138 Royal Society, 



For a long time in the history of terrestrial magnetism the varia- 

 tion alone was attended to. The consideration of the dip was then 

 superadded ; but the observation of this element being more difficult 

 and delicate, our knowledge of the actual and past state of the dip 

 over the earth's surface is lamentably deficient. It has lately ap- 

 peared, however, that this element can be observed with considerable 

 approximation, though not with nicety, at sea, so that no reason sub- 

 sists why materials for a chart of the dip analogous to that of varia- 

 tion should not be sj^stematically collected. Lastly, the intensity 

 has come to be added to the list of observanda ; and from the great 

 facility and exactness with which it can be determined, this branch 

 of magnetic knowledge has in fact made most rapid progress. 



These three elements, the Horizontal Direction, the Dip, and 

 the Intensity, require to be precisely ascertained before the magnetic 

 state of any given station on the globe can be said to be fully deter- 

 mined. Nor can either of them, theoretically speaking, be said to 

 be more important than the othei"s, though the direction, on account 

 of its immediate use to navigators, has hitherto had the greatest 

 stress laid upon it, and been reduced into elaborate charts. A chart 

 of the lines of total intensity has been recently constructed by Major 

 Sabine. 



All these elements are, at each point, now ascertained to be in 

 a constant state of fluctuation, and affected by those transient and 

 irregular changes wliich are above alluded to ; and the investigation 

 of the laws, extent, and mutual relations of these changes is now 

 become essential to the successful prosecution of magnetic discovery, 

 for the following reasons. 



1st. That the progressive and periodical being mixed up with 

 the transitory changes, it is impossible to separate them so as to ob- 

 tain a correct knowledge and analysis of the former, without taking 

 express account of and eliminating the latter, any more than it would 

 be practicable to obtain measures of the sea-level available for an 

 inquiry into the tides, without destroying the irregular fluctuation 

 produced by waves. 



'2ndly. That the secular magnetic changes cannot be concluded 

 from comparatively short series of observations without giving to 

 those obser\'ations extreme nicety, so as to determine with perfect 

 precision the mean state of the elements at the two extremes of the 

 period embraced, which, as already observed, presupposes a know- 

 ledge of the casual deviations. 



3rdly. It seems very probable that discordances found to exist 

 between results obtained by different observers, or by the same at 

 different times, may be, in fact, not owing to error of observation, 

 but may be due to the influence of these transitory fluctuations in 

 the elements themselves. 



4thly and lastly. Because the theory of these transitory changes 

 is in itself one of the most interesting and imjDortant points to which 

 the attention of magnetic inquirers can be turned, as they are no 

 doubt intimately connected with the general causes of terrestrial 

 magnetism, and wUl probably lead us to a much more perfect know- 

 ledge of those causes than we now possess. 



