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i CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 229The drops of rain rapidly increase in size while falling throughthe saturated atmosphere ;and during this process as well asby the formation of dew, the heat which retained the waterin the gaseous form, and was insensible while doing so, isliberated, and thus helps to keep up the high temperature ofthe air. This production of heat is almost always going on.In fine weather the nights are always dewy, and the diagramon the preceding page, showing the mean monthly rainfall atBatavia and Greenwich, proves that this source of increasedtemperature is present during every month in the year, sincethe lowest monthly fall at the former place is almost equal tothe highest monthlyfall at the latter.It may perhaps be objected that evaporation must absorbas much heat as is afterwards liberated by condensation, andthis is true ;but as evaporation and condensation occur usuallyat different times and in different places, the equalising effectis still very important. Evaporation' occurs chiefly duringthe hottest sunshine, when it tends to moderate the extremeheat, while condensation takes place chiefly at night in theform of dew and rain, when the liberated heat helps to makeup for the loss of the direct rays of the sun. Again the mostcopious condensation both of dew and rain is greatly influencedby vegetation and especially by forests, and also by thepresence of hills and mountains, and is therefore greater onland than on the ocean, while evaporation is much greater onthe ocean, both on account of the less amount of cloudyweather and because the air is more constantly in motion.This is particularly the case throughout that large portion ofthe tropical and subtropical zones where the trade-winds constantlyblow, as the evaporation must there be enormouswhile the quantity of rain is very small. It follows, then,that on the equatorial land-surface there will be a considerablebalance of condensation over evaporation, which musttend to the general raising of the temperature, and, owing tothe condensation being principally at night, not less powerfullyto its equalisation.General Features of the Equatorial ClimateThe various causes now enumerated are sufficient to enableus to understand how the great characteristic features of the