We have already seen where the rainless portions of the earth are, and why they are so; because those lying north of the northern limit of the equatorial rainy belt were yet too far south to be covered by the line of extra-tropical rains; or in other words, too far south to be uncovered by the surface N. E. trades and the longitudinal magnetic currents, and to be covered by the counter-trades in contact, or nearly so with the earth, and influenced by the perpendicular north polar magnetic currents. Thus we have seen that the rains of Southern Mexico were summer rains, due to the northern extension of the equatorial rainy belt; those of California were winter rains, due to the southern extension of the extra-tropical rains following the N. E. surface trades. We have also briefly alluded to the fact that either side of the equatorial rainy belt, evaporation is going on for months under a vertical sun, without precipitation--unless it be from an occasional brief storm of great intensity which originates in that belt at the line of it, and passing on in the counter-trade, reverses, for the time being, by its concentrated and powerful action, like a magnetic body introduced into the field of another magnet, the surface-trades. Mere evaporation then, does not produce the storm, or shower, or rain, where most active in the dry torrid zone. It may be said that those dry portions are, for the time being (as the rainless portions of the earth are continually), within the operation of the surface-trades, and that therefore the evaporated moisture is carried away by them toward the equatorial rainy belt. Precisely so; but why carried away? Why should it not condense, occasionally, at least, and drop the rain as it passes along, if a great supply of moisture from excessive evaporation could furnish rain. Perhaps it may still be said it is going from a cold to a warm section. This is not true, as we have shown. But, it may be said that the rainless regions at any rate receive no moisture, and therefore can not supply any by evaporation. This would not meet the case, as it would still be true that when the rainy belt has left a given spot, the dry weather sets in with excessive evaporation, and the north-east trades in summer, blowing from the countries lying north of the rainless regions, and which have been supplied during the interval by the extra-tropical rains, and are loaded with evaporation, are passing over the rainless regions on their way to enter the central belt. So blow the N. E. trades from the Mediterranean, and the Barbary States _over the Desert of Sahara_ and into the rainy belt south of it; but drop no moisture on their way, because exposed to no magnetic currents of an opposite polarity.