Mr. W. Brown on the Oscillations of the Barometer. 255 



varices along the line A B, except that as it approaches B, 

 from the great extension of the minimum depression in the 

 north, the point A may move more slowly than the point C; 

 near this time, however, the north wind begins to set in di- 

 rectly in front of the storm from north-east, and then the re- 

 storation of the atmospheric pressure proceeds throughout. 



But not only does the minimum of the barometer occur first 

 at the point C, its depression is also sometimes the greatest at 

 this point; thus at 9 a.m. on the 1 1th, its height at Cork was 

 28*93, at Belfast 29*10, and at Plymouth (to the south-east) 

 29*11. Thus it appears that the barometer in this instance 

 was about one-tenth of an inch lower at a station similar to 

 the point C than at the point d*. In § 2 it is stated merely 

 that the minimum height of the barometer is near the point 

 of meeting of the opposite currents, in order to simplify the 

 reasoning of that paragraph ; but it is evident that it will be 

 rather to the south of it, for at this point the resistance of the 

 air in front of the south wind at the surface of the earth, is 

 either equal to its force, in which case it advances no further, 

 or if not equal to it, it is yielding to it and retreating. In 

 either case the opposition will cause more or less condensation 

 where it is immediately felt, but the opposition decreases to- 

 wards the higher regions of the atmosphere; hence (§ 2) the 

 upper current, whose force is only exhausted by the de- 

 struction of its momentum, carries off the air from the point 

 d (which, for the sake of illustration, let us suppose the limit 

 of the south wind), and the positions south of it, so as still to 

 carry on the reduction of pressure; but in the greatest degree 

 a little south of d (or at C), on account of the condensation 

 decreasing from d towards C. Now in storms in which the 

 advancing portion is insignificant, the minimum pressure will 

 be very near the point C of fig. 1 ; but in such as those now 

 in consideration, it is apparently at a great distance, though 

 the difference of the pressure at the point of the meeting of 

 the currents, and that of the minimum pressure, is very slight. 

 The distance, however, is in a great degree only apparent, 

 being occasioned by the nature of the advance of the storm. 

 17. In high latitudes, the warmer regions, except when they 



* This appears to me a strong confirmation of the belief, that the origin 

 of the south wind is that upon the supposition of which this theory is 

 founded, for how upon any other than that of air descending from a cur- 

 rent which flows by an acquired velocity, could a current flow from one 

 station, C (fig. 3), to another, d, of a colder temperature and greater pres- 

 sure? In this paragraph, as also in those which follow it, it will be ob- 

 served that 1 have been obliged to depart from the form of the reasoning 

 in the others, that of simple deduction from the principles stated at the 

 outset, or from the results of previous paragraphs. 



