D1UE]S"AL OSCILLATIONS OF BAEOMETEE. 



235 



sink. But while the pressure of the air diminishes, the quantity 

 of watery vapour augments rapidly, and its pressure added to 

 that of the air, produces a sort of temporary wave, after which 

 the barometric column continues to fall, to rise again with the 

 nocturnal cold. If the pressure of the watery vapour disappeared 

 from the atmosphere the barometer would rise regularly in all seasons 

 towards the middle of the night, and would be at its lowest towards 

 the middle of the day. This is shown by the following figure, repre- 

 senting the barometric oscillation of the dry air at the port of Apen- 



I 922 H1S7- 



«5.6a3in» 7 



ag :i3 ina ^ 





Noon 3^ 6^ B^ Miiight lo> 18'" 



Fig. 100.— Pressure of dry air at Apenrade. 



Spring 

 Winter 



Arerage for 

 tixe 'year 



Summer 

 AuJLiimn, 



2L^ Noon 



rade on an estuary of the Baltic. In very dry countries, such as 

 Eastern Siberia, the pressure of the watery vapour is too slight to 

 counterbalance the action of temperature, and in consequence only 

 two oscillations occur during the four-and- twenty hours, a fall with 

 the increase of temperature, and a rise with the cold of night. 



The diurnal movements of the barometer are much more regular and 

 more easily ascertained in the equatorial regions and near the level 

 of the sea than under high latitudes and in the interior of continents. 

 This is because over the tropical seas the alternations of temperature, 

 evaporation, and precipitation succeed each other, like all other ph5:si- 

 cal phenomena, with greater regularity than in other parts of the 

 globe. Besides, it was in the equatorial seas that the diurnal oscilla- 

 tion was observed for the first time, and it was in these same latitudes 

 that Humboldt was able to ascertain the hours exactly. In the tem- 

 perate regions these regular movements of the barometric column are 

 in a great measure hidden by the abrupt leaps of the mercury, obey- 

 ing the constant variations of the atmosphere ; it is, therefore, only 



