320 Royal Society : — 



limited for the most part to one only of the three elements required 

 in a complete investigation. When the German Association com- 

 menced its operations, the Declination was the sole element for which 

 an apparatus had been devised capable of recording its variations 

 with the necessary precision. To meet the deficiency in respect to 

 the horizontal component of the magnetic force, M. Gauss constructed 

 in 1837 his bifilar magnetometer, which was employed at Gottingen 

 and at some few of the German stations, concurrently with the 

 Declinometer, in the term observations of the concluding years of 

 the Association. But an apparatus for the corresponding observa- 

 tion of the vertical portion of the Force was as yet wholly wanting ; 

 without such an apparatus as a companion to the bifilar, no deter- 

 mination could be made of the perturbations or momentary changes 

 of the magnetic Dip and Force : and without a knowledge of these no 

 satisfactory conclusion in regard to the real nature, amount and 

 direction of the perturbing forces could be expected. The ingenuity 

 of Dr. Lloyd supplied the desideratum by devising the vertical force 

 magnetometer, which, with adecpiate care, has been found scarcely, if 

 at all, inferior to the bifilar in the performance of its work. The 

 scheme of the British Observatories was thus enabled to comprehend 

 all the data required for the investigation of the casual disturbances, 

 whether that investigation was to be pursued as before by concerted 

 simultaneous observations at different stations, or, as suggested in the 

 Report, by the determination of the laws, relations and dependencies 

 of the disturbances at individual stations obtained independently and 

 tvithout concert loith other observers or other stations. Thus, in 

 reference to these particular phenomena, the British system was both 

 an enlargement and an extension of the objects of the German Asso- 

 ciation ; but it also embraced within its scope the determinations 

 with a precision, not previously attempted, of the absolute values of 

 the three elements, and of the periodical and progressive changes to 

 which they are subject ; premising however, and insisting with a 

 sagacity which has been fully justified by subsequent experience, on 

 the necessity of eliminating in the first instance the effects of the 

 casual and transitory variations, as an indispensable preliminary to a 

 correct knowledge and analysis of the progressive and periodical 

 changes. A further prominency was given to investigations into the 

 particular class of phenomena which form the subject of this paper, 

 by the declaration that " the theory of the transitory changes is in 

 itself one of the most interesting and important 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 will probably lead us to a much more perfect knowledge of these 

 causes than we now possess." 



The instructions contained in the Royal Society's Report for the 

 adjustments and manipulation of the several instruments provided for 

 these purposes were clear, simple and precise. In looking back upon 

 them after the completion of the services for which they were 

 designed, it is impossible to speak of the instructions otherwise than 

 with unqualified praise. But the guidance afforded by the instruc- 

 tions terminated with the completion of the observations. To have 



