252 
MAGNETIC SURVEY IN NORTH AMERICA. 
of the Force, as it has been now computed from seventy-eight stations visited in 1843 
and 1844, and surrounding the maximum in all directions, the coincidence with the 
central point of the closed curve of \' 7 , as drawn in the map of 1837, could scarcely 
have been more perfect. 
Materials for the extension of the isodynamic lines to the north and east of the 
present survey will shortly be supplied, — to the north by the Expedition under Sir 
John Franklin, — and to the east by Lieut. Moore of the Royal Navy, whose magnetic 
observations in Her Majesty’s ship Terror form an important portion of the Survey 
accomplished by the Expedition under Sir James Ross ; and whose subsequent obser- 
vations in the Pagoda, in conjunction with those of Lieut. Clerk, R.A., in the com- 
pletion of that survey, will form the VUIth number of the Contributions. Under the 
direction of the Lords of the Admiralty, Lieut. Moore has proceeded in the present 
summer to Hudson’s Bay, in one of the vessels of the Hudson’s Bay Company in 
which a passage has been kindly granted him, for the purpose of obtaining a suite of 
determinations intermediate between those which form the subject of the present com- 
munication, and those which are now in progress by the officers of the Arctic Expe- 
dition. The magnetic lines surrounding the one of the two points of maximum inten- 
sity which is on British territory, will thus be completely delineated : and I may be 
permitted to express a hope, founded on the unsparing liberality of the Russian 
Government in other branches of magnetical research, that the lines which surround 
the other point of maximum intensity in the northern hemisphere, which is within the 
Russian dominions, may ere long be determined with an equal completeness. The 
geographical longitude of the Siberian maximum is approximately known from the 
observations of Hansteen, Due and Erman ; but an equally correct knowledge of its 
latitude appears to require an extension of the researches to the shores of the Polar 
Ocean. 
The intensity of the magnetic Force at its point of maximum in North America is 
1’878 in the arbitrary scale; or 14*2 1 in absolute measure, of which the units are, — 
of mass a grain, of time a second, and of space a foot, in British weights and measures. 
The intensity of the Force near the corresponding point in the southern hemisphere 
may be taken approximately, from a group composed of the daily determinations made 
by Sir James Ross’s Expedition from the 19th to the 27th of March 1841, between 
the latitudes of —58° and —64° 26', and longitudes of 128° 40' E. and 148° 20' E., 
the track of the Expedition when crossing the southern isodynamic ellipse of 2 000 
about midway between the extremities of its principal axis. The mean of the results 
of this group is 2-059, or in round numbers, 2*06 in lat. —64° and long. 137°‘5, or in 
absolute measure 15-60. Neither the position of this maximum, nor the value of the 
Force, can be regarded as determined with as much precision as we may consider 
those at the northern maximum now to be; but we may conclude with certainty, 
that at the present magnetic epoch the Force at the southern maximum is consider- 
