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



the beginning of the rise of the barometer, frequently some 

 time previous to the setting in of the northerly current. 



7. But an atmospheric pressure above the mean will also 

 result from the opposition of these currents, but an opposition 

 differing from that of § 2 in this respect; that whereas in 

 that case the force of the descending current, originally much 

 the greatest, is reduced at the place of its termination to an 

 equality with that of the opposite one by the diminution of 

 the pressure of the atmospheric columns composing it; in 

 this, where a rise of the barometer takes place, the force of 

 the lower, or current of gravity, is equal or superior to that 

 of the descending one when at its full pressure, and the former 

 is advancing upon the latter. Let then the two currents 

 so circumstanced meet, as in fig. 1, at C, there will be at 

 this station either simply a condensation of the air produced 

 by the pressure of the two currents, or the air of the lower 

 one will ascend, carrying with it an impetus which would 

 tend to carry it on still in its first direction. In the case of 

 § 2, air brought to the place of meeting is carried off in the 

 upper current by the force of that current, but in this, if the 

 air of the lower current does in this way ascend, it will simply 

 check that flowing above in the contrary direction, and cause 

 an accumulation of air to take place exactly similar to that of 

 water occasioned by partially damming a stream. But even 

 when the force of the former is in some degree inferior to that 

 of the latter, it may yet be sufficient to retain so much air of 

 the upper current by its opposition as to accomplish the same 

 effect though in a less degree: thus the pressure will be re- 

 presented by the dotted line a gb in fig. 1, and an elevation 

 of the barometer will ensue in the localities where the south 

 wind is blowing, as well as in those which have the north ; 

 hence great elevations of the barometer occur with south 

 winds as well as north. 



8. The foregoing paragraph is intended to explain the 

 great atmospheric pressure sometimes produced by strong 

 north-east winds, and calms or very gentle breezes, as one or 

 other of these is produced (§ 12) at places situated near the 

 collision of the two currents, especially when the effect is in- 

 creased, as shown in a previous essay, by reduction of tempe- 

 rature; but increase of pressure will arise from other causes; 

 as in some locality sufficiently far to the north of a strong 

 southerly wind to be out of reach of its influence in depressing 

 the barometer, and upon which the air from the depression 

 flows; though probably in this case the barometer will not 

 rise in a great degree, on account of the air, whose removal 

 causes a deficit of pressure in one locality, being so extensively 



