MADE BY CAPTAIN BACK DURING HIS LATE ARCTIC EXPEDITION. 
387 
72° 55 '* by the times of vibration in the meridian and at right angles to it, which 
differ but little from the first determination from Captain Back’s observations. 
Looking to the geographical positions of some of the stations. Rock Rapid, Point 
Beaufort, Montreal Island, and Point Ogle, we should be led to infer that the small 
difference in the dip at Rock Rapid, Point Beaufort, and Montreal Island, and the 
considerable difference between the dip at the two latter stations and that at Point 
Ogle, must be due to errors of observation, or to some local cause influencing the 
direction of the needle. The results at all the other stations would lead us to expect, 
at Point Beaufort and Montreal Island, a dip but little under 89° ; and it will be seen 
that the observations for determining the intensity by the times of vibration of hori- 
zontal needles, indicate that such should be nearly its amount. 
Independently of the reduction of Captain Back’s observations, and ascertaining 
the degree of reliance which might be placed on the results I have deduced, I had 
another and more general object in the long discussion into which I have entered, 
that of pointing out the kind of tests which may be applied to dip observations, in 
order to ascertain whether the observations are consistent with themselves, and by 
this means whether the results deduced from them are worthy of confidence. I have 
not yet had leisure to enter upon such an inquiry with many observations ; but in 
some which I have examined I find far greater discrepancies in values of the angle y 
than the greatest which I have noticed in the present instance. On some future oc- 
casion I may perhaps enter more fully upon such an inquiry, but for the present I 
shall leave it, and proceed with Captain Back’s observations. 
II. Observations of the Variation of the Magnetic Needle. 
As these observations are published in Captain Back’s Narrative, it would be 
superfluous to introduce them here in the form of a table, particularly as they are 
given in a subsequent table, in which I have instituted a comparison between results 
deduced from them, in conjunction with the dip observations which I have detailed, 
and certain theoretical results, with the view of ascertaining how far these observa- 
tions tend to support the theory. I shall therefore here simply refer to the table at 
p. 390 for the variation at the different stations of observation. 
III. Comparison of the Observations of the Dip and Variation of the Needle with 
theoretical results. 
On the hypothesis of two magnetic poles symmetrically situated in a diameter of 
the earth and near to its centre, the poles of verticity and of convergence will coincide, 
and the tangent of the dip will be equal to twice the tangent of the magnetic latitude. 
Although, viewing all the phenomena on the whole surface of the earth, such an hypo- 
thesis is clearly inadequate to their explanation, it is interesting to inquire how far it 
may be consistent with those observed on a limited portion. For such an inquiry, 
* An Account of Experiments to determine the Figure of the Earth, &c., p. 476. 
3 d 2 
