224 TROPICAL NATURE 



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 6 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- 



