316 
MAGNETIC SURVEY IN NORTH AMERICA. 
stations present discordances much exceeding the ordinary limits ; in most of these 
cases similar discrepancies occur in the observations of the Inclination also, whence 
it may be inferred that they are occasioned by station-error. Setting aside these 
seventeen results, there remain 272 independent determinations at 219 stations distri- 
buted over a portion of the earth’s surface, extending in latitude above twenty-eight 
degrees, and in longitude above fifty degrees. 
At thirty-five of Captain Lefroy’s stations, results were obtained both by the 
magnets of the unifilar magnetometer, and by the statical needles. Omitting Pierre 
au Calumet, where much local disturbance prevailed, there are thirty-four stations 
at which the values of the total Force in absolute measure derived from the two me- 
thods admit of being compared ; when this is done it is found that the values obtained 
by the horizontal method are in excess at fifteen stations, and in defect at an equal 
number, the results being identical at the remaining four stations. The sum of the 
differences in excess is 1*95, and in defect 1‘22 ; the difference of these two numbers 
divided by 34 (the number of stations), gives ‘0215 as the average excess of the ab- 
solute determinations, or about ‘0015 of the whole force. As the two methods of 
experimenting are perfectly independent of each other, having no single element in 
common, such an agreement is a very satisfactory confirmation of the general merits 
of both, and testifies, far more than any verbal expressions, in praise of the unre- 
mitting care with which the observations were conducted and executed. The sum 
of the differences in excess and defect, taken without reference to signs, is 3T 7, 
which divided by 34 gives an average difference at each station of 0‘99 between the 
two methods. As in all probability the differences which appear in the results of the 
two methods in such very high magnetic latitudes are chiefly attributable to observa- 
tion-error in the Inclinations, of which the secants are employed in the deduction of 
the total Force from the horizontal components, it may be proper to notice that an 
error of 0‘09 in the total Force is equivalent, when the Inclination is 80°, to an obser- 
vation-error of less than 4' in the Inclination : and when all the circumstances are 
considered under which the observations of Inclination were made, an average error 
of 4' in determinations, which rarely admitted of confirmation on a second day, will 
by no means appear an extraordinary amount. 
As a considerable portion of the statical determinations of the Force, and of the 
observations of Inclination, were entrusted to Bombardier Henry, I may take this 
occasion to insert, as I have great pleasure in doing, the following extract from a 
communication from Captain Lefroy : — “During the twenty months which I passed 
in the Hudson’s Bay Territories, Bombardier Henry was my only English assistant: 
his excellent conduct and cheerful endurance at times of considerable inconvenience 
and hardships, did as much credit to his character as a non-commissioned officer of 
artillery, as the interest with which he devoted himself to the observations entrusted 
to him did to his zeal and intelligence.” 
