232 THE ATMOSPHERE AND METEOROLOGY. 



CHAPTER III. 



MEAN PRESSURE OF THE ATMOSPHERE UNDER VARIOUS LATITUDES.— DE^'SITY OF 

 THE AIR IN THE NORTHERN HEMISPHERE.— DIURNAL OSCILLATIONS OF THE BARO- 

 METRICAL COLUMN. — ANNUAL OSCILLATIONS. — IRREGULAR VARLATIONS. — ISO- 

 BAROMETRIC LINES. 



The atmosphere is of such mohillty that its weight, measured in an 

 exact manner by the column of mercury in the barometer, varies 

 incessantly all over the earth. The various meteoric changes, from 

 cold to heat, from dryness to moisture, augment or diminish the pres- 

 sure of the air, and in consequence a corresponding oscillation is 

 produced in the mercury contained within the tube of the instrument. 

 Now, any volume of mercury being about 10,500 times heavier than 

 the same volume of air taken at the level of the ocean, we must con- 

 clude from this that every movement of the barometric column re- 

 veals a change 10,500 times greater in aerial space. 



When air is heated either by the direct influence of the sun or by 

 a current of higher temj^erature, its particles expand, become rela- 

 tively lighter, ascend into space, and then spread out laterally. The 

 pressure then diminishes, and in consequence the column of mercury 

 in the barometer must fall. The contrary takes place when the air is 

 condensed by cold, and when the aerial masses flow together to fill up 

 the space ; the weight of the atmosphere is increased, and the level of 

 the mercury rises in the instrument. This is the reason why the fall 

 of the barometer indicates generally an increase of temperature, while 

 a diminution of heat is marked by the contrary phenomenon. The 

 barometer and thermometer oscillate in inverse ways. It is true 

 that the air can absorb so much the more watery vapour the warmer 

 it is, and in this way the pressure which is diminished on one side by 

 the ascent, and the lateral flow of the aerial fluid, is augmented on the 

 other by the increase of vapour contained in the atmosphere ; air be- 

 coming colder, on the other hand, loses its capacity of dissolving the 

 watery vapour, and grows lighter in proportion. Thus the pheno- 

 mena counterbalance each other, and it is not without numerous ob- 



