Terrestrial Magnetism. 1 45 



Messrs. Dunlop and Sulivan are discussed in this view ; and the 

 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 Navy, 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 

 Pile/' 



March 26. — The reading of a paper, entitled, " Researches in 

 Electricity, Seventeenth Series : on the source of power in the Vol- 

 taic Pile." By Michael Faraday, Esq., D.C.L., F.R.S., &c, was 

 resumed and concluded. 



In a postscript, the author states that he has since found a pas- 

 sage in Dr. Roget's treatise on Galvanism, in the Library of Useful 

 Knowledge, published in January 1829, in which the same argu- 

 ment respecting the unphilosophical nature of the contact-theory is 

 strongly urged. 



" Were any further reasoning necessary to overthrow it, (namely, 

 the voltaic theory of contact) a forcible argument might be drawn 

 from the following consideration. If there could exist a power, 

 having the property ascribed to it by the hypothesis, namely, that 

 of giving continual impulse to a fluid in one constant direction, 

 without being exhausted by its own action, it would differ essentially 

 from all the other known powers in nature. All the powers and 

 sources of motion with the operation of which we are acquainted, 

 when producing their peculiar effects, are expended in the same 

 proportion as those effects are produced ; and hence arises the im- 

 possibility of obtaining by their agency a perpetual effect, or, in 



Phil. Mag. S. 3. Vol. 17. No. 108. Aug. 1840. L 



