732 



The first naturalists * who remarked the 

 great regularity of the ascending and descend- 

 ing movement of the barometer within the 

 tropics, were struck with the inequality which 

 they observed in the extent of the variations 

 between two consecutive days, alike calm and 

 serene. It happens, for instance, that the co- 

 lumn of mercury attains a greater height than 

 usual at the hours of the maximum of the even- 

 ing ; that it diminishes very little during the 

 night and till 4 h in the morning ; that the ba- 

 rometer rises much more from 4 h till 9 h in the 

 morning than it descends from 9 h in the morn- 

 ing till 4 h in the afternoon, and that this play of 

 unequal movements continues during several 

 days. A general tendency is then observed 4- 

 to the increase or diminution of the column of 

 mercury, without the periodicity or alternancy 

 of the variations being disturbed. They are 



* Literary Journal of the Hague, 1722, p. 234. Thibault 

 de Chanvalon, Voyage to Martinique, p. 135 (23). 



t Within the tropics, this tendency modifies the extent of 

 the horary variations, which remains the principal and most 

 sensible phenomenon j in Europe, on the contrary, when the 

 barometer has a general tendency to descend during several 

 days, the lowering is simply slower, or stopped at the 

 epochas of the maxima. The principal and most sensible 

 phenomenon is then the tendency of the column of mercury 

 to sink 5 and the atmospheric tides are manifested only by 

 modifying it a little at the approach of the limit-hours. 



