TIDES IN THE AIR. 175 



meter indicates the highest pressure, or, as we 

 might say, the flow of the tide, at 9, or 9^, A.M. 

 and 10, or lOf , P.M. ; and twice a day the lowest 

 at 4, or 4, P.M., and 4, A.M., or nearly the hottest 

 and coldest hours in the round of the twentyrfour. 

 It is highly remarkable that this periodic rise 

 and fall in the atmospheric ocean, takes place in 

 the torrid zone of America, without sensible 

 disturbance by elevation, winds, storms, or rain, 

 or earthquakes. Such, in fact, is the regularity 

 of this phenomenon, that the hour may almost be 

 told by looking at the column of mercury instead 

 of the clock ! It has also been observed, that in 

 Europe the same phenomenon takes place, but 

 is affected to a great extent by the season. In 

 winter, the highest pressure takes place about 

 9, A.M., and the lowest about 3, P.M., after which 

 the pressure again increases up to 9 in the 

 evening. In the summer, these periods are 

 slightly different. It is, however, very difficult 

 to distinguish the occurrence of these aerial 

 tides, in consequence of innumerable accidental 

 causes, which disturb their indications. 



The explanation of the phenomenon arises 

 out of what has been said as to its periods. At 

 the hottest part of the day, the air expanding 

 ascends, and passes over into neighbouring re- 

 gions, and the barometer at the same time falls. 

 This is the " ebb " of the tide. At the coldest 



