t CLIMATE AND ASPECTS OF THE EQUATORIAL ZONE 223 
and a large portion of this heat is given out at night and thus 
keeps up the high temperature of the air when the sun has 
ceased to warm the earth. In the temperate zones, on the 
other hand, the stratum of uniform earth -temperature lies 
very deep. At Geneva it is not less than from thirty to 
forty feet, and with us it is probably fifty or sixty feet, and 
the temperature found there is nearly forty degrees lower 
than at the equator. This great body of cool earth absorbs 
a large portion of the surface heat during the summer, and 
conducts it downwards with comparative rapidity, and it is 
only late in the year (in July and August), when the upper 
layers of the soil have accumulated a surplus store of solar 
heat, that a sufficient quantity is radiated at night to keep up 
a rather high temperature in the absence of the sun. At the 
equator, on the other hand, this radiation is always going on, 
and earth-heat is one of the most important of the agencies 
which tend to equalise the equatorial climate. 
Influence of the Aqueous Vapour of the Atmosphere 
The aqueous vapour which is always present in consider- 
able quantities in the atmosphere, exhibits a singular and 
very important relation to solar and terrestrial heat. The 
rays of the sun pass through it unobstructed to the earth ; 
but the warmth given off by the heated earth is very largely 
absorbed by it, thus raising the temperature of the air ; and 
as it is the lower strata of air which contain most vapour, 
these act as a blanket to the earth, preventing it from losing 
heat at night by radiation into space. During a large part 
of the year the air in the equatorial zone is nearly saturated 
with vapour, so that, notwithstanding the heat, salt and 
sugar become liquid, and all articles of iron get thickly coated 
with rust. Complete saturation being represented by 100, 
the daily average of greatest humidity at Batavia reaches 96 
in January and 92 in December. In January, which is the 
dampest month, the range of humidity is small (77 to 96), 
and at this time the range of temperature is also least ; while 
in September, with a greater daily range of humidity (62 to 
92) the range of temperature is the greatest, and the lowest 
temperatures are recorded in this and the preceding month. 
It is a curious fact that in many parts of England the degree 
