On a Method for computing Magnetic Declination. 183 



in attempting to refer the fects observed by Mr. Faraday* to 

 the optical axis, inverts the right course of proceeding ; the 

 attraction or repulsion of this axis being a secondary result, 

 depending first of all upon the magnetism or diamagnetism 

 of the substance, and secondly upon the manner in which 

 either force is modified by the peculiar structure of the crystal. 



The conducting power, so to speak, of Iceland spar for 

 both magnetism and diamagnetism appears to be in directions 

 perpendicular to the lines of cleavage. If these views be cor- 

 rect, the optical axis can no longer be regarded as the prime 

 agent in the production of the phaenomena which we have 

 been considering; we shall no longer seek the explanation of 

 new facts in the hypotheses of new forces, but rather in modi- 

 fications of the old. 



Marburg, January 1850. 



XXV. Further Illustrations of a Method for computing Mag- 

 netic Declination, on the principle proposed by Professor 

 Gauss. By Samuel BESwicKf. 



IN my former communication I gave two illustrations of a 

 new method whereby the mean declinations at Greenwich 

 and St. Helena, for the past year 1849, were computed with 

 the utmost exactitude. But the utility of this new method 

 consists, not only in its simplicity and precision, but also in 

 its applicability to all places in the two hemispheres, and to 

 all epochs, past, present, and to come. To obtain the decli- 

 nation all over the world, at an epoch so distant as the time 

 of Columbus (see the first illustration), or of one hundred, or 

 even one thousand years in the future, there needs no addi- 

 tional data, nor any alteration in the method : there are only 

 the same number of items : and the result is obtained equally 

 easy, and in the same time, as .if the declination was being 

 obtained for the present year. Time and place make no dif- 

 ference in the length or nature of the computation. 



If, therefore, this paper be favoured with the attention of 

 the Committee appointed to conduct the. co-operation of the 

 British Association in the system of Simultaneous Magnetical 

 and Meteorological Observations,! trust they will kindlynotice, 

 not only the above observations, but also the following ad- 

 vantageous characteristic of this new method over all others, 

 viz. to compute the declination all over the world, for any 

 epoch, however remote the time may be, not a single observa- 



* Phil. Mag., Jan. 1849, p. 75. 

 t Communicated by the Author. 



