292 Royal Society. 



restrial magnetism in a given place, viz. Declination, Inclination, and 

 Intensity, the first soonest engaged the attention of philosophers, the 

 second much later, and the third has only at a very recent period 

 become an object of investigation and experiment. This progressive 

 interest is chiefly to be accounted for by the circumstance, that while 

 the variation of the compass offered the greatest interest, as applied 

 to the purposes of navigation and geodesic operations, the dip was 

 looked upon as more nearly allied to it than was the intensity of ter- 

 restrial magnetism. To the natural philosopher, those three elements 

 are absolutely of the same import, inasmuch as our knowledge of the 

 general system of terrestrial magnetism will ever remain imperfect, 

 until an equal share of attention has been bestowed on its separate 

 branches. 



For the first light thrown upon this subject we are indebted to the 

 Baron Humboldt, whose attention was particularly directed to it 

 during all his travels, and who has furnished a considerable series of 

 observations, from which the gradual increase of this intensity, from 

 the magnetic equator of the earth towards the magnetic poles, has 

 been deduced. Many observers have since followed the footsteps of 

 that great naturalist ; and almost every part of the world to which, in 

 recent times, travellers have penetrated, has furnished its quota of 

 materials, from which already Hansteen (to whom this branch of phi- 

 losophical inquiry is under great obligation) has been enabled to 

 attempt the construction of an iso-dynamical chart. 



The mode adopted in all these observations consists in disturbing the 

 equilibrium of one and the same magnetic needle in places the compa- 

 rative intensity at which is to be determined, and in exactly measuring 

 the duration of its oscillations. This duration is indeed, cceteris paribus, 

 dependent on the magnitude of the arc; but in such a manner, that 

 however small the arc becomes, it still approaches a determined limit, 

 loosely called the duration, and to which, the arc of oscillation being 

 known, the really observed duration may easily be reduced. The in- 

 tensity of terrestrial magnetism is thus inversely proportional to the 

 square of the duration of oscillation of the same needle, or directly 

 so to the square of the number of oscillations in a given time ; and 

 the result relates to the whole force, or to the horizontal portion of it, 

 according as the needle has been caused to vibrate, in the plane of 

 the magnetic meridian, round a horizontal axis, or, in a horizontnl 

 plane, round a vertical axis. 



It is evident that the admissibility of this method entirely rests on 

 the assumption of the unchanged magnetic state of the needle em- 

 ployed. If a properly-magnetized and carefully-preserved needle of 

 good hardened steel be made use of for the experiments, and these 

 do not take up too long a space of time, the danger to be apprehended 

 from such alteration may not, indeed, be considerable ; and the ob- 

 server may rest the more satisfied in this respect, if, on returning to the 

 first place, he find the time of the vibration to be the same ; but expe- 

 rience teuches us that this result cannot by any means be calculated 

 upon 5 neither can it be denied, that in resorting to such a proof we 

 are only reasoning in a circle. It was known indeed, long ago, that 



