Mr Lawson on the Trade-Winds at Barbadoes. 145 



flow with greater velocity than it does in the adjoining parts, 

 the quantity of air in a vertical column of the atmosphere, at 

 that point, must be proportionately diminished ; tlie atmo- 

 spheric pressure, then, would consequently be less than all round, 

 and the air would flow inwards towards it from the neighbour- 

 hood. To the westward, the air being no longer supported 

 as usual, the wind would back to NNE. and N., while, to the 

 eastward, it would equally veer to ENE., and a little more to 

 the southward to E. These currents flowing rapidly from 

 various points, joining that which was previously moving from 

 the NE., would gradually assume the same direction, still re- 

 taining their onward velocity however, and would hence pre- 

 vent the restoration of the atmospheric pressure. The trade 

 to the SE. of this point, then, impelled by the excess of pres- 

 sure then over that to its NW., would also flow inwards at 

 first from ESE., and farther to the south from SE. In this 

 manner only could the change above mentioned have origi- 

 nated, though the moon, on passing the lower meridian, shifted 

 the inner margin of the trade to the northward (19, 21), 

 and the wind accordingly remained at SE. all the subsequent 

 day. 



25. Immediately beyond the polar margins of the trades, 

 the winds are more or less variable ; but, owing to the exist- 

 ence of the equatorial current from the SW. high in the at- 

 mosphere, which frequently becomes directed downwards to 

 the earth's surface, the general tendency of the lower portions 

 of the atmosphere there is to move from the westward. When 

 a current moving from the SW., having its velocity dimi- 

 nished by friction on the earth's surface, is subjected to the 

 indraught to the equator which exists about the outer mar- 

 gin of the trade, a greater or less amount of it will be drawn 

 towards the equator again, still retaining its westerly motion, 

 and will thus constitute a NW., NNW., or N. wind, to the 

 north (and, in like manner, a NW. current will be changed to 

 a SW., SSW., or S. breeze, to the southward) of the line, 

 according to the relative amount of its retained and acquired 

 velocities. This current, from its high temperature, and dew- 

 point, would naturally float higher in the atmosphere than the 



VOL. XXXIX. NO. LXXVII. — JULY 1845. K 



