222 TROPICAL NATURE 



produces any burning of the skin, while in the tropics at 

 almost any hour of the day, and when the sun has an eleva- 

 tion of only 40 or 50, exposure to it for a few minutes will 

 scorch a European so that the skin turns red, becomes painful, 

 and often blisters or peels off. Almost every visitor to the 

 tropics suffers from incautious exposure of the neck, the leg, 

 or some other part of the body to the sun's rays, which there 

 possess a power as new as it is at first sight inexplicable, for 

 it is not accompanied by any extraordinary increase in the 

 temperature of the air. 



These very different effects, produced by the same amount 

 of sun-heat poured upon the earth in different latitudes, is 

 due to a combination of causes. The most important of these 

 are, probably, the constant high temperature of the soil and 

 of the surface-waters of the ocean, the great amount of 

 aqueous vapour in the atmosphere, the great extent of the 

 intertropical regions which cause the winds that reach the 

 equatorial zone to be always warm, and the latent heat 

 given out during the formation of rain and dew. We will 

 briefly consider the manner in which each of these causes 

 contributes to the high degree and great uniformity of the 

 equatorial temperature. 



Influence of the Heat of the Soil 



It is well known that at a very moderate depth the soil 

 maintains a uniform temperature during the twenty -four 

 hours, while at a greater depth even the annual inequalities 

 disappear, and a uniform temperature, which is almost exactly 

 the mean temperature of the air in the same locality, is con- 

 stantly maintained throughout the year. The depth at which 

 this uniform temperature is reached is greater as the annual 

 range of temperature is greater, so that it is least near the 

 equator, and greatest in localities near the arctic circle, where 

 the greatest difference between summer and winter tempera- 

 ture prevails. In the vicinity of the equator, where the 

 annual range of the thermometer is so small as we have seen 

 it to be at Batavia, the mean temperature of about 80 Fahr. 

 is reached at a depth of four or five feet. The surplus heat 

 received during the day is therefore conducted downwards 

 very slowly, the surface soil becomes greatly super -heated, 



