THE 



PHILOSOPHICAL MAGAZINE 

 AND JOURNAL. 



I. On Terrestrial Magnetism. 



To Mr. Tilloch, 



Sir, — It happens but seldom that a single chapter of a trea- 

 tise on any subject is so completely disengaged from the rest of 

 the work as to allow of its being published separately. There are 

 usually so many references, and dependences of one part on an- 

 other, that it will not admit of being detached. The following, 

 however, is an exception. It forms the tenth chapter of the 

 third volume of the Treatise on Experimental and Mathematical 

 Philosophy, lately published at Paris by M. Biot of the Insti- 

 tute &c. : a work of very distinguished merit, having a consi- 

 derable portion of its contents treated in a manner altogether 

 new, and containing deductions from the experiments of a most 

 interesting and important nature. 



The study of terrestrial magnetism has latterlv declined very 

 much in this country; which is readily accounted for by the un- 

 certainty attending most of its conclusions. It excited great 

 interest, and indeed was one of the principal objects of pursuit, 

 in England, about the end of the seventeenth and the com- 

 mencement of the eighteenth century. 



Amongst a number of persons who have exerted themselves 

 in this department of knowledge we are particularly indebted to 

 Dr. Halley, who recommended the matter to the earnest atten- 

 tion of Government, and after unwearied solicitatiim obtained per- 

 mission for a ship to be fitted out and sent with him on a voyage 

 for the purpose of making magnetical observations. In this he re- 

 peatedly traversed the Atlantic Ocean, and enriched various parts 

 of nautical science with the fruits of his labour, as is abundantly 

 testified by the numerous papers which he afterwards published 



Vol. 49. No. 225. Jan, 1S17. A 2 in 



