Professor De la Rive on Electricity. 269 



north and south. Since the publication of M. de Humboldt'^s 

 observations about the beginning of the present century, a great 

 number of travellers and naturalists have made and published 

 observations of a similar kind. Of these reseaches, so assidu- 

 ously prosecuted, or still going forward, we can cite only a very 

 few of the most remarkable. 



The numerous voyages of discovery which have been under- 

 taken within the last few years, and especially towards the North 

 Pole, have enabled M. Hansteen, from the observations of navi. 

 gators, and from those which he himself has made, especially in 

 Siberia, to draw ingenious conclusions concerning the position 

 of the magnetic poles, and the isodynamic lines, that is, those 

 lines which pass through all those points where the magnetic in- 

 tensity is the same. The arrangement of these lines, which, 

 for a long time, was imagined to be that of small circles nearly 

 parallel to the magnetic equator, is not so regular ; on the con- 

 trary, it seems to indicate the existence, in each hemisphere, of 

 two distinct magnetic poles, each of which forms a centre, around 

 which the isodynamic curves are distributed sufficiently regular- 

 ly. These poles are situated, so far as the northern hemisphere 

 is concerned, the one in Hudson's Bay, and the other in Siberia : 

 the latter of these appears to be somewhat more feeble than the 

 former. 



The observations of Captains Sabine and Freycinet, and es- 

 pecially those of Captain Duperrey have supplied a great number 

 of data, as valuable for their great accuracy, as for the choice 

 localities in which they have been made. Those of Captain Du- 

 perrey, taken in the voyage of discovery which he made in La 

 Coquilky during the years 1822-23-24 and 1825, have enabled 

 us to determine the real position of the magnetic equator, viz. of 

 the curve which passes through all the points where the needle 

 exhibits no inclination or dip, but remains perfectly horizontal. 

 It is known that the general direction of this curve, is that of a 

 great circle of the terrestrial sphere, forming an angle of some 

 degrees with the terrestrial equator. This was known as early 

 as 1780, and more accurately determined by M. Morlet, in con- 

 sequence of the observations of M. de Humboldt. The obser- 

 vations of Captain Duperrey shew, that it is liable to a movement 

 of change, which, from year to year, progressively transports it 



VOL. XVII. NO. XXXII. APRIL 1834. T 



