326 
SIR RODERICK I. MURCHISON’S ADDRESS. [May 24, 1858- 
properly measured by the vertical extent of elevation or depression 
combined with the horizontal extent along the great circle. M. de 
Francq, however, has taken only the linear horizontal extent as the 
measure in question. It is for the natural philosopher and the 
geologist, rather than for the geographer, to pronounce on the 
soundness of the physical views on which these researches are 
founded ; but the facts respecting the distribution of land and sea, 
of mountains, plains, and rivers, with which these investigations 
may make us acquainted, as well as the laws according to which 
they may be grouped and classified, are equally interesting to the 
geographer, whatever may be the physical principles on which such 
researches are professedly founded. 
It would be impossible for me to enter into any detailed analysis 
of the examinations which M. de Francq has made of the pheno- 
mena along an immense number of great circles. I can offer but 
the briefest outline of them. In order to render the investigation 
as impartial as possible, he has fixed upon eight equidistant points 
on the Equator, beginning with the meridian of Paris. He takes 
through each of these points 36 great circles equidistant by 5° from 
each other, thus forming four systems ( roses' ) of divergent great 
circles, each system passing through two opposite points of the eight 
above mentioned. He then examines the horizontal extent, along 
each great circle, of the lines of elevation ( arcs d’ enhaussement) ; 
along the remaining portion of the circle there will generally, of 
course, be depression. All dry land is considered as belonging to 
elevation , but the whole bed of the ocean is not regarded as belong- 
ing to depression ; for lines along shallow coasts, ranges of islands, 
&c., which are only slightly and partially immersed beneath the 
surface of the sea, are also regarded as lines of elevation, being supposed 
in fact to lie above a certain mean surface, to which elevation and 
depression are referred. Moreover, these great circles frequently 
pass across regions which are nearly or altogether unknown, in 
which case he calculates the lengths of the lines of elevation in such 
regions on the supposition of their being proportional to the lengths 
of similar lines along the known portion of the great circle, and 
adopts these calculated lengths as the most probable lengths of the 
unknown lines in question. Proceeding on these suppositions, he 
finds (1) that on all those great circles along which the lines of 
elevation defined by the existence of dry land form together an arc 
of less than about 100°, there exist submarine lines of elevation, 
which, together with the terrestrial ones and those which are as^ 
