302 
MAGNETIC SURVEY IN NORTH AMERICA. 
“ Edmonton , August 17, 1844. — The bifilar was adjusted, but the state of the weather prevented the con- 
tinuance of the readings for so long as twenty-four hours, and the value of X is reduced to the mean of twelve 
hourly readings only. 
“ Athabasca. — There are three separate determinations at this station with bars 30, 31 and 17, viz. in October 
1843, and in March and July 1844 ; the two former at the beginning and termination of a series of hourly 
observations of the bifilar, &c., made during the winter of 1843, and the latter after my return to Athabasca, 
from Mackenzie’s river. Three spare bars of various lengths. No. 23, 29 and 20, were also employed in 
October 1843 ; but as the results obtained from them were calculated with an assumed temperature coefficient, 
and their moments of inertia are less accurately known than those of bars 30, 31 and 17, to introduce the 
values of X given by them into the general mean would only vitiate more accurate results, and they are added 
for purposes of illustration only ; the general mean at the station is the mean by all the observations with bars 
30, 31 and 17, except one of bar 17, in which the accuracy of the observed times of vibration appeared doubt- 
ful, and which is not included in the Table. 
“ Vermilion, July 12, 1844. — The bifilar was placed in adjustment, and the several parts of the observation 
are reduced to the mean of the corresponding readings. 
“ Dunvegan, July 23, 1844. — The bifilar was placed in adjustment, and hourly readings taken on the 23rd 
and 24th of July, the term day of the month. The absolute intensity is reduced to the mean reading of the 
bifilar on the 23rd, which differs but little from the mean of thirty-two hourly observations. 
“ Big Island, June 20, 1844. — The place of observation was a small fishing-station near the Big Island on 
Great Slave Lake, and named after it, but not actually upon it. The values of X are deduced from experiments 
of deflection only, applying the mean values of m given by the observations of June 12 and 23. The bifilar 
was not observed. 
“ Fort Simpson, May 2 and June 12, 1844. — The experiments of May are reduced to the mean reading of 
the bifilar given by the hourly observations of nine days, from April 27 to May 8. Those of June are reduced 
to the mean of the readings taken during the experiments ; the values of X obtained from the experiments with 
the spare bars 20 and 23 have not been included in the mean, for the reason given in the remarks on the ob- 
servations at Athabasca. 
“Fort Good Hope. — The observation at this, the most northern station of the series, was made at midnight 
on the 29th and 30th of May, by the soft and beautiful twilight of that season and latitude. The night was 
calm, and free from any magnetic disturbance. The results are reduced to the mean of twenty hourly obser- 
vations of the bifilar magnetometer. The value of X by the observation of No. 30, when vibrated in the 
stirrup, is apparently too small. The suspension thread broke after 100 vibrations had been observed, occupy- 
ing only ll m 20 s , being too short a time to give an accurate result in so high a magnetic latitude : in taking 
the general mean I have allowed it only half the weight of the other results.” — J. H. Lefroy. 
