SUPPLY AND WASTE OF HEAT. 13 



almost neutralised the usual effects of tropical lieat 

 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 

 of an unusually 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 blow- 

 ing 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 reservoirs are both leaky, 

 but while the one loses at the rate of nine hundred 

 gallons in the twenty-four hours the other loses at the 

 rate of eleven hundred gallons in the same time, sup- 

 posing that both are kept exactly half full and thus 

 subjected to the same uniform water-pressure. If now 

 both are left to be supplied by the above-mentioned 

 pipes the result will be, that in the one which loses by 

 leakage less than it receives the water will rise day by 



