462 Mr. W. Brown on the Influence of Atmospheric Currents 



hence, even when the south wind is blowing with some strength, 

 there may be a rise of the barometer to above its mean eleva- 

 tion; for if we suppose the place of calm to be removed to H, 

 and the heights of the columns to arrange themselves as shown 

 by the dotted outlines, the pressure of E F will be above its 

 mean, although the south wind is blowing at F. 



The elevation, however, which the barometer sometimes at- 

 tains during the prevalence of these extremely light winds, is 

 greater than can be justly due to this cause alone ; and it is 

 maintained with such constancy, that it is quite evident it is 

 not the counter effect of an extreme reduction in some more 

 southern region, arising from a sudden and violent disturbance 

 of the equilibrium of the atmosphere. But the almost uni- 

 versal attendant of such greatly increased pressure under these 

 circumstances, is a temperature much below that of the season. 

 For the sakeofgi'eater clearness I will take a particular instance. 



The one I have selected, and it was the first I happened 

 to meet with of a pressure much above the mean, is from 

 Howard's Journal, published in the 'Annals of Philosophy.' 

 It occurred in January 1826. The mean height of the 

 barometer for four days, during two of which the wind was 

 north-west, and the other two south-west, was 30-690 inches. 

 The mean temperature of those days was 24-7°, and the mean 

 temperature of the month 33° ; a cold therefore of 8° below 

 the temperature of the season. 



Now the wind for some days previously had varied very 

 little from west, and at the time of this great elevation the 

 weather was exceedingly fine ; hence we may infer that this 

 cold was produced by local causes, probably by radiation un- 

 impeded by clouds, its cooling effect being unmitigated by the 

 precipitation of vapour. If then we suppose that the mean 

 temperature of the month, 33°, was that due to the latitude, 

 the temperature was at this time relatively 8° below that of the 

 latitudes on each side north or south. 



Tl>e temjierature then at F, a portion of the earth of more 

 or less extent, is 8° below that which it would be in a regularly 

 progressive decrease from B to H ; its height will therefore be 

 diminished in proportion, and there will be a rapid decrease in 

 the height of the atmosphere from A to E, and a proportion- 

 ally slight one from E to G ; hence the upper current will be 

 accelerated in the former part and retarded in the latter. If, 

 however, the lower current maintained its course with a rela- 

 tive velocity, there would be no increase of height in the 

 column E F, and consequently no increase of pressure ; but 

 in this case the efflux of llie air from its base is either wholly, 

 or almost whcrflly, prevented by the opposition of the equa- 



