121 



Le Monnier, in his work on the Laws of Magnet- 

 ism*, observes how desirable it would be, to know 

 the relation between the oscillations of the same 

 needle at Peru, and in the north of Europe ; but 

 a note added to his magnetic planisphere^ shows, 

 that he had not a very accurate idea of the causes, 

 which modify the intensity of the total force. 

 According to Cavendish ;{;, this intensity must be 

 the same over the whole surface of the Globe ; 

 and the opinion of this g reat natural philosopher 

 must have had a very considerable influence on 

 those, who had not had an opportunity of inter- 

 rogating nature by direct experiments. 



In this state of uncertainty, the Academy of 

 Sciences at Paris very earnestly exhorted Mr. 

 La Pe rouse to make, during the course of his 

 voyage round the world, experiments on the in- 

 tensity of the magnetic forces. " It is known," 

 say the commissioners in the instructions com- 

 municated to the naturalists engaged in the ex- 

 pedition, " that at Brest, Cadiz, TenerifFe, Goree 

 on the coast of Africa, and Guadaloupe, the in- 

 tensity is sensibly the same. It would be inter- 

 esting to repeat these experiments, reckoning 

 the magnetic force by the duration of the oscil- 

 lations of a good dipping needle, at sea, in very 



* Introduction, p. 25. 



f M6m. de Paris, 1786, p. 43. 



% Phil. Trans., 1778, p. 390. 



§ Voyage de La Perouse, t. i, p. 160. 



