Royal Sociclij. 525 



angles of inclination to the horizon, are principally attributable to a 

 want of absolute permanence in its axis of motion. In the present 

 case, the centre of gravity of the needle being nearly coincident with 

 the axis, a very minute derangement in that axis would cause a con- 

 siderable change in the value of the angle y ; so that the existence 

 of differences in the values of this angle do not warrant the inference 

 that the needle itself received any serious injury during the expedi- 

 tion ; to which, indeed, from the care taken of it by Captain Back, 

 it could not well have been liable. 



The second part of the paper relates to the observations of the va- 

 riation of the magnetic needle, which are already published in Capt. 

 Back's narrative, and which are here introduced for the purpose of 

 applying them, in conjunction with the observations of the dip, detailed 

 in the preceding part, to a formula deduced from theory, with the view 

 of ascertaining how far they may tend to support that theory. 



The third section is devoted to the comparison of the observations 

 of the dip and variation of the needle with theoretical results of a 

 more general kind. The observations made by Captain Back are pe- 

 culiarly adapted for verifying the hypotheses on which the theories of 

 terrestrial magnetism rest, and that theory, in particular, which as- 

 sumes the existence of two magnetic poles, symmetrically situated in 

 a diameter of the earth, and near to its centre : for, on this hypothesis, 

 the poles of verticity and of convergence will coincide ; and the tan- 

 gent of the dip will be equal to twice the tangent of the magnetic 

 latitude. In no case has a progress towards the magnetic pole been 

 made so directly, and to such an extent, as in the present expedition ; 

 whether that point be considered as the point of convergence of mag- 

 netic meridians, or that at which the direction of the force is vertical. 

 It is deducible from the theory that the product of the tangent of the 

 dip by the tangent of the polar distance is equal to two : and there- 

 fore, if the distance of the pole of convergence from two stations be 

 determined by means of the observed variations at those stations, we 

 may estimate, by the approximation of this product to the number 

 two, in each case, the degree of coincidence which exists between 

 theory and observation. A table is then given, exhibiting the several 

 data on which this comparison is made, and the results deduced from 

 them. From an inspection of the numbers in the column which indi- 

 cate the deviations from theory it appears that there is not, in o-eneral, 

 that accordance between the observations and the theory which might 

 reasonably have been expected ; and that although that theory may 

 serve as a first approximation, yet it requires to be considerably mo- 

 dified to reconcile it with the observations. Hence the author arrives 

 at the general conclusion that, unless considerable errors have crept 

 into the observations of either the dip or the variation, the theoretical 

 pole of verticity does not coincide with the pole of convergence, even 

 when the positions of these points are deduced from observations made 

 at very limited distances from those poles. 



" On the Safety-valve of the right Ventricle of the Heart in Man j 

 and on the gradations of the same apparatus in Mammalia and Birds." 

 By J. W. King, Esq. Communicated by Thomas Bell, Esq., F.R.s! 



