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



of a wind or storm* is at once attained at the point C, which is 

 there supposed to be that of the furthest extent of the descend- 

 ing current from the^rs^ place of its descent, and the place of 

 the barometric minimum in the parts then occupied by the wind. 

 The pressure continues to decrease for some time by the pro- 

 gressive motion of the storm. It is evident that air having begun 

 to descend at A and to flow forward with considerable velocity 

 in the same direction as before its descent, the lower air on the 

 south of A must at once begin to flow towards A to supply its 

 place ; but as this air is either at rest or in a state of motion 

 in another direction, it cannot at once begin to flow with suf- 

 ficient velocity to supply the deficiency ; therefore the rare- 

 faction thus produced will cause the upper current to descend 

 into it, and thus the space upon which it flows will gradually 

 extend itself backward from A, or southward. But at the 

 same time that this is going on behind A, the advanced por- 

 tion of the descending current has begun to retreat from C ; 

 for its force there, at first superior to that of the opposite one, 

 is at last overcome by it, and the heavy current of cold air 

 then advances upon the receding wind, flowing with a force 

 in some degree proportioned to the degree of the rarefaction, 

 and restores the air to its ordinary pressure t» according to 

 one general law of storms, that when the wind changes from 

 south to north the barometer begins to rise. Thus the point 

 of minimum pressure, C, or the furthest advanced portion of 

 the storm, and the point of its first occurrence or descent, A, 

 both move in one direction, from north to south, but not 

 equally ; for it is obvious that the portions of air which de- 

 scend after the first have an advantage over the latter in this, 

 that the opposition in front being partially removed, by the 

 first portions and the diminution of pressure begun, they flow 

 towards a rarefaction ; and thus the force of the storm and 

 the diminution of pressure at C are increased, and the motion 

 of this point southward is retarded, whilst the wind is pro- 

 gressing on the south from A. But this disproportionate 

 motion of the two extremities cannot remain, for with the in- 

 crease of the rarefaction there is an increase of the force of the 



torrents of rain which frequently fall during their occurrence seem to ma- 

 nifest that the only difference between this and the fore-mentioned cases 

 is in intensity. 



* The only distinction here inferred between wind and storm is that of 

 force; in many instances I use the term storm, because of the phenomena 

 being sufficiently striking only when the wind has great force. 



f In the hurricanes of the tropics, the returning current is a second 

 storm, and it is sometimes so in those of high latitudes; but in the latter 

 the rarefaction of the atmosphere is so extended, that the restoration of the 

 pressure is frequently very gradual and produced by moderate winds. 



