256 MAGNETIC SURVEY IN NORTH AMERICA. 
appears to me, an importance beyond those where the magnetic direction, the resultant 
of those forces, may happen to coincide with the direction of the gravitating force. 
In like manner, I consider that the line of least intensity, (or the line on which in 
every meridian the magnetic Force, having progressively diminished from the high 
latitudes of the one hemisphere, attains a minimum, and commences a progressive 
increase to the high latitudes of the other hemisphere,) deserves to be considered as 
the separating line between the northern and southern magnetic hemispheres, more 
properly than does the line in which the resultant direction of the magnetic forces 
happens to be perpendicular to the direction of the gravitating force. It may be a 
question whether this association of the phenomena of independent forces may not 
have been too exclusively dwelt upon, and have thus become in some respects preju- 
dicial to the advancement of terrestrial magnetism ; but the view of the philosopher 
is imperfect, who, in the contemplation to which he subjects the magnetic phenomena, 
in the hope of being conducted by the sure path of induction to a knowledge of their 
laws, omits to give a due consideration, either to those relations which have the ad- 
vantage of being purely magnetical, or to those coincidences or contrasts which the 
magnetic phenomena may present in respect to the phenomena of gravitation, or to 
those of any other of the great physical agents at the surface of our planet. 
Magnetic Inclination. 
The observations of the Inclination made in Lieut. Lefroy’s survey are contained 
in § 12. The number of determinations amounts to 179 (including 8 by Lieut. 
Younghusband, R.A. with the same instruments, and 5 by Dr. Rae, an officer of 
the Hudson’s Bay establishment, furnished with an Inclinometer by Barrow), and 
the number of stations to 162. The general table, which includes the Inclinations 
observed by gentlemen of the United States, contains 450 determinations made at 
335 stations. These were all observed between the years 1835 and 1846, and by far 
the greater number between 1839 and 1846. I have not attempted to introduce a 
correction for secular change, as the difference of epoch is small, and the rate of 
secular change is far from being even approximately known : without doubt also it 
varies in different parts of the wide district comprehended in this survey. At Toronto, 
its amount is so extremely small as to be scarcely appreciable by means of the most 
careful and multiplied observations continued for several years. The earliest obser- 
vation of the Inclination in the United States, from which a satisfactory conclusion 
in respect to secular change may be derived, appears by Mr. Loomis’s investigation*, 
to have been made by myself in 1822, in the garden of the Lunatic Asylum, near 
New York. By comparing the result which I then obtained (73° 05') with those of 
Locke, Lefroy and Renwick, at the same spot in 1841, 1842 and 1844 (72° 42 ,- 5), 
w r e find a diminution in twenty years amounting to 22'*5, or rather more than 1' an- 
* Silliman’s Journal, 1 842, Art. IX. I believe that I may also claim the credit of having made in the same 
year the earliest determination of the magnetic force in the United States. 
