140 Royal Society. 



who were employed in them, makes it the duty of the person charged 

 with their superintendence, to spare no pains to place before the 

 Fellows, on suitable occasions, the results of researches designed to 

 obtain a foundation of facts, on which a correct theory of the mag- 

 netic variations might be framed, and an insight be gained into the 

 nature of the physical agency by which they are produced. 



" In this first stage of scientific inquiry, when we have only the 

 phsenomena themselves to guide us in their classification, or to indi- 

 cate by apparent correspondences the existence of some causal con- 

 nexion of which we have no other knowledge than that which the 

 observations themselves may afford, the first difficulty to be met 

 consists, in disentangling from the complication in which the mag- 

 netic variations proceeding from different causes first present them- 

 selves, the effects which may appear to be due to certain amongst 

 them ; and in presenting these in some methodical order or arrange- 

 ment, which may best assist the physicist or the mathematician in 

 his conception of the problem or problems, to the solution of which 

 he may desire to apply himself. 



" The first and most obvious separation of the magnetic variations 

 is into those which are presented at one time at diflferent parts of 

 the earth's surface, and have special reference therefore to space ; and 

 those which present themselves at different times at one and the 

 same place, and have special reference therefore to time. It is the 

 object of magnetic surveys to collect the facts of the first, and of 

 magnetic observatories the facts of the second, of these primary divi- 

 sions. The present communication belongs to the second, and re- 

 gards the variations depending upon time at a single station (St. 

 Helena). 



" Still, however, the phsenomena even at a single station are too 

 complicated for ready comprehension, and stand in need of further 

 subdivision. This is most satisfactorily effected by the customary 

 separation into three classes, or elements as they are frequently 

 termed, the Declination, the Inclination, and the Intensity of the 

 Directive Force. The discussion is limited on the present occasion 

 to a single element, the Declination, and to a portion only of the 

 results obtained by the observations of that element at St. Helena." 



After premising a description of the instrument with which the 

 observations were made, and of the mode of observing and of record- 

 ing the observations, which is omitted here because it may be found 

 in the Introduction to the fiirst volume of the ' St. Helena Magnetical 

 Observations,' the author proceeds to the conclusions which he de- 

 sires to notice, and to the manner in which these have been obtained, 

 which we follow, by adopting, as nearly as may be convenient, his 

 own words. 



"Before we attempt to examine those periodical variations, or 

 fluctuations about a mean value, which, from their having for periods, 

 for example, the solar year or the solar day, we naturally refer to 

 causes depending in some way upon the earth's place in its orbit re- 

 latively to the sun, or to the earth's revolution round its axis, it is 

 desirable to examine, and if practicable, to eliminate the effects of a 

 variation which we have reason to believe belongs intrinsically to the 



