MAGNETIC SURVEY IN NORTH AMERICA. 
only as conveyed a knowledge of the modes of observation severally employed, and 
reduced the whole to a common unit. The observations thus combined gave for the 
highest isodynamic lines of the northern hemisphere, closed and irregularly elliptical 
curves, extending across the North American Continent nearly in a north-west and 
south-east direction, and having their central point, or the point of maximum of Force, 
approximately in 52° north latitude, and 270° east longitude*. 
To confirm this previously unsuspected characteristic of the magnetic system of the 
globe, — to establish beyond a question so remarkable a fact in Physical Geography, 
— to fix within narrow limits the geographical situation of the point of maximum 
corresponding to a particular epoch, — to ascertain with the precision of modern in- 
struments and methods the intensity of the magnetic Force at its point of maximum, — 
and to assign the form and geographical localities of the isodynamic curves adjacent 
to that point, — were objects which presented themselves amongst the most important 
desiderata for our present knowledge, and as likely to have a peculiar value at a future 
period in respect to the ^Etiology of the science ; when, like the earlier determina- 
tions in astronomy, these researches might serve to elucidate the laws of those secular 
changes, which, in our present ignorance of the causes of the earth’s magnetism, 
seem even more mysterious than the apparently complex relations of contemporary 
phenomena. 
The report referred to contained accordingly a suggestion of the expediency of 
measures being taken to procure a magnetic survey of that portion of the North 
American Continent, which is comprised within the isodynamic line of T8 in the 
arbitrary scale. This suggestion was treated as second in importance only to the 
recommendation which it was the primary object of that report to urge, and which 
has since been carried into execution ; viz. a magnetic survey of the high latitudes of 
the southern hemisphere by a British naval expedition. 
The extensive territory over which it would be necessary to travel, in order to 
execute the proposed survey in North America, forms a part of the British dominions 
on that Continent, and is annually traversed during the summer months by the boats 
of the Hudson’s Bay Company conveying merchandize. A navigation conducted on 
lakes and rivers is better suited for the safe transport of small and delicate instru- 
ments, than where the travelling has to be performed exclusively on land. The good 
offices which the Hudson’s Bay Company had contributed to the success of the geo- 
graphical expeditions, undertaken by the British Government, for the purpose of 
tracing the American rivers and coasts of the polar sea, and their liberality in origi- 
nating expeditions of the same nature, and in executing them at their own cost and 
by their own officers, justified the hope that assistance might be given by the Hud- 
son’s Bay Company, which should render an undertaking feasible, which undoubtedly 
would not have been so without their aid. 
With this expectation, and having reason to believe that leave of absence from 
* Vide Map 2 of the Report referred to. 
