PDF - Wallace Online

222 TROPICAL NATUREproduces any burning of the skin, while in the tropics atalmost any hour of the day, and when the sun has an elevationof only 40 or 50, exposure to it for a few minutes willscorch a European so that the skin turns red, becomes painful,and often blisters or peels off. Almost every visitor to thetropics suffers from incautious exposure of the neck, the leg,to the sun's rays, which thereor some other part of the bodypossess a power as new as it is at first sight inexplicable, forit is not accompanied by any extraordinary increase in thetemperature of the air.These very different effects, produced by the same amountof sun-heat poured upon the earth in different latitudes, isdue to a combination of causes. The most important of theseare, probably, the constant high temperature of the soil andof the surface-waters of the ocean, the great amount ofaqueous vapour in the atmosphere, the great extent of theintertropical regions which cause the winds that reach theequatorial zone to be always warm, and the latent heatgiven out during the formation of rain and dew. We willbriefly consider the manner in which each of these causescontributes to the high degree and great uniformity of theequatorial temperature.Influence of the Heat of the SoilIt is well known that at a very moderate depth the soilmaintains a uniform temperature during the twenty -fourhours, while at a greater depth even the annual inequalitiesdisappear, and a uniform temperature, which is almost exactlythe mean temperature of the air in the same locality,is constantlymaintained throughout the year. The depth at whichthis uniform temperature is reached is greater as the annualrange of temperature is greater, so that it is least near theequator, and greatest in localities near the arctic circle, wherethe greatest difference between summer and winter temperatureprevails. In the vicinity of the equator, where theannual range of the thermometer is so small as we have seenit to be at Batavia, the mean temperature of about 80 Fahr.is reached at a depth of four or five feet. The surplus heatreceived during the dayis therefore conducted downwardsvery slowly, the surface soil becomes greatly super -heated,