146 Magnetism of the Earth. 
The observations that M. Hansteen has already made in the first: 
year of his undertaking, and the conclusions which they establish in 
regard to the direction assumed by the isodynamic curves, or curves 
of equal magnetic intensity, are in the highest degree curious and im- 
portant. In the letter with which he has favored me, he has taken 
the trouble to communicate his observations in full detail, and has 
expressly permitted me to make every use of them that I may think 
roper, “ especially when it may sega to new undertakings, 
and accordingly forward the science.” Having been requested by 
you to superintend the construction in this country, of a part of the 
magnetic instruments, designed for the expedition now preparing by 
the government of the United States, for scientific researches in the 
southern hemisphere, I cannot anticipate a more favorable opportu- 
nity of turning to good account the information of which M. Han- 
steen has so liberally made me the depository. Since analogy would 
lead us to expect that a corresponding system of magnetism prevails 
on the two hemispheres of our globe, a knowledge of the arrange- 
ment of the system in the northern hemisphere, may prove an im- 
portant guide and direction for corresponding researches in the 
southern; whilst the example of M. Hansteen’s undertaking may 
stimulate, and his success is well calculated to encourage, those who 
are about to enter on a career honorable alike to themselves, and to 
the government under whose instructions they are employed. 
For some years past it has been the opinion of several persons, 
who have attentively considered the subject, that a knowledge of the 
general system of the magnetism of our globe is more likely to be 
attained by experiments on the relative intensity of the magnetic at- 
traction in different parts of the earth’s surface, than by observations 
on the dip or variation of the needle. In conformity with this opin- 
ion, M. Hansteen, (without neglecting to observe, on all occasions, 
the three phenomena conjunctively,) has applied himself especially 
to trace the lines connecting these places on the globe, where a 
needle, — suspended in the magnetic direction, and drawn a cer- 
tain number of degrees from rest, is found to make an equal number 
of vibrations around its point of rest in a given time. It was to be 
expected that these lines of equal i intensity would arrange themselves 
systematically around the point or points in each hemisphere, where 
the intensity was greatest; and on the supposition that two such 
points would be found, opposite to each other on the globe, one in 
the northern and the other in the southern hemisphere, that the iso- 
