OSCILLATIONS OF THE BAROMETER. 35 



the tidal action of the sun, by acting alternately in 

 opposition to, and in conjunction with the course of 

 the currents which result from the axial rotation of 

 the earth, causes an increase of atmospheric pressure 

 between the hour of sunrise and noon, and a decrease 

 of pressure between noon and the hour of sunset ; so 

 also, in a manner precisely analogous, must the tidal 

 action proceeding from astral attraction tend to cause 

 an increase of atmospheric pressure between the hour 

 of sunset and midnight, and a decrease of pressure 

 between midnight and the hour of sunrise. And 

 also, from the more concentrated action of the solar 

 force, the extremes of oscillation resulting from its 

 action should be greater than those resulting from 

 the action of astral gravitation : which is shown on 

 the table by a greater extreme of rise and depression 

 occurring in the day time, than occurs at night. 



Since, as far as those diurnal oscillations of the 

 barometer which we have under consideration are 

 concerned, they result, in the temperate zones, from 

 the upper currents of the atmosphere alternately 

 pressing upon or being raised upwards from the 

 lower strata; whereas within the tropics both the 

 upper and the lower strata are similarly acted upon. 

 Therefore we should expect to find those diurnal 

 oscillations more regular and more clearly apparent 

 in the tropical than in the temperate zones. And 

 this appears to be in accordance with observed facts. 

 For Sir John Herschel, referring to this diurnal 



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