AND THE INTENSITY OF MAGNETICAL FORCE, 313 
The point, line, or surface of maximum intensity, is the subject now of no small inte- 
rest, and my observations, together with Lieutenant Lefroy’s, as alluded to in Colonel 
Sabine’s letter, seem to settle the question that it is at or near to Lake Superior; but 
whether within the limits of the United States, or of British America, is not positively 
determined. So abrupt and irregular are the magnetic changes, in very small distances 
in the region of the lake, that a true mean result requires many varied observations. It 
is like determining the mean level of a very uneven and broken country by taking random 
elevations; as by descending upon various unknown points in the night, with a balloon, 
and determining their altitude barometrically, without being able to discriminate whether 
you are upon a hill or in a valley; a species of surveying evidently requiring a multitude 
of observations to furnish a correct mean. 1 have drawn the chart of this part according 
to the present conditions of knowledge, and leave it open to such modifications as future 
developments may demand. I have represented an “axis” of total intensity, as suggested 
by Colonel Sabine’s letter, uniting the point of maximum intensity discovered by Lieu- 
tenant Lefroy, in the line of his journey, with that discovered by myself in a line inter- 
secting that through which he passed, but extending scarcely beyond it. My desire to 
return and multiply observations in that region this summer is almost uncontrollable, but 
my private means will not permit it, and I despair of any assistance from government, or 
from any other source. While Great Britain is sending out naval expeditions, and 
encompassing the earth with her magnetical researches, I feel impatient that there is no 
means by which I can be aided in completing a kind of magnetical reconnoisance of our 
own country. I do not ask to be paid for it, I merely ask to be helped a little ;—I am still 
willing to make a personal sacrifice. 
“The line of equal variation,” placed on this chart, is not the result of my own obser- 
vations, but is copied from a chart by Professor Loomis, published in Silliman’s Journal, 
It is introduced merely to illustrate the popular definitions with which this paper com- 
mences. ‘The line of equal dip is also introduced for popular explanation, but it was 
the result of my own experiments. More lines of equal dip might have been introduced, 
but I have chosen to designate the dip in numbers, and leave the chart clear of the confu- 
sion necessarily arising from the intersection of different systems of lines. 
After all, I fear that my readers will suppose that I attach more importance to the 
chart lines than they are worth. This point will be determined by the support which they 
derive from the observations; and it will be seen that they represent to some extent rather 
what is desirable than what has been attained. The southern line, 994, has a number of 
observations to support it from New York to the Mississippi. The second one, 1004, has 
- scarcely more than two. The third, 1014, has two groups, one in Iowa and the other 
about Lake Erie. The fourth, 1024, is sustained by one group, only. The fifth, 1034, has 
only two observations near to it, at Mackinaw and at the Sault St. Marie. The sixth, 
1044, is determined by the observations at La Pointe, while the other observations along 
the line are too low. The axis or maximum line, is sustained by the mean of numerous 
observations at Copper Harbour, and by one observation on Isle Royale. It may be, too, 
that the high intensity observed at Copper Harbour will not be sustained by that of the 
surrounding regions, when it would appear, to speak figuratively, that at Copper Harbour 
there is an isolated mountain, an outlier of high intensity. Although to make an aceu- 
