70 SEVENTH REPORT—1837. 
that of 50° of dip, successively intersects in its eastern pro- 
gress all the lines of dip between 52° and 73°, with which 
latter it coincides in lat. 60° and long. 10°; it then again de- 
scends, intersecting successively, a second time, the same lines 
of dip, until it touches that of 57° in long. 70°. When it is 
seen that the same line of intensity successively coincides with 
the lines of dip of twenty different degrees, it must be admit- 
ted that their systems are not parallel, and that the conclusion 
was justly drawn, that the facts could not be represented by.an 
hypothesis in which the intensity should vary as any function 
of the dip. A conclusion by no means at variance, however, 
as has been erroneously imagined, with their having a causal 
connexion. 
Nor is the fact of non-parallelism confined to the northern 
hemisphere; on the contrary, the southern hemisphere ex- 
emplifies it in a still more striking degree. Thus we have in 
South America the line of unity under a dip of 0, as observed by 
M. de Humboldt in Peru; and at the Cape of Good Hope, the 
same line of unity under a dip exceeding 50°, as shown by the 
concurrent observations of Captains de Freycinet and Fitz Roy; 
whilst at Port Desire and atthe Falkland Islands, these officers 
found an intensity of 1:36, with nearly the same dip as had 
been found at the Cape of Good Hope accompanying an inten- 
sity less than unity. 
‘In M. Erman’s dip-lines (Plate I.), which represent his 
own recent observations, and are quite independent of pre- 
existing evidence, we see the same double flexure, of which 
the importance, in its bearing on physical causes as well as 
on empirical laws, was pointed out in the Fifth Report of the 
British Association, page 67. This double flexure takes place 
also in the intensity lines, but ina more marked degree. In 
both series of lines the radii vectores drawn from the geo- 
graphical pole have two maxima and two minima; a line joining 
the parts of each curve which approach nearest to one another, 
i.e. at the points of minima, will divide the area into two un- 
equal portions, the larger comprehending the American, and 
the smaller the Siberian centre of attraction. But there is a 
distinction in this respect between the two series of curves of dip 
and intensity, which has been pointed out by M. Erman, and 
is illustrated by the annexed diagram (Plate II.), taken from his 
paper in the Annalen der Physik, vol. xxi. The diagram re- 
presents the northern hemisphere, on which the curves of in- 
tensity of 1:45 and of 75° of dip are drawn. The longitudes 
of the maxima of both these curves are nearly the same; but 
not so those of the minima. In the curve of dip, the minima 
