Mr. W. Brown on the Oscillations of the Barometer. 24<7 



north current, which will after a time carry backward the 

 point C with increasing velocity, and gradually put a period 

 to the storm*. 



This is more conspicuous in storms of temperate regions 

 than in those of the tropics, because the former consist, as it 

 were, of one deep wide depression of the atmosphere, the pro- 

 gressive motion of the storm being, so far as regards the 

 movement from north to south, apparently in great measure 

 an enlargement of this depression southward, whilst the latter 

 occasion a much smaller one. 



4>. As a direct consequence of the foregoing, and as also 

 proved by P. J. Espy, the greatest reduction of atmospheric 

 pressure in storms is not where the wind is most violent, but 

 where its velocity is reduced by the resistance in front of it; 

 and the depression of the barometer at any given place de- 

 pends on its position with regard to the place of the mini- 

 mum, or the point C, fig. 1, as well as on the violence of the 

 storm. 



5. A further consequence of the same result is, that a con- 

 siderable diminution of the pressure of the atmosphere, and 

 consequently, fall of the barometer, takes place on the loca- 

 lity where a north-east wind is blowing, when this wind is 

 immediately on the north of the northern range of the south 

 wind which occasions the fall, (as at the point E in the cur- 

 rent from B towards C), though, as explained in § 2, to a 

 less extent than in the localities occupied by the south wind. 

 This result explains a fact of very frequent occurrence, the 

 falling of the barometer during a north-east wind. 



6. As the impetus of the south wind may have reduced the 

 elasticity or pressure of the air, in the column C c at any eleva- 

 tion of the atmospheric columns, below that of the air of the 

 same elevation on the north ; a very rapid increase of the 

 pressure of the atmospheric columns on the north may give 

 so great a check to the upper current at C, as to cause the 

 air to accumulate so rapidly, that the increase of pressure 

 or rise of the barometer extends to a great distance on the 

 south ; but it is yet evident, from the state of the atmospheric 

 columns shown by the figure, that the south wind will con- 

 tinue to flow from A because of the greater pressure there; 

 but the barometer will rise on account of the accumulation of 

 air taking place at C and extending in a diminishing degree 

 towards A ; hence a frequent phenomenon, the rise of the 

 barometer during the continuance of the south wind ; as also 



* The deflection from south by the rotation of the earth is for the pre 

 sent left out of consideration, see § 15. 



