23G 



THE ATMOSPHEEE AND METEOEOLOGT. 



after a longer or shorter series of days, or even weeks, that meteorolo- 

 gists can, by establishing averages, reveal normal oscillations analo- 

 gous to those which occur at the equator. In the high mountain 

 regions it is still more difficult to ascertain the regular succession of 

 the barometric waves, for the changes which occur in the lower strata 

 of the air are only felt later, and are variously mixed in the higher 

 strata. Thus the rising of the barometer which takes place towards 

 ten in the morning at Zurich, does not occur on the summit of the 

 Kighi till two o'clock in the afternoon, and only at three o'clock on 

 the Faulhorn ; often the depression of the barometric column does 

 not even make itself felt in the afternoon on these heights, and each 

 day presents but a single great oscillation. 



The annual variations of the pressure of the air present alterna- 

 tions analogous to those of the diurnal variations. In the tropical 

 countries, where tlie seasons follow one another with great regular- 

 ity, and in the interior of continents, the air of which contains 

 but a slight quantity of watery vapour, the mercury of the baro- 



»VurJ 



Fig. 101.— Monthly variations in the pressure of the atmosphere at Cairo, Calcutta, Berlin, 

 St. Petersburg, Benares, Paris, and Halle. 



meter gradually sinks from winter to summer in inverse proportion 

 to the heat, and re-ascends with the cold from summer to winter. 



