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 annually, on the average, for ten months as the arrow flies. The arrow from the next most prevalent quarter is half-feathered, provided 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, with them in the southern hemisphere.
267. The offices of sea and air in the 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 example. We see them day
