348 On the Circulation ^c. of the Atmosphere of the Atlantic. [June 15, 



currents to keep up the heaping ; hut he may naturally ask, how do I know 

 that an upper current comes from the pole? First, because the prevailing 

 surface winds in high latitudes blow towards the pole, which air must re- 

 turn ; and secondly, because the trade-winds are composed of cool dry air, 

 which could not have come from the equator : here there is pretty good 

 evidence that two upper currents come to the surface of the earth in these 

 zones where the air is heaped up, and again, that in dipping to the surface 

 by some unknown means they cross each other, as Maury conjectured. 



Perhaps a few words may be desirable as to the manner in which the 

 westerly winds which blow in high latitudes appear to draw the air from 

 the heaps above mentioned. Here we will refer to our experience in 40 

 S. lat., where the normal circulation of the air is less interfered with by the 

 land. 



This parallel of latitude is subject to a series of gales which commence 

 at N. and end at N.W. or W. As the north wind sets in, the barometer 

 falls, the air becomes warm, damp, and cloud v ; the wind gradually draws 

 round to the N.W., after a time rain accompanies the wind, the barometer 

 continues to fr.ll, often fast, until in a heavy shower of rain the wind shifts 

 to the west, when the barometer immediately rises, generally followed by a 

 strong breeze from the westward, which decreases as the quicksilver rises, 

 very often settling down into a calm. After a few hours the north wind 

 sets in again, with a falling barometer, and a repetition of the whole series 

 takes place. 



One is naturally led to ask why the trade-wind draws air from this heap 

 in a regular continuous stream, when these gales are fitful. May it not be 

 because in the direction in which the trade moves the meridians diverge 

 and give plenty of room for the flow, whereas the westerly winds have 

 converging meridians which seem to check the progress of the air. These 

 fitful gales have always led me to think that the air was checked in its 

 course. If further south, say in 50 S. lat., the wind continues steady from 

 the west (as Maury leads us to suppose is the case), then this zone of -40 

 seems to act as a reservoir for the westerly winds, being constantly refilled 

 and steadily drawn off, only the stream into the reservoir is freer than that 

 which runs out. 



Now if we consider that these gales are composed of the warm damp 

 air which come to this heap from the equator above the S.E. trades, de- 

 scending to the surface of the earth and travelling towards the pole, their 

 westing is accounted for by the change in the diameter of the circular 

 route which the air has to describe in accompanying the earth in its revo- 

 lution. These gales changing from N. to N.W. and W. have been treated 

 as the N.E. quarters of southern hemisphere cyclones ; and we read in the 

 'Nautical Magazine ' of a ship's having hove to to allow one of them to 

 pass ; but if, as we suppose, they form part of the normal circulation of the 

 air, it seems useless to heave to to avoid them. The source of these gales 



