228 Proceedings of the British Association. 



and the posterior troughs of these waves and their amplitudes, 

 as to the number of miles traversed and times of traversing, from 

 which he deduces a velocity of transit varying from 25 to 3 1 miles 

 per hour: and in concluding his Report Mr. Birt remarked, that 

 it is characteristic of waves that different systems pass onwards 

 without destroying one another ; each wave of each system pursues 

 its own path although crossed by others ; and it can be followed 

 in all its individuality. In this inquiry three systems of waves have 

 been detected, or at least three barometric maxima ; these maxima 

 have been found to move across the area in three different directions, 

 having on each side a diminution of pressure. The progress of each 

 of these maxima appears to have been independent of the others : 

 thus, at the opening of the observations, the line of greatest diminu- 

 tion of pressure on the English area was from Glasgow to St. 

 Catherine's Point; at a later period, the observations indicated 

 the direction of maxima at right angles to this line, and that a line 

 cutting this line of maxima transversely passed through Geneva 

 and Brussels, nearly parallel to the former ; it was in this direction 

 the wave was considered to have been moving. The barometric 

 phenomena in this direction progressed slowly. While these move- 

 ments were proceeding over the area, the barometric differences 

 between Scilly and Longstone increased ; the latter station exhibited 

 a much less pressure than the former. At length, a decided line 

 of maximum pressure is traced from Dublin to Geneva, after which 

 the barometric affections at the stations at Scilly and Longstone 

 are reversed, Scilly being the lowest and Longstone the highest. We 

 have, therefore, a cause simultaneously operating on the barometer 

 with that which produced the movement from Glasgow to St. 

 Catherine's Point and from Brussels to Geneva, but distinct as the 

 phenomena progressed in different directions. During the period 

 that these two distinct, but contemporaneous causes are in operation, 

 producing certain barometric phenomena in certain directions, and 

 from the last of which we should expect at certain stations — Scilly 

 and Longstone, for instance — a rising barometer, we actually find 

 it falling rapidly, but not without exhibiting the same phenomena 

 that characterize this fall as resulting from a wave. A decided 

 line of maxima is observed ; and in the same line, at a subsequent 



