370 Mr Lawson on the Trade- Winds of Barbadoes. 



were either almost quiescent, or moving gently from the 

 NE., the moon passes the meridian in such a declination as to 

 cause a rapid flow of air to any point considerably to the 

 southward, that must of necessity be supplied by the NE. 

 and NW. currents, already so often mentioned, and it is ob- 

 vious that the NW. current, and the upper portions of the 

 NE., will have acquired a high velocity long before the lower 

 portion of the trade, which is in immediate contact with the 

 earth's surface, and much retarded by its friction thereon, 

 can be materially accelerated. The moment this flow com- 

 menced, the absolute quantity of air in a vertical column of 

 the atmosphere, within its limits, would begin to diminish, 

 and, consequently, the barometer to fall, though for hours 

 afterwards, perhaps, there might be nothing in the state of 

 the wind on the earth's surface to excite attention. 



53. As the lowest portions of the atmosphere have always 

 a higher temperature within the tropics than those a little 

 higher, the former would gradually rise, while the latter 

 would fall downwards and occupy their place, but, as soon 

 as the ascending portions reached the elevation of the cur- 

 rent moving to the southward (52), they would be rapidly 

 carried avv^ay by it; and, as the interchange continued, por- 

 tions of the current itself would be deflected downwards in 

 place of the warmer ascending columns, and, retaining their 

 original velocity, would be experienced below as strong gusts^ 

 coinciding in direction with the current from which they were 

 derived. 



54. A few such gusts, within a short distance of each other, 

 would throw the intervening quiescent air into violent ed- 

 dies, and destroy the equilibrium which had previously ex- 

 isted among its various portions, and thereby still farther 

 facilitate its ascent in the atmosphere, while its place on the 

 surface of the earth would be occupied by portions of the 

 current, already flowing rapidly over it, rather than by the 

 adjacent portions of the lowest strata, which hitherto have 

 not had their velocity sufficiently increased to flow in readily. 

 The absolute quantity of air in the vertical column of the 

 atmosphere over this point would be thus farther reduced, 

 and the barometer would descend to a corresponding extent. 



