MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 389 
already determined from the constant value of the angle y ; in the fifth column, the 
numerical value of the product tan & . tan <p ; and in the sixth column, the differences 
between this product and the number 2, that is, the error of the theoretical result. In 
d § 
the three following columns are given, first, the value of deduced from the ob- 
servations ; secondly, the value of - co& - ~ to which the former should, according 
to the theory, be equal ; and thirdly, the difference between the values of these frac- 
tions, or the error of the theoretical result. In this Table I have not included the 
observations at Point Ogle, because, from the greatly diminished horizontal force 
acting upon the needle, I do not consider that the variation could have been deter- 
mined with any precision ; indeed, the amount of the daily variation, in such a posi- 
tion, would, supposing that the variation could be accurately determined at any 
instant, render the hour at which the observation was made a matter of the first im- 
portance, the position of the magnetic pole resulting from an observation made at 
one period of the day being necessarily very different from that deduced from obser- 
vations made at others *. This remark would in a great degree apply to the observa- 
tions at Point Beaufort and Montreal Island, but these places are more conveniently 
situated with respect to Rock Rapid, for a comparison of the observations, than Point 
Ogle : indeed, the variation at Rock Rapid would assign a position to the south of 
Point Ogle for that of the magnetic pole, which position is quite at variance with the 
observations at the latter station. 
* This is a consideration which does not appear to have occurred either to Sir John Ross or Captain Jambs 
Ross in assigning a position to the magnetic pole. Taking a mean of Sir John Ross’s daily variations at 
Victory Harbour for the month of April 1830, we have the variation at noon 100° 53' W., and at midnight 
85° 22' W. ; giving a diurnal variation of more than 15°. Assuming the dip here 88° 55', as determined by 
Captain James Ross, the distance of the pole of verticity from Victory Harbour would be 2° 9' 57" nearly : 
consequently the situation of the pole at midnight would be 35' 5", or rather more than 40 miles distant from 
its position at noon. It appears to have been considered that the true position of the magnetic pole has been 
determined within much narrower limits than such an interval. Taking this view of the subject, it may be an 
inquiry worth entering upon, to ascertain whether the extent of the diurnal variations observed by Captain 
Foster at Port Bowen corresponds to the same orbit, if I may use the expression, of the magnetic pole on the 
earth’s surface as that observed by Captain Back at Fort Reliance; whether these correspond with the orbit 
which would result from the diurnal variation given by Sir John Ross ; and also whether the several times of 
the maxima and minima are in accordance with the same motion, whether uniform or not, of the pole in one 
orbit. This is an inquiry upon which my present engagements do not admit of my now entering, but I pro» 
pose doing so as soon as I have the requisite leisure. 
