30 ANNUAL OSCILLATION OP THE BAROMETER. 



barometer according with the oscillations of the earth 

 in its orbital motion: the barometer falling in the 

 northern hemisphere when that hemisphere is in ad- 

 vance in the earth's orbital motion, and at the same 

 time rising in the southern hemisphere : and in like 

 manner, falling in the southern hemisphere when that 

 hemisphere is in advance in the earth's orbital motion, 

 and at the same time rising in the northern hemi- 

 sphere. This, it will be observed, is an ascertained 

 phenomenon which is clearly in accordance with what 

 theory would lead us to expect under the action of 

 the forces which cause the annual tide and its con- 

 comitant counter-tide. The force of attraction, which 

 draws the greater mass of air into the hemisphere 

 which is in advance, tends to prevent atmospheric 

 pressure downwards, and therefore causes a low baro- 

 meter : whereas the pressure of the vis inertiae cur- 

 rents causes a high barometer in the opposite hemi- 

 sphere. 



185. Besides these annual variations we should 

 also expect that, at least in the equatorial regions, the 

 barometer should be lower on the average at sunrise 

 than at sunset : for the line of sunrise on the surface 

 of the earth is in advance as the earth moves in its 

 orbit, and is therefore under the atmospheric tide 

 raised by attraction ; whereas the line of sunset is 

 under the atmospheric tide caused by vis inertiae, which 

 tends to cause an increase of pressure on the baro- 

 meter. From the diurnal oscillations of the barometer 



