Mr. W. Brown on the Oscillations of the Barometer. 249 



spread by the flowing of the upper current on others ; the 

 reason why elevations of a degree corresponding to depressions 

 at any given place have never been found by comparative ob- 

 servations*. 



9. An elevation of the barometer may also be the conse- 

 quence of a previous reduction ; for let the pressure be re- 

 duced, as at C, fig. 1, the returning air or northerly current 

 which sets in after the cessation of the southerly one, will oc- 

 cupy a portion at least of the higher regions of the atmosphere 

 where it is not wont to flow, as is evident from the figure. 

 On the restoration therefore of the usual pressure, the north 

 current will be blowing not only in its own proper region, but 

 also in part of that which properly belongs to the southerly one, 

 and will continue there some time by reason of its acquired 

 velocity after the original impulse has ceased to act ; and thus 

 the upper current, not resuming at once the whole of its ac- 

 tion, and consequently the air not being allowed to flow from 

 the upper parts of the atmospheric columns as rapidly as it 

 is brought to the lower, will accumulate. In like manner, the 

 elevation of the barometer may be the cause of giving to the 

 upper current a great velocity, for an elevation being any- 

 where produced, the force by which the lower current causing 

 it was urged on, must sooner or later be overcome by the in- 

 crease in the pressure of the air towards which it flowed. 

 But this current being overcome, the overplus of pressure 

 will then increase the velocity of the upper one, and probably 

 determine the flow of the air at the surface of the earth in 

 the same direction ; which indeed is frequently the way in 

 which an elevation of the barometer subsides; and in the ob- 

 servations given in this essay, in which are included two pe- 

 riods of stormy weather, both began with the occurrence of 

 a southerly storm after a high barometer. 



10. The direction of the wind when one current alone pre- 

 vails, is determined by the relative situations of the warm air 

 and cold, and the deflection of the current thus produced, by 

 the rotation of the earth, as the "trade winds" and mon- 

 soons of the tropics, and the north-east wind of high latitudes ; 

 but when the opposite currents come into collision, the direc- 

 tion is the resultant of their forces, and thus in the latter re- 

 gions we have winds from every point of the compass, as has 

 been pointed out by Prof. Kaemtz. 



11. The action of these currents meeting together and pro- 

 ducing the various winds may be considered as follows : — The 

 south-west and north-east winds blowing from two stations, A 

 and B, and meeting together at a station between them, C, 



* Daniell's Meteorological Essays. 



