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this respect, to those of the magnetic inclina- 

 tion ; and the celebrated astronomer * who 

 alone on the continent of Europe marks the 

 latter, measuring- daily, during several hours, 

 the amplitude of the elongations of the mag- 

 netic needle, will tell us, in publishing his pre- 

 cious observations, what patience and long 

 assiduity such a species of labor requires. 

 I advise the traveller, when he arrives within 

 the tropics, to certify by observations during a 

 day and night without discontinuing, whether 

 the epochas of the limits are effectively, in the 

 spot where he would fix his stay, 21 h -22 h ; 

 4 h -5 h ; 10 h -ll h ; 15 h -16\ This previous labor will 

 ascertain for him the periods of the day and 

 night when he must be found near his instru- 

 ment; in order to occupy himself with the dif- 

 ferent parts of the problem of horary variations; 

 for instance, to examine, in observing every ten 

 minutes, if the maximum be attained at 9 h , or 

 at 9£ h in the morning ; if the mercury remain 

 stationary, and how long a time that state lasts ; 

 if the tides of the night are stronger than those 

 of the day, &c. 



I. Continuity of the Movements. What is 

 at first most striking in the phenomenon of the 

 horary variations between the tropics, is the 



* M. Arago. 



