254? Mr. W. Brown on the Oscillations of the Barometer. 



the line Fc A b, the pressure increasing from B towards nil 

 parts of that line. Under these circumstances, then, the ad- 

 vancing storm occasioned by the greater height of the atmo- 

 spheric columns about C than of those about B, occurs, and 

 reduces the barometer at C, so as to make it the place of the 

 greatest depression, the line c C now representing the line 

 H A of fig. 2. But whilst the south current is thus setting 

 in from C, the north-east is blowing from b, and the two 

 meet somewhere between these points ; and there, as at D, 

 fig. 3, the wind is south-east (§ 11), though south-west in the 

 localities south of it ; but as the direction of the current 

 which carries off the air so as to produce the depression is 

 from south-west, the depression will be produced in that 

 direction also, or along the line C B. Now if we consider 

 the storm to have advanced from any line drawn from C 

 towards the circumference of the depression as C c, it is 

 evident that whatever part of this we suppose a particular 

 portion of the current to set out from, its condition will 

 be the same, that of having at the time when the pressure 

 of the atmosphere is reduced to the lowest point, a greater 

 atmospheric pressure on the north-west side, or along lines 

 drawn from it to the circumference cAb, as Ca and C A, 

 because of that side being always adjacent to atmospheric co- 

 lumns less and previously affected by the storm ; hence on the 

 line Cc, the occurrence of the minimum in order of time will 

 be in the direction from c to C, or that of the receding storm 

 (§ 15). Let then the barometer be reduced to its minimum 

 height at C (supposed the place where the storm is most in- 

 tense), or to the limit of its equality with the resistance of the 

 air in the direction from A to C, and let the air at d be re- 

 duced to the same degree of rarefaction, or to one (as after- 

 wards to be noticed) not quite so great. These stations are 

 subject to the influence of the pressure increasing along the 

 line C A ; hence from any station E on that line, the air will 

 tend to flow towards them, as indicated by the arrows at E, 

 the effect of which in the case of C is to oppose the southerly 

 current, but in that of d rather to assist it ; hence whilst at C 

 it is overcome and the wind sets in from north-west, it conti- 

 nues its course at d. The commencement of the restoration 

 of the pressure of the atmosphere advancing thus from C to- 

 wards D, whilst its advance from west is, as before stated, 

 along the line c C or A C, it is evident that every point of the 

 line C D B in succession from C will be related to a line par- 

 allel to A C, precisely as the point C is related to the line 

 A C at the time of the setting in of the north wind ; and the 

 restoration of the pressure will commence as the line A C ad- 



