466 Mr. W. Brown on the Itifluence of Atmospheric Currents 



Now it is evident, that when the barometer has been greatly 

 depressed by the velocity of the south wind, the density of the 

 air will be so diminished by the great reduction in its pressure, 

 that the tendency of the denser air of the polar or northern 

 columns, pushed back by this v/ind, will be greatly increased, 

 and, therefore, so soon as the strength of the south wind is 

 overcome, the north wind, though still deflected by its remain- 

 ing force (its course being north-west), rushes forward to re- 

 store the air to its former density and elasticity; and hence the 

 stronger the wind the more rapid the rise of the barometer. 



The phsenomena here described are so constant and so well 

 known, that their truth does not seem to require confirmation ; 

 for this, however, I need only to refer to the description of 

 storms in high latitudes given by Col. Reid*. One short ex- 

 tract will suffice. " On the south coast of England violent 

 gales usually set in with the wind about south, and veer by the 

 west towards north-west. The barometer, falling at the com- 

 mencement, rises as the wind becomes northerly. In the cor- 

 responding latitude, in the southern hemisphere, this order, 

 as regards both the wind and barometer, is reversed." This 

 description, as may be seen by reference to this work, applies 

 generally to the storms of temperate regions. 



It is evidently impossible to obtain, by calculation, the ex- 

 treme limits to which the depression of the barometer might 

 be carried ; but it is easy to show, that, within the limits which 

 might be assumed on this theory, there is ample room for the 

 greatest reduction ever observed. 



Let us suppose that a violent storm occurs at some latitude 

 whose temperature at the time is 40°, and allowing nothing for 

 the expansion which must immediately ensue on the diminu- 

 tion of the pressure of the atmosphere by a portion of its height 

 being removed, supposing its density to remain as before, and 

 that its height is reduced to that of the northern columns whose 

 temperature is 7°. The densities of air under equal press- 

 ures at these temperatures, corrected for the presence of aque- 

 ous vapour, are 1 0*000 and 10*722, which, taking the mean 

 pressure of the column of 7° temperature at 29*900 inches, 

 will give 27*886 inches for the pressure of the column at 40°; 

 so that it is obvious, that considering the progressively in- 

 creasing reduction in density, which will go on as the height 

 of the latter is diminished, and that the whole mass of the at- 

 mosphere is moving in one direction from this locality, whilst 

 a disproportionate supply of air is moving towards it from 

 columns on the equatorial and longitudinal sides, the depres- 

 sion may be very rapid and very great without the proportion- 

 * Law of Storms, chap. ix. 



