224 
TROPICAL NATURE 
I 
of humidity, as measured by the comparative saturation of the 
air, is as great as that of Batavia or even greater. A register 
kept at Clifton during the years 1853-1862 shows a mean 
humidity in January of 90, while the highest monthly mean 
for the four years at Batavia was 88 ; and while the lowest 
of the monthly means at Clifton was 79*1, the lowest at 
Batavia was 78*9. These figures, however, represent an 
immense difference in the quantity of vapour in every cubic 
foot of air. In January at Clifton, with a temperature of 35° 
to 40° Fahr., there would be only about 4 to 4| grains of 
vapour per cubic foot of air, while at Batavia, with a tem- 
perature from 80° to 90° Fahr., there would be about 20 
grains in the same quantity of air. The most important fact, 
however, is, that the capacity of air for holding vapour in 
suspension increases more rapidly than temperature increases, 
so that a fall of ten degrees at 50° Fahr. will lead to the con- 
densation of about 1| grain of vapour per cubic foot, while a 
similar fall at 90° Fahr. will set free grains. We can thus 
understand how it is that the very moderate fall of the ther- 
mometer during a tropical night causes heavier dews and a 
greater amount of sensible moisture than are ever experienced 
during much greater variations of temperature in the tem- 
perate zone. It is this large quantity of vapour in the 
equatorial atmosphere that keeps up a genial warmth 
throughout the night by preventing the radiation into space 
of the heat absorbed by the surface soil during the day. 
That this is really the case is strikingly proved by what occurs 
in the plains of Northern India, where the daily maximum of 
heat is far beyond anything experienced near the equator, 
yet, owing to the extreme dryness of the atmosphere, the clear 
nights are very cold, radiation being sometimes so rapid that 
water placed in shallow pans becomes frozen over. 
As the heated earth, and everything upon its surface, does 
not cool so fast when surrounded by moist as by dry air, it 
follows that even if the quantity and intensity of the solar 
rays falling upon two given portions of the earth’s surface are 
exactly equal, yet the sensible and effective heat produced in 
the two localities may be very different according as the 
atmosphere contains much or little vapour. In the one case 
the heat is absorbed more rapidly than it can escape by radia- 
