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TROPICAL NATUREIn the north temperate zone, on the other hand, the windsare always cool, and often of very low temperature even inthe height of summer, due probably to their coming fromcolder northern regions as easterly winds, or from the upperparts of the atmosphere as westerly winds and this constant;supply of cool air, combined with quick radiation through adryer atmosphere, carries off the solar heat so rapidly that anequilibrium is only reached at a comparatively low temperature.In the equatorial zone, on the contrary, the heataccumulates, on account of the absence of any medium ofsufficiently low temperature to carry it off rapidly, and it thussoon reaches a point high enough to produce those scorchingeffects which are so puzzling when the altitude of the sun orthe indications of the thermometer are alone considered.Whenever, as is sometimes the case, exceptional cold occursnear the equator, it can almost always be traced to the influenceof currents of air of unusually low temperature. Thusin July near the Aru islands, the writer experienced a strongsouth-east wind which almost neutralised the usual effects oftropical heat, although the weather was bright and sunny.But the wind, coming direct from the southern ocean duringits winter without acquiring heat by passing over land, wasnecessarily of a low temperature. Again, Mr. Bates informsus that in the Upper Amazon in the month of May there is aregularly recurring south wind which produces a remarkablelowering of the usual equatorial temperature. But owing tothe increased velocity of the earth's surface at the equator asouth wind there must have been a south-west wind at itsorigin, and this would bring it directly from the high chainof the Peruvian Andes during the winter of the southernhemisphere. It is therefore probably a cold mountain wind,and blowing as it does over a continuous forest, it has beenunable to acquire the usual tropical warmth.The cause of the striking contrast between the climates ofequatorial and temperate lands at times when both arereceiving an approximately equal amount of solar heat mayperhaps be made clearer by an illustration. Let us supposethere to be two reservoirs of water, each supplied by a pipewhich pours into it a thousand gallons a day, but which runsonly during the daytime, being cut off at night. The reser-