Mr. W. Brown on the Oscillations of the Barometer. 251 



the lower one, both the depressions and elevations of the ba- 

 rometer are the greatest in winter. 



14. The forces which urge on these currents are accelera- 

 ting forces ; but the lower current being exposed to the fric- 

 tion occasioned by its flowing along the surface of the earth*, 

 as also to checks given by inequalities of temperature, its force 

 is unequal to that of the upper one ; hence storms are by 

 far the most frequent and violent from south. 



15. The progressive motion of a south-west wind or storm 

 has been in part previously considered (§ 3), but requires fur- 

 ther notice ; it will be according to figure 2 of Plate V. Let 

 the upper current descend upon a station A (the top of the 

 page in this figure and those of § 16 and 1 7 being supposed the 

 north) with more or less force ; as shown in § 3 and the essay 

 on " The Storms of Tropics," the storm moves or recedes 

 from A, in the direction from B to A ; but blowing from 

 south, it is carried by the rotation of the earth towards east, 

 or as if impelled in the direction C A; hence its actual path 

 is the resultant of these motions, or that shown by the arrow 

 at A. In this direction, therefore, the storm will arrive at 

 the places which it visits, or as the line A D moving in a di- 

 rection perpendicular to its length ; hence also H A will be a 

 section of the two currents at their place of meeting, and con- 

 sequently the line or parallel of the line of the minimum at- 

 mospheric pressure (§ 3), extending in the same direction, or 

 that shown by the arrow. The progressive motion however 

 will not be in the same direction throughout; for let A be the 

 place at which the current descends on the arrival of the 

 storm, it will advance a certain distance along the surface. In 

 the progressive motion just considered, there is a compara- 

 tively rapid motion from west, because although this motion 

 is opposed by the air in front of it, yet it is principally on the 

 east and by air at rest, for as the storm recedes and one por- 

 tion of air descends behind the previous one, the opposition 

 on the north is in part removed by the first descending air 

 from that which descends after it; but when the air, as in this 

 part of the storm, advances from south to north, this opposi- 

 tion is felt at every degree of its progress ; hence the path 

 taken by the wind in this case is simply the resultant of the 

 directions of the two forces, but that from south-west being 

 much the strongest, it is from a point much nearer to this 

 than to that from which the opposite force is directed. Now 



* See Phil. Mag., October 1843, "On the Storms of the Tropics," p. 277. 

 I may here correct an error in the note on that page : it stated, that omitting 

 the two months in which the change of the monsoons occurs, the differ- 

 ence of the atmospheric pressure of the two seasons at Canton is nearly 

 one-third of an inch : it ought to have been nearly half an inch, or 0*44. 



