THE ATMOSPHERE. 103 



their length and their feathers the mean annual duration from 

 each quadrant. Only the arrows nearest to the axis in each belt 

 of 5^ of latitude are drawn with such nicety. The largest arrow 

 indicates that the wind in that belt blows annual I3', on the 

 average, for ten months as the arrow flies. The arrow from the 

 next most prevalent quarter is half-feathered, jDrovided the 

 average annual duration of the wind represented is not less than 

 four months. The unfeathered arrows represent winds having an 

 average duration of less than three months. The arrows are on 

 the decimal scale ; the longest arrow — which is that representing 

 the south-east trade-winds between 5° and 10° S., where their 

 average duration is ten months — being half an inch. Winds that 

 blow five months are represented by an arrow half this length, 

 and so on. The half-bearded arrows are on a scale of two for 

 one. It appears, at first, as a singular coincidence that the wind 

 should whirl in these discs about the poles as it does in cyclones, 

 viz., against the hands of a watch in the northern, T\T.th them in 

 the southern hemisphere. 



267. The offices of sea and air in tlie physical economy. — To act 

 and react upon each other, to distribute moisture over the surface 

 of the earth, and to temper the climate of different latitudes, it 

 would seem, are two of the many offices assigned by their 

 Creator to the ocean and the air. When the north-east and 

 south-east trades meet and produce the equatorial calms (§ 212), 

 the air, by the time it reaches this calm belt, is heavily laden 

 with moisture, for in each hemisphere it has travelled obliquely 

 over a large space of the ocean. It has no room for escape but 

 in the upward direction (§ 223). It expands as it ascends, and 

 becomes cooler ; a portion of its vapour is thus condensed, and 

 comes down in the shape of rain. Therefore it is that, under 

 these calms, we have a region of constant precipitation. Old 

 sailors tell us of such dead calms of long continuance here, of 

 such heavy and constant rains, that they have scooped up fresh 

 water from the sea to drink. The conditions to which this air is 

 exposed here under the equator are probably not such as to cause 

 it to precipitate all the moisture that it has taken up in its long 

 sweep across the waters. Let us see what becomes of the rest ; 

 for Nature, in her economy, permits nothing to be taken away 

 from the earth which is not to be restored to it again in some 

 form, and at some time or other. Consider the great rivers — the 

 Amazon and the Mississippi, for examj)le. We see them day 



