TROPICAL NATURE 



In the north temperate zone, on the other hand, the winds 

 are always cool, and often of very low temperature even in 

 the height of summer, due probably to their coming from 

 colder northern regions as easterly winds, or from the upper 

 parts of the atmosphere as westerly winds ; and this constant 

 supply of cool air, combined with quick radiation through a 

 dryer atmosphere, carries off the solar heat so rapidly that an 

 equilibrium is only reached at a comparatively low tempera- 

 ture. In the equatorial zone, on the contrary, the heat 

 accumulates, on account of the absence of any medium of 

 sufficiently low temperature to carry it off rapidly, and it thus 

 soon reaches a point high enough to produce those scorching 

 effects which are so puzzling when the altitude of the sun or 

 the indications of the thermometer are alone considered. 

 Whenever, as is sometimes the case, exceptional cold occurs 

 near the equator, it can almost always be traced to the in- 

 fluence of currents of air of unusually low temperature. Thus 

 in July near the Aru islands, the writer experienced a strong 

 south-east wind which almost neutralised the usual effects of 

 tropical heat, although the weather was bright and sunny. 

 But the wind, coming direct from the southern ocean during 

 its winter without acquiring heat by passing over land, was 

 necessarily of a low temperature. Again, Mr. Bates informs 

 us that in the Upper Amazon in the month of May there is a 

 regularly recurring south wind which produces a remarkable 

 lowering of the usual equatorial temperature. But owing to 

 the increased velocity of the earth's surface at the equator a 

 south wind there must have been a south-west wind at its 

 origin, and this would bring it directly from the high chain 

 of the Peruvian Andes during the winter of the southern 

 hemisphere. It is therefore probably a cold mountain wind, 

 and blowing as it does over a continuous forest, it has been 

 unable to acquire the usual tropical warmth. 



The cause of the striking contrast between the climates of 

 equatorial and temperate lands at times when both are 

 receiving an approximately equal amount of solar heat may 

 perhaps be made clearer by an illustration. Let us suppose 

 there to be two reservoirs of water, each supplied by a pipe 

 which pours into it a thousand gallons a day, but which runs 

 only during the daytime, being cut off at night. The reser- 



