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



agreeably with this, we find that the direction of the wind in 

 such cases is generally at first from S.S.E., but as the storm 

 continues, it changes to S. or S.W. ; according to the explana- 

 tion given of the lateral motion of the receding wind (one por- 

 tion removing the opposition for that behind). Therefore let 

 us suppose the direction of the advancing air to be from due 

 south or along the line A B; then a, b, c will be stations at 

 which it arrives in its progress; but at the time it reaches any 

 of them the storm will have moved on a certain distance from 

 A in the direction of the arrow ; therefore suppose A to be 

 moved back along the line H A, then the wind will arrive 

 on each point of the line E A from a point immediately 

 south of it on the line H A ; and if the intervals of time which 

 have elapsed on its arrival at stations equidistant with a, 6, c, 

 from the line H A in a direction due north, or parallel to 

 A B, be severally represented by the lines af t b e, and c d, 

 parallel to the arrow, the wind will arrive at the stations a, b, 

 c, or every point on the line A B, asf, e, d, or the correspond- 

 ing points of the line E A, supposing it to move together 

 with the line D A. Thus as fresh portions of air will advance 

 as the point A moves forwards, the storm may be represented 

 by a moving body of air, within which the wind is S.W., S. or 

 S.E., and whose progressing front has the shape DAE; and 

 we may name that part of it represented by A E the advancing 

 portion*, and that by D A the receding portion, according to 

 the nature of the motion. 



16. In storms of the tropical regions, and in those of high 

 latitudes commencing with the usual atmospheric pressure, 

 the former of these is apparently insignificant ; but in some 

 cases this portion of the storm extends over so great a space, 

 and the phenomena presented by it are so peculiar, that it 

 will require a distinct consideration. It is evident that its ex- 

 tent, or the distance to which the wind advances, will depend 

 not only on its force, but also on the greater or less resistance 

 of the air, or in other words, the greater or less pressure of 

 the atmosphere in front of it (the resistance from the differ- 

 ence of temperature being supposed the same in all cases); 

 hence we find that the cases in which it is traced in the fol- 

 lowing observations to a considerable distance, and to which 

 this paragraph is intended principally to apply, are those in 

 which the height of the barometer has been much reduced by 

 * This must not be confounded with the second period of storms, in 

 which the north wind advances upon the receding south. For a descrip- 

 tion of the winds in both the parts, A C and C B, fig. 1, of a progressing 

 body of air, see this Magazine, vol. xxiii. p. 214; if the several directions 

 there given be reversed, it will apply to this case. It is to tropical storms 

 that we must look for an exhibition of these phenomena in their greatest 

 simplicity. 



