to any 
one of these results over the other two, and as, moreover, they are much more accord- 
ant with each other than might reasonably have been expected, we appear to be justi- 
fied in taking 1*020 as probably a very near approximation to the true ratio of the 
total magnetic force at Norway House to unity at Toronto. If the Force at Toronto 
be taken at 1*836 in the arbitrary scale, then the value of the force at Norway House, 
in terms of the same scale, is 1*873 : which has been considered the value whenever 
Norway House has been used as a base station. 
For the ratio of the total Force at Fort William we have the result of the two series 
of observations with F. A. at that station on the 29th of May 1843, one by Lieut. 
Lefroy and the other by Bombardier Henry, with the same weights which had been 
used at Toronto in the early part of the same year (Table XIV.) ; these give the value 
