146 Magnetism of the Earth. 



The observations that M. Hansteen has akeady made in the first 

 year of his undertaking, and the conclusions which they establish in 

 regard to the direction assumed by the isodynaraic 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 

 proper, " especially when it may encourage to new undertakings, 

 and accordingly forward th6 science." Having been requested by 

 you to superintend the construction in this country, of a part of tlie 

 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 tliese places on the globe, where a 

 needle, freely 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 intensity would arrange tlieraselves 

 systematically around tlie 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- 



