16 TROPICAL NATURE, AND OTHER ESSAYS. 



low altitudes. The drops of rain rapidly increase in 

 size while falling through the saturated atmosphere ; 

 and during this process as well as by the formation of 

 dew, the heat which retained the water in the gaseous 

 form, and was insensible while doing so, is liberated, and 

 thus helps to keep up the high temperature of the air. 

 This production of heat is almost always going on, In 

 fine weather the nights are always dewy, and the diagram 

 on the preceding page showing the mean monthly rain- 

 fall at Batavia and Greenwich proves that this source 

 of increased temperature is present during every month 

 in the year, since the lowest monthly fall at the former 

 place is almost equal to the highest monthly fall at the 

 latter. 



It may perhaps be objected, that evaporation must 

 absorb as much heat as is afterwards liberated by con- 

 densation, and this is true ; but as evaporation and 

 condensation occur usually at different times and in 

 different places, the equalising effect is still very 

 important. Evaporation occurs chiefly during the 

 hottest sunshine, when it tends to moderate the extreme 

 heat, while condensation takes place chiefly at night in 

 the form of dew and rain, when the liberated heat helps 

 to make up for the loss of the direct rays of the sun. 

 Again, the most copious condensation both of dew and 

 rain is greatly influenced by vegetation and especially 

 by forests, and also by the presence of hills and moun- 

 tains, and is therefore greater on land than on the 

 ocean ; while evaporation is much greater on the ocean, 

 both on account of the less amount of cloudy weather 

 and because the air is more constantly in motion. 

 This is particularly the case throughout that large 



