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XlVlll 



with them on terms of the most intimate 

 friendship, I have had the means of con- 

 sulting them daily on objects of chemistry, 

 natural history, and several branches of the 

 mathematics. I have already mentioned 

 in the collection of my astronomical ob- 

 servations what I owe to the friendship of 

 Mr. Arago, who, after having terminated 

 the measure of the meridian of Spain, has 

 been exposed to so many dangers ; and 

 who unites the talents of an astronomer 

 with those of a geometrician and a natu- 

 ralist. At the period of my return I 

 discussed particularly with Mr. Gay-Lussac 

 the different phenomena of meteorology 

 and physical geology, which I had amassed 

 in my journey. For eight years past we 

 have usually dwelt under the same roof in 

 France, Germany, and Italy ; we have 

 witnessed together one of the great erup- 

 tions of Vesuvius ; and have joined our 

 labours on the chemical analysis of the 

 atmosphere, and the variations of terrestrial 

 magnetism. I have been enabled to avail 

 myself of the profound and ingenious views 

 of this chemist, in correcting my ideas re- 



