386 
MR. CHRISTIE ON THE MAGNETICAL OBSERVATIONS 
values of the terrestrial intensity, as deduced from the times of vibration of the 
needle, and from the observed angles of inclination to the horizon, are principally 
attributable to a want of absolute permanence in its axis of motion. With respect 
to the amount of the change in this axis, it is to be observed, that in this needle the 
centre of gravity was nearly coincident with the axis ; and, consequently, that a very 
minute derangement of the axis would cause a considerable change in the value of 
the angle y. We must not therefore infer, in consequence of the differences in the 
values of this angle exhibited in the preceding Table, that the needle itself received 
any serious injury during the expedition. To such it could not be liable, being, during 
the whole expedition, under the personal charge of Captain Back, who, aware of the 
importance of preserving- this and other needles as nearly as possible in the same 
state, carried them himself with the same care as he bestowed on the chronometers. 
Having now fully discussed the observations which Captain Back made with the 
dipping needle, it is proper that I should state how far I consider that the dip is de- 
termined by them at the several stations. If we refer to the Table at p. 384, it will 
be seen that, excepting the New York, Montreal, and Fort Alexander observations, 
the differences between the dips as determined by means of the constant value as- 
sumed for the angle y, and as determined from the relative terrestrial intensities, 
deduced from the times of vibration of the same needle, are generally much within 
the limits of the errors to which observations of the dip, with our present instruments, 
are generally found liable. I think therefore that we are to consider the results not 
only as entitled to the confidence which is generally given to those deduced from 
observations carefully made with good instruments, but that the differences in the 
last column of the table, in general, fairly exhibit the amount of the error in each 
case, by which the result may be affected. In the cases of New York, Montreal, and 
Fort Alexander this is so considerable that much uncertainty must attach to the 
results determined by means of the constant value assumed for the angle y. With 
respect to the dip at these places as determined by means of the intensity, it may be 
considered that the same degree of uncertainty may not attach. It is, however, sin- 
gular that the results at New York and Fort Alexander in this case differ more widely 
from preceding determinations than in the other. Sir John Franklin* determined 
the dip at New York in 1825 to be 73° 27' 3", which differs only 37 ' 45" from the dip 
determined from Captain Back’s observations by means of the constant value of y, 
but differs 2° 14' 34" from the result obtained by means of the intensity. At Fort 
Alexander Sir John Franklin’s determination of the dip in 1825 was 7 8° 47' 8", dif- 
fering only — 5' 29" from the result from Captain Back’s observations in the first 
case, but +41' 5" from the result in the second. In December 1822 Captain Sabine 
determined the dip at New York to be+ 73° 5' by means of a Meyer’s needle, and 
* Narrative of a Second Expedition to the Shores of the Polar Sea, Appendix, p. cxxxvi. 
f An Account of Experiments to determine the Figure of the Earth, &c., p. 474. 
