66 INTRODUCTION. 



can scarcely balance his needle, because tlie effects 

 of gravity always mingle with those of a weak 

 magnetism, which inheres in every piece of 

 wrought iron. On long voyages, it might be use- 

 ful to reverse the poles at every change of 10° or 

 20° of latitude, and to correct in proportion the 

 intervening observations. On a voyage to the 

 north pole it will suffice to perform this operation, 

 if possible, at the commencement of the voyage, 

 and at the most northerly observation. It is, like- 

 wise, a very good way to take two or more needles, 

 and make with each of them the two observations 

 mentioned under N° 1. 



There is another method of determining the dip 

 of the magnetic needle, by the estimation of the 

 magnetic power of the earth, in different latitudes, 

 by the dipping needle, that is, by counting the vi- 

 brations which the needle makes, first, in the mag- 

 netic meridian, and then, in the direction from 

 east to west, in a given time ; for instance, in ten 

 minutes. Let the number of the first be M, that 

 of the latter, P ; then, according to Laplace, Sin. I 

 = Ll. vSupposing we had found in Kamtschatka 

 the number of vibrations which the dipping needle 

 makes in the direction of the magnetic meridian, 

 or M, in ten minutes, to be = 250, and the num- 

 ber of vibrations which it makes in a direction per- 

 pendicular to the magnetic meridian, or P = 238, 

 then we have the Sine of the dip I = — ', and the 

 calculation is as follows : 



