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MAGNETISM. 



period in the history of the mariner's compass. In fact, what may be not im- 

 properly called magnetic geography has been, and still is, a subject of profound 

 interest, as well to the merchant as to the philosopher. 



It has been already stated that the discoverer of the dip found that at London 

 a magnetic needle, free to move on an axis perpendicular to the magnetic me- 

 ridian, presented its north pole downward, forming an angle of above seventy- 

 one degrees. If the instrument be carried northward, it is found that the dip 

 gradually increases ; and, on reaching a certain region near the pole, the nee- 

 dle would become vertical, the dip being then ninety degrees, and its north 

 pole pointing downward. At such a place, the common compass needle, mov- 

 ing on a vertical support, would lose its directive power, and rest indifferently 

 in any position. A place where these effects would be produced is called a 

 northern magnetic pole. 



If, on the other hand, the dipping needle were carried toward the equator, 

 the magnitude of the dip would be gradually diminished, until, on arriving at a 

 certain region near the equator, the needle would become horizontal, and the 

 dip would become nothing ; and if the dipping needle could be carried round 

 the globe, always following such a course as would allow it to retain its hori- 

 zontal position, its course traced on the globe would be the magnetic equator. 



The magnetic equator does not coincide with the equator of the globe, nor 

 is it a great circle of the earth. It never departs from the equator, however, 

 more than twelve or thirteen degrees. 



If, after passing the magnetic equator, the dipping needle be carried south- 

 ward, its south pole will dip or be directed downward ; and this dip will in- 

 crease in magnitude as the needle approaches the south pole. A place near 

 that pole, where the needle becomes vertical, is a southern magnetic pole. 



The first national project to determine the elements of magnetic geography 

 was undertaken by the British government about the year 1700, when the cele- 

 brated Halley was commissioned to make a voyage with the view to collect 

 the necessary observations. The results obtained by him were, however, de- 

 prived of the chief part of the advantages which ought to have attended them, 

 because of the absence of uniformity in his instruments, and the neglect of 

 making proper comparisons of them with others. 



Since that period, observations have been made and recorded in all exten- 

 sive voyages, and the data for the determination of the elements of this part 

 of physical geography have been greatly augmented. 



