ATMOSPHERIC PRESSURE. 309 



larity of .this ebb and flow of the aerial ocean undisturbed 

 either by storm, tempest, rain, or earthquake, both on the 

 coasts, and at elevations of nearly 13000 English feet above 

 the sea, where the mean temperature sinks to 7 Cent. 

 (44. 6 Eah.) The amount of horary oscillation decreases 

 from the Equator to 70 N. lat. (where we have very accu- 

 rate observations made by Bravais ( 383 ) at Bosekop), from 

 1.3-2 to 0.18 French lines (0.117 to 0.016 English inches). 

 It has been supposed that much nearer to the pole the mean 

 height of the barometer is really less at ] A.M. than at 

 4 h. P.M., so that the hours of maximum and minimum are 

 inverted ; but this can by no means be concluded from Parry's 

 observations at Port Bowen in 73 14' N. 



Owing to the effect of the ascending current, the mean 

 height of the barometer is rather less at the equator and ge- 

 nerally under the tropics, than in the temperate zone, ( 384 ) and 

 it appears to reach its maximum in Western Europe in the 

 parallels of 40 to 45. If with Kamtz we combine, by 

 isobarometric lines, those places which present the same 

 mean difference between the monthly extremes of the ba- 

 rometer, we obtain curves whose inflections and geographic 

 positions furnish important conclusions respecting the in- 

 fluence which the form of the land and the distribution of 

 land and sea exercise on the oscillations of the atmosphere. 

 Hindostan, with its lofty mountain chains and triangular 

 peninsulas, the eastern coast of the New Continent, 

 where the warm current of the Gulf Stream is deflected 

 to the east by the banks of Newfoundland, shew greater 

 isobarometric fluctuations than do the West Indies and 

 Western Europe, Pievailing winds exercise a principal 



