Ill 



the magnetic meridian makes with the meridian 

 of the place ; thirdly, the horary variations of 

 the variation ; fourthly, the intensity of the 

 magnetic forces, measured by the duration of 

 the oscillation of a horizontal or vertical needle *. 

 The extent of the surface of the Globe, in which 

 I have been enabled to determine the magnetic 

 phenomena with the same instruments, and 

 employing similar methods, was one hundred 

 and fifteen degrees in longitude, and is com- 

 prised between fifty-two degrees north and 

 twelve degrees south latitude. This vast re- 

 gion is so much the more interesting, as it is 

 traversed by the magnetic equator ; so that the 

 point where the dip is nought, having been de- 

 termined by land, and by astronomical means, 

 we may, with respect to the two Americas, con- 

 vert, with precision, the terrestrial latitudes into 



* When we measure the intensity by the oscillations of a 

 needle in a horizontal plane, we must correct the results by 

 the dip observed in the same place. This correction is un- 

 necessary, when we employ a dipping needle, which oscillates 

 in the plane of the magnetic meridian. The number also of 

 these oscillations, compared with the number of those which 

 the same needle makes in a plane perpendicular to the mag- 

 netic meridian, determines the dip of the place. This method 

 of finding the dip by an instrument without a divided limb 

 affords more precision near the magnetic equator, than in the 

 northern regions : it served to verify the exactness of a part 

 of my observations, published before my return to Europe by 

 Mr. de La Lande. (Journal de Phys., vol. xlix, p. 429.) 



