/ 



114 



and worthy of exercising the sagacity of geome- 

 tricians. 



Though our passage from Corunna to Cumana 

 lasted thirty-seven days, I could collect, during 

 this space of time, only twelve good observations 

 of the magnetic dip. I had caused an addition 

 to be made to Borda's compass, by an able artist 

 of Madrid, Mr. Megni£, of a suspension with a 

 double movable ring, like that known under the 

 name of Cardan's suspension. By these means 

 the instrument might be tied by a very long cord 

 to a part of the poop, which appeared nearly free 

 from iron, and where small portions of this metal 

 were very equally distributed. I ascertained the 

 advantages of this position by determining the 

 dip during a dead calm, on the deck, and in se- 

 veral parts of the vessel below. During the 

 course of those observations, I found the direc- 

 tion of the magnetic meridian in seeking the 



of November, 1812, 68<> 42'. The first of these two experi- 

 ments was made by Mr. Arago and myself; the second by 

 Mr. Arago alone. The particular observations did not differ 

 three or four minutes. The poles of the needle were changed 

 several times; and all imaginable precautions were employed, 

 in the use of Borda's compass, to avoid errors. From these 

 observations it results, that from 1775 to 1805 the dip dimi- 

 nished at London 4 7 V yearly : at Paris the annual dimi- 

 nution, from 1798 to 1812, was 4' 54". I should think it 

 hazardous to go back to anterior times, when the instruments 

 were too imperfect, and when observers employed too little 

 nicety in their magnetic experiments. 



