Magnetism of the United States audits Vicinity. 197 



five observations of Major Graham, the southern half of this coun- 

 try is still an untrodden field. Cannot some volunteers be found 

 to do for the south, what has already been done for the north ? 



The observations of the magnetic intensity, possess, if possible, 

 a greater interest than those of the dip. The region of greatest 

 intensity for the northern hemisphere has now been surveyed ; 

 and what is the result ? The greatest intensity anywhere ob- 

 served in the northern hemisphere is 2*099 ; the intensity in Peru, 

 as observed by Humboldt, being called unity. This intensity in 

 Peru was formerly supposed to be the least which would be found 

 in any part of the globe ; but an intensity has since been found 

 as low as -743 ; so that the greatest magnetic intensity now 

 known upon any part of the globe, is not quite three times the 

 least. It was formerly attempted to explain the phenomena of 



terrestrial magnetism by supposing an enormous magnet to be 



situated within the earth, having one pole in the continent of 

 North America, and the other south of New Holland. But if the 

 poles of such a magnet approached very near the surface of the 

 earth, the force of its attraction should there be many thousand 

 times greater than it is at the equator. If it is proposed to ex- 

 plain the observed phenomena by a permanent magnet situated 

 within the earth, then we must conclude that its poles are so far 

 below the surface that we are very little nearer them at one point 

 of the earth's surface than at another ; we must place the poles 

 fall three thousand miles below the earth's surface. Then to 

 explain the high intensity observed in Siberia, we must introduce 

 a second magnet whose axis makes a considerable angle with the 

 former. But all this has been attempted in vain. Such hypoth- 

 eses will serve to explain the observed phenomena only in a 

 very rude and inadequate manner. We must wait patiently until 

 { he entire globe has been surveyed, and then we may expect to 

 ^e the Newton who shall develop the grand law of terrestrial 

 ma gnetisrn. Within a few years we have made rapid progress 

 towards obtaining the requisite materials for this research. For 

 &e most interesting part of this continent, the survey may be pro- 

 nounced nearly complete. If we call the intensity under the 

 equator in South America unity, this intensity slowly increases 

 5 s we travel northward, amounting to 1-4 among the West India 

 Islands, while at New York it amounts to 1-8. Lines passing 

 trough all those places on the earth where the intensity is the 

 same, are called lines of equal intensity, or isodymmtc lines. 

 *he particular object of the expedition undertaken by Lieut. 

 Ufr oy, was to trace out the line of 1-8, and determine the posi- 

 t^f* of the point of greatest intensity upon this continent. This 

 ob Ject has been accomplished. The isodynamic line of 18 is a 

 closed curve of an oval shape, extending somewhat below lat. 

 40 ° in the longitude of Cincinnati, and reaching off nearly to 



