1835.] Magnetic Intensity of the Earth. 61 



two points which appear to be the centres of the magnetic 

 force. If we call these magnetic poles, then the earth 

 possesses four poles. If the magnet is placed in the north- 

 ern hemisphere to the westward of one of these points, the 

 north pole is directed to the eastward ; and if to the east- 

 ward, it points to the west. When the intensity of the 

 degree of the inclination is investigated round the pole of 

 the earth in parallel circles, we find that it continues to 

 increase until the meridian is attained in which one of these 

 points lies, where it reaches its maximum. 



An important point to determine with regard to the mag- 

 netism of the earth, is the degree of its strength or intensity. 

 For a long time it was disputed by the learned whether the 

 magnetic force was equally strong over the whole earth's 

 surface, or was different in different places. Mallet who 

 was sent in 1769, to observe the transit of Venus at Ponoi 

 in Russian Lapland, (Nov. Comm. Petrop. torn, xivj allowed 

 magnetic needles of 6" to move through an arc of 20° to 24°, 

 and found' that for the four first oscillations 14" less were 

 required than at St. Petersburg. The difference, however, 

 was so small that no inference could be drawn from it. 



The French Academy gave instructions to the mathema- 

 ticians along with La Perouse in 1785 — 88, to investigate 

 the subject, and Lamanon* in a letter from St. Catharina 

 stated that he had made a number of observations; but 

 these as well as his subsequent ones were unfortunately 

 lost by the disastrous shipwreck. Captain (afterwards Ad- 

 miral) De Rossel made observations between 1790 and 1793 

 at Brest, Teneriffe, Amboyna, Java and Van Dieman's Land, 

 and ascertained, that if we reckon the intensity at Amboyna 

 in the neighbourhood of the equator unity, then it will be 

 at Teneriffe 1*3, at Brest 1*4, and at Van Dieman's Land T6. 

 Hence, it is inferred that the magnetic force at the equator 

 is smaller than towards the pole. But as there is a difference 

 of 5° of latitude between Brest and Van Dieman's Land, 

 the former being in 48° and the latter in 43°, and as the 

 intensity is greater at the latter than at the former place, 

 it is evident that the intensity does not alone depend on the 



* This ill-fated individual who filled the situations of Natural Philosopher, 

 Mineralogist, and Meteorologist, was murdered hy the natives of Macuna, 11th 

 December, 1787. See Dillon's Voyage — Edit. 



