273 Proceedings of the British Association 



These instruments had been described at former meetings: but it was 

 deemed advisable to subject them to a practical trial to test their effi- 

 ciency as a means of recording the magnetic phenomena in any part 

 of the world where science might require them to be better known. 

 Before, however, it could be known in what degree they might be use- 

 ful either as auxiliary or as supersedieg altogether the method of ob- 

 servation previously in use, it was necessary that three instruments 

 should be provided, — one for the variations of declination and two for 

 those of the horizontal and vertical components of the forces. The 

 limited funds at the disposal of the Association for the purposes of the 

 Kew Observatory barely enabled them to complete two of these instru- 

 ments between the years 1846 and 1850; and the third must have yet 

 been wailed for had not the President and Council of the Royal Society, 

 seeing the importance of the object, appropriated a portion of the sum 

 placed at their disposal by government to this purpose. The three in- 

 struments being now completed, Mr. Ronalds proposed to the Council 

 that a six months' trial should be made of the working of these instru- 

 ments precisely as they would be in an observatory, and that every 

 item of expense incurred in that practical trial should be carefully 

 noted and faithfully reported. The Council of the Royal Society hav- 

 ing approved of this trial, a grant was made from the donation fund of 

 lOOZ. The six months' trial commenced in April last, and when it 

 shall be completed a full report will be made to the Council of the 

 Royal Society. In the mean time it was thounrht that a brief report of 

 the progress so far made would be acceptable. 



In Mr. Ronald's instruments the magnetic variations are recorded 

 either on silvered plates or on prepared paper. The silvered plates 

 have the advantage of greater sensibility, and in consequence require 

 a less time of exposure to the light, — so that movements of a more 

 rapid character or more transient duration may be recorded by them; 

 also, they produce more sharply defined traces; they are free from the 

 defects arising from inequalities of surface, and the stretching and 

 shrinking of the paper by being welted and dried. The paper on the 

 contrary, has the advantage that the original tracings and records of 

 the variations can be preserved, — a matter impossible with the plates, 

 except in some very rare and interesting cases. In coming to a just 

 conclusion in this comparative estimate of the merits of each method, 

 the practical object to be attained should be kept in view. No doubt 

 a mere inspection of the actual tracings as shown either on the plates 

 or the paper possesses an interest both to the uninstructed and to those 

 more familiar with the phenomena; and amidst the great variety of 

 them some may have light thrown on them by a close inspection and 

 consideration of the trace itself. But as in astronomy the great advance 

 of the science has followed the combination of measuring apparatus 

 with optical power, — so in terrestrial magnetism the various periodical 

 and other causes which in their joint action produce the variations, re- 

 quire that the traces of them should undergo tabulation as an indis- 

 pensable preliminary to their practical application. In Mr- Ronald's 

 plan the tracings when taken on the silvered plates are tabulated with 

 tolerable rapidity, and are then copied by hand with a graver^s tool on 

 sheets of transparent gelatine paper, — an operation requiring about a 



