222 TROPICAL NATURE I 
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, 
