760 



ascertained if it be produced on extended table- 

 lands, as well as in necks or narrow passages. 



4°. We see every where (as may be supposed) 

 that the variations slacken near the concave 

 and convex summits of the curve they repre- 

 sent ; that is, when the barometric heights at- 

 tain alternatively their maximum and their mi- 

 nimum ; and in some parts of the earth the 



place at the same hour at Saint Bernard and Geneva. (Bibl. 

 Univ., Tom. xxii, p. 111). These uncertainties on the 

 isochronism of the oscillations can never be removed, till we 

 possess the mean observations made at the limit-hours, for 

 Geneva and Saint Bernard, Milan and the village of Sim- 

 plon, and for Trento and Inspruck. It may also happen 

 that the necks of land situated on the top of the Alps, and 

 surrounded with lofty summits, retard and modify the pe- 

 riods of the maxima and the minima, and that this local in- 

 fluence ceases in table-lands of greater extent. In order to 

 know if a want of isochronism is manifest in the torrid zone, 

 in certain circumstances, I have recently engaged MM. 

 Boussingault and Rivero to observe their barometers simul- 

 taneously at Santa Fe de Bogota, and at la Chapelle de 

 Notre Dame de Guadaloupe, which seems as if it were fixed 

 to a rock almost perpendicularly above the town, with a 

 difference of height of 322 toises. Mr.Dnniel {Meteor. Essays, 

 1823, p. 260), thought he recognized in the observations 

 made during the last voyages to the polar regions, especially 

 in Melville Island, and at the Rocky Mountains, that the 

 barometer rises in 74° of latitude, when it falls in 41°. 

 That learned naturalist appears to attribute this phenome- 

 non to atmospheric currents, of which it is not easy to verify 

 the existence. 



