213 



value of results obtained under circumstances of due precaution is 

 pointed out by their success. 



The position of the lines on the land portion of the map is derived 

 from 120 determinations in various parts of Europe, Africa, and 

 America, between the years 1834 and 1839, of which about the 

 half are now first communicated. 



The series of Messrs. Dunlop and Sulivan contain also observa- 

 tions of the magnetic intensity made at sea ; Mr. Dunlop's by the 

 method of horizontal vibrations, and Lieut. Sulivan's by the instru- 

 ment and method devised by Mr. Fox. The degree of precision 

 which may be obtained by experiments thus conducted, is shown by 

 the comparison of these observations with each other, and with the 

 isodynamic lines previously derived from observations made on 

 land. 



The first section of this paper concludes with discussions on the 

 relative positions of the lines of least intensity and of no dip, and 

 of the secular change which the latter line has undergone in the ten 

 years preceding 1837. 



In the second section, the observations of Mr. Dunlop are combined 

 with recent observations on the coasts of Australia, by Captains 

 Fitz Roy, Bethune, and Wickham, of the Royal Nzlyj, to furnish a 

 first approximation to the position and direction of the isodynamic 

 lines over that portion of the Indian ocean which is comprised be- 

 tween the meridian of the Cape of Good Hope and New South 

 Wales. 



A paper was also in part read, entitled " Experimental Re- 

 searches in Electricity, seventeenth series." By Michael Faraday, 

 Esq. D.C.L., F.R.S., &c., On the source of power in the Voltaic 

 PHe. 



March 26, 1840. 



The MARQUIS of NORTHAMPTON, President, in the Chair. 



• The reading of a paper, entitled " Researches in Electricity, 

 Seventeenth Series : on the source of power in the Voltaic Pile." 

 By Michael Faraday, Esq., D.C.L., F.R.S., &c., was resumed and 

 concluded. 



In this series, the author continues his experimental investigation 

 of the origin of electric force in the voltaic pile. Having found 

 abundant reason, in the experiments already described, to believe 

 that the electricity of the pile has its origin in the chemical force of 

 the acting bodies, he proceeds to examine how the circumstances 

 which can affect the affinity of substances for each other, influence 

 their power of roducing electric currents. First, with relation to 

 heat : — circuits were made of a single metal and a single fluid, and 

 these were examined with a view to ascertain whether, by applying 



