Mr Lawson on the Trade-Winds of Barbadoes, 369 



where this was only to a moderate extent. Facts will be 

 hereafter adduced in support of these positions, which are 

 inexplicable on the ordinary theory of rotation. 



49. It will now be necessary to point out the circum- 

 stances under which these atmospheric currents come to de- 

 viate from their ordinary course, and to present the violence 

 of the hurricane, and, also, to explain the gradual fall and 

 subsequent rise of the barometer, and the central calm which 

 is observed during the storm. 



50. Hurricanes seem to originate in two ways, which it is 

 necessary to distinguish ; in one, the air near the inner mar- 

 gin of the trade-wind is drawn to the southward with great 

 velocity ; and the other phenomena of the hurricane gradually 

 supervene ; in the other, when the inner margin of the trade- 

 wind is about lat. 12° or 13° N., an indraught of air takes 

 place over the northern part of the South American coast 

 during the day or evening, and the air to the north of it 

 flows towards the point of least pressure, to restore the equi- 

 librium. The velocity this air acquires will always cause it 

 to flow past the point of least pressure, when it will accu- 

 mulate to the southward until the increased pressure be suf- 

 ficient to counterbalance the momentum of the current, so 

 that the current from the northward will continue towards 

 this point, only the point itself will clearly be gradually re- 

 moved to the north. In from 8 to 12 hours, the diminution 

 of pressure would begin to be sensible at Barbadoes. 



51. It is obvious, from what has already been stated, that 

 the moon's attraction (35, et seq.) on the atmosphere, within 

 the tropics, is quite sufficient, at certain seasons, to increase 

 the velocity of the currents already existing, or even, when 

 exercised in the proper direction, to overcome them at any 

 given point, and to originate others with an opposite ten- 

 dency. Such changes will, naturally, be much more frequent 

 and abrupt about the inner margin of the trade-wind than 

 elsewhere, though, when local circumstances are favourable, 

 it is not unreasonable to suppose they might occur at any 

 point in its course. 



52. If, when the lower portions of the atmosphere about 

 the inner margin of the trade-wind, in August or September, 



