i 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 spec. 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 



