384 



HYDROGRAPHY. 



seeking the heated or rarefied areas. Knowing where the areas of the 

 heated waters are, where calms usually take place, it will at once be 

 perceived how these vexatious visitations to the navigator are to be 

 avoided, and they will serve as a means by which he will be enabled 

 to pursue his voyage and preserve his wind. All that is required of 

 him is to regard those parts of the ocean where the high temperatures 

 exist at certain seasons of the year, and so to navigate his vessel as to 

 pass either on one side or the other, by which he may be assured that 

 he will be able to take advantage of the current of air that may be 

 flowing towards them. 



It now remains to treat of the variable winds of the temperate 

 zones, or those spaces which I have included between the latitudes of 

 30° and 60° north and south. It is well known in these northern 

 and southern zones, in both hemispheres, that the winds prevail from 

 the belt of calms on the borders of the Trades in the summer months, 

 and towards them in the winter months. This is the general course 

 of the variables. By the old theory as quoted, it will be recollected 

 that these winds were assumed as the return current towards the 

 Poles, and that their direction resulted from the action of the rotary 

 motion of the earth, and the flow to the north and south was the 

 return current to the Poles, which they had supplied to the Trades ; 

 and were it not for this interchange, the wind, it is argued, " would 

 cease to blow for want of air to make wind of." The modification of 

 this system, since adopted, I regard as equally inconsistent with what 

 takes place, viz., that the heated air which rises from within the Tropics 

 flows over towards the calm areas on the outer borders of the Trade 

 Winds, there meets with another upper current from the Poles, de- 

 scends to the surface, and reissues in two surface currents in opposite 

 directions — the one to the south, the other to the north — the former 

 to supply the Trades, the latter to restore to the Poles what had been 

 taken from them for that purpose. This modification of the old 

 theory removes one obstacle, — that of the winds coming in actual 

 contact on every successive parallel ; but it creates another, and, in 

 my opinion, a greater one, viz. : a meeting and crossing at the area of 

 calms of opposite currents of air of different temperatures without 

 commingling. I can see no reason why the air should flow towards 

 these belts by the upper current, descend, and reissue in different and 

 opposite directions, affected, as it is represented to be, by the rotary 

 motion of the earth, united with a flow to and from the Poles, whilst 



