310 PHYSICAL GEOGRAPHY. 



influence on the diminution of the pressure of the atmo- 

 sphere ; in accompaniment with which, according to Daussy, 

 as we have already noticed, the mean level of the sea 13 

 raised. ( 385 ) 



As the most important fluctuations of atmospheric pres- 

 sure, whether regular and periodical, or irregular or acci- 

 dental and then often violent and dangerous, ( 386 ) havefor their 

 principal cause, like all other phsenomena of weather, the 

 heating power of the sun's rays, it has long been customary 

 (partly as proposed by Lambert) to compare the direction of 

 the wind with the height of the barometer, and with changes 

 of temperature and the increase or decrease of moisture. 

 Tables of atmospheric pressure accompanying different 

 winds, which have received the name of barometric 

 windroses, afford a deep insight into the mutual rela- 

 tion of meteorological phsenomena. ( 387 ) Dove, with ad- 

 mirable sagacity, has recognised in the " law of rotation" in 

 both hemispheres, which he has propounded, the cause of 

 many important processes and extensive movements in the 

 aerial ocean. ( 388 ) The difference of temperature between 

 the equatorial and the polar regions of the globe produces 

 two opposite currents; one in the higher portion of the 

 atmosphere, the other near the surface of the earth. The 

 difference between the rotatory velocity at the poles and at 

 the equator causes the air flowing from the poles to undergo 

 an easterly, and the equatorial current a westerly deflection : 

 on the opposition of these two currents, on the place where 

 the upper one descends to the surface of the earth, and on 

 the alternate displacement of one by the other, depend the 

 principal phenomena of atmospheric pressure, of the heating 



