MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 413 
that on calling Captain Back’s attention to the circumstance, and referring to the 
register of the observations, he could recall no circumstance that could induce him 
to think that in these observations the reading of the needle was beyond 90° on the 
northern limb, though he perfectly remembered remarking that such was the case at 
Point Ogle, and that the angle registered was that on the southern limb, and so 
marked. 
I have already stated my opinion of the inapplicability of Hansteen’s method of 
determining the intensity, to cases where the dip is great — but my objections to the 
method are by no means limited to such cases — and I may notice another source of 
error, besides that to which I have adverted, to which results deduced from the times 
of vibration of a horizontal needle are in these cases peculiarly subject, — local influence. 
The directive force acting upon the horizontal needle is, in these cases, greatly dimi- 
nished, and consequently the relative effect of an extraneous force is, with little ex- 
ception, increased ; so that masses of rock which are magnetic, although without 
polarity, may, by their position, exert an influence on the time of vibration of a hori- 
zontal needle, which would be altogether insensible on that of the dipping needle. 
That the times of vibration of the horizontal needle at Point Beaufort may have been 
thus affected, is not altogether improbable. On the observations there Captain Back 
has this remark: “Instrument in perfect adjustment; stand on shingle at the base 
of a gneiss rock three or four hundred feet high.” There is no remark on the posi- 
tion of this rock with respect to the needle, and I cannot now consult Captain Back 
on the subject; but that in particular positions such a mass would affect the hori- 
zontal needle, and even the dipping needle, is more than probable. At Montreal 
Island the stand was “ placed on firm sand, about sixty yards from some low rocks”; 
so that here there does not appear any particular cause for suspecting local influence. 
It is, however, proper to give a remark of Captain Back’s relative to these observa- 
tions : “No. 3 vibrated slowly, and on the first trial stopped dead at 10°. Two 
kettles, twenty yards off, were then taken further away, and I took off my brace- 
buckles ; it then vibrated regularly, but made a long rest at each extreme, as if 
disposed to remain there. We have only made twelve miles N.W. f W. from the last 
place, so that one would imagine some local cause, such as the rock, to account for 
the difference in the interval of vibrations.” But I must now close my remarks on 
the observations at these two stations, although I may, very unwillingly, leave some 
degree of uncertainty attached to the results deduced from them. 
Whatever uncertainty may attach to the results of the observations at Point Beau- 
fort and Montreal Island, every circumstance tends to confirm the correctness of 
the observations at the more important station. Point Ogle, and to indicate the very 
near approach which was here made to the northern pole of verticity. If any doubts 
could be entertained with regard to the amount of the dip at this station, as deter- 
mined from the observations, they must be completely removed by Captain Back’s 
remarks on the difficulty of adjusting the horizontal needles, — which, it will be borne 
