124 



the regulaiity of an isochronous pendulum. In 

 general the rate, which the magnetic variations 

 and the dip foiow in different longitudes, ap- 

 pears more regular in the basin of the sea, than 

 on continents where the inequalities of the sur- 

 face, and the nature of the rocks of which the 

 surrounding mountains are composed/ cause 

 frequent anomalies. As to the duration of the 

 oscillations, it sometimes undergoes irregulari- 

 ties, even in the middle of the seas* ; no doubt 

 because the stratum of water is too thin, to pre- 

 vent the needle from being affected by the un- 

 equal distribution of the magnetic forces in the 

 interior of the Globe. The mathematical theory 

 of the tides, it is true, makes it probable, that 

 the mean depth of the ocean is at least four 

 leagues-^- ; but we know, from the aerostatic 

 voyage of Mr. Gay-Lussao, that in rising perpen- 

 dicularly from the surface of the Earth 3600 

 toises, no sensible change is perceived in the in- 

 tensity of the magnetism. We cannot therefore 

 admit, that the sea is much deeper in those lati- 

 tudes, where, under the same magnetic parallel, 

 we see the number of oscillations diminish. 

 I did not regret my hot having embarked the 



* See, in the Journal, my observations made in 34° 30^ 

 and 14° J 5' of north latitude. 



f From the small height of the tides, in open seas, and the 

 ratio of the density of the sea to that of the land. (LaPlace ? 

 in the Mem. (le Paris, 1776, p. 218.) 



i 



