464 Mr. W. Brown on the Lifluence of Atmospheric Currents 



It is in some measure already accounted for in the preceding 

 remarks, because the south wind, that which depresses the 

 barometer, coming from warmer regions, brings with it a larger 

 quantity of vapour than its decreasing temperature will sup- 

 port, and consequently a precipitation takes place. But the 

 north wind, coming from colder climates, is comparatively 

 dry ; and thus the barometer rises when the weather is fine, 

 and falls when it is the contrary. 



But the connexion here noticed cannot be wholly explained 

 in this way, because it almost invariably happens, that a south 

 wind with rain causes a greater depression of the barometer 

 than the same wind without rain. 



This, however, as has been fully shown by Professor Daniell, 

 is precisely what ought to take place, because the latent heat 

 emitted by the vapour in condensing into clouds, together with 

 that absorbed by the clouds themselves from the sun's rays, 

 instead of allowing it to penetrate to the surface of the earth, 

 will, by expanding the upper strata of the atmosphere, dimi- 

 nish the density of the columns in which the precipitation takes 

 place to a yet greater degree below that of those to the north 

 of them, thus increasing their height, and consequently the 

 elasticity of their upper strata, so as to retard the influx of air 

 from the equatorial, or higher columns, and urge on the efflux 

 towards the polar or lower ones ; and it is also obvious that 

 a slight depression of the barometer will be occasioned, even 

 when the lower current is flowing in due course, because the 

 expansion being confined to the higher strata of the atmo- 

 sphei*e, the temperature of the lower is not immediately affected 

 by it, and therefore the lower current which depends upon 

 this temperature, and the elasticity of the whole column, is not 

 accelerated ; so that the increased efflux of the air from the 

 higher regions of the atmosphere will occasion a decrease in 

 the pressure ; and a fall of the barometer, as is frequently the 

 case when rain falls with a northerly wind, will be the con- 

 sequence. 



This reasoning also applies to the explanation of the fact, 

 that at those places which are peculiarly situated with regard 

 to the pi'ecipitation of moisture, more especially in mountain- 

 ous localities, the lines showing the oscillations of the baro- 

 meter, deviate in a greater degree from the mean than those 

 drawn at stations within a comparatively short distance, and 

 differently circumstanced, and present also small irregularities 

 not perceptible in the latter, although a general accordance 

 between the lines is maintained*. 



Without noticing any particular deviations which may be 



• Daniell's Meteorological Essays, p. 562. 



