234 



THE ATMOSPHERE AND METEOEOLOGY. 



according to the hours and seasons ; it obeys the rhythm of time as 

 well as that of space. Every day the aerial mass oscillates twice 

 in inverse directions. In the morning towards four o'clock the 

 barometric column presents a first minimum of height, but it rises 

 gradually, and towards ten o'clock in the morning it attains its high- 

 est elevation; afterwards the pressure of the air diminishes till towards 

 four o'clock in the evening, the time when the barometer is at its 

 minimum of height. The column of mercury then begins to rise till 

 ten o'clock at night, to sink again for six hours ; the periods of the 

 day during which these changes occur are known under the name of 

 ** tropical hours." The curve of variations, as we see by the accom- 

 panying figure, is much more regular in the equatorial than in the 

 temperate zone. 



29522 



29.528^ 



Abo 



Ctrmana 



^ Halle 



Ocean EquatmiiJ 



9^ Mdm^M 15^ ' 18^ 



Noon 



Fig. 99.— Tropical hours of the Equatorial Ocean, of Cumana, of Halle, and Abo. 



"What is the cause of this double daily oscillation ? Many meteoro- 

 logists formerly recognised in these movements of the barometer regu- 

 lar tides, similar to those in the ocean, and, like them, obeying the 

 combined influences of the sun and moon. But these oscillations al- 

 ways occur, on an average, at the same hours, and do not present at 

 the epoch of syzj^gies and quadratures phenomena corresponding to 

 those of the ebb and flow.* The researches of Aime, of Flanger- 

 gues, and other natural philosophers have, it is true, established the 

 existence of an aerial tide ; but the amplitude of this movement 

 is very slight in comparison with that which occurs between the trop- 

 ical hours. It is, therefore, by the combined influence of the heat of 

 the day, and the pressure of watery vapour, that we must explain, 

 with Dove, the two movements of rising and falling, which take 

 place every day in the column of mercury. Commencing in the cold 

 hours of the morning, the gradual increase of temperature must 

 result in expanding the atmosphere, and making the barometer 

 * See above the section eutitlod, Tlie Tides. 



