Mr. W. Brown on the Oscillations of the Barometer. 243 



From this may be deduced the following results, which may 

 be applied to the explanation of the general atmospheric phe- 

 nomena of all latitudes of both hemispheres, though when the 

 cardinal points are referred to, they are given in terms adapted 

 only to the northern, and to extra- tropical latitudes; for the 

 sake of convenient reference to each, they are placed in num- 

 bered paragraphs. 



1. An equality in the pressure of the atmosphere can only 

 be maintained by the flow of these currents in their proper, 

 that is, in their original positions, the one above and the other 

 below; but on the descent of the upper current, which still, 

 by its momentum, maintains wholly, or in part, its original 

 direction, the lower is either more or less retarded, or entirely 

 pushed back : thus whilst the air is carried away, either from 

 the upper parts, or whole of the atmospheric columns, the 

 flow to the lower is prevented, and consequently a diminution 

 of air, or decrease in the atmospheric pressure, takes place in 

 the regions where the descending current prevails on the sur- 

 face of the earth : hence the great oscillations of the barome- 

 ter in high latitudes, or the region of the " variable winds," 

 and the maintenance of the equality of the atmospheric pres- 

 sure in the region of the " trade winds," the ascent and de- 

 scent of the air taking place at the extremities of the latter, 

 and consequently without interruption to the course of the 

 currents. 



2. In the extent of the descending current, and for a space 

 in front of it, following the line of its direction, the pressure 

 of the atmosphere upon the surface of the earth will be distri- 

 buted according to the curved line a deb, Plate V. fig. 1; 

 and the lines A a, Dd and Cc, &c. will represent the pressure 

 at the several points on which they are drawn, C being sup- 

 posed the point at which the descending current terminates 

 and meets the opposite or lower one, as shown by the arrows, 

 whilst the southerly current is blowing above the north from 

 c to b; the minimum pressure will be near C, or the place of 

 meeting of the two currents. That this will be the case is 

 evident from the consideration, that the descending current 

 advances by reason of the superiority of its force to that of 

 the lower one, which it drives back; but this superiority is 

 constantly diminishing by the rarefaction of the air produced 

 by its flowing from c to b, which rarefaction at last reduces 

 the force of this current below that of the resistance of the 

 opposite one in front of it ; hence it is clear that where the 

 descending current terminates in advancing from the place 

 of its descent, or is overcome, as at C, the pressure must be at 

 the minimum, increasing from this point in both directions, 



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