with Geographic Botany. — Humidity. 317 



an insensible state. Ne::tj by changes occurring here, it as- 

 sumes a visible form and returns to the earth as rain. Again, 

 whilst on the earth it has to fulfil a variety of uses, furnishing 

 all organized beings with moisture, feeding especially the vege- 

 table kingdom with large quantities, supplying numerous lakes 

 and rivers, and multitudes of streams in all parts of the world, 

 the greater number of which descend to the ocean. Thus it 

 happens that the ocean and organized matter are the last stages 

 in its migration, and hence the chief sources of evaporation. 



Humidity or moisture may then be conveniently studied 

 under three conditions ; — 1st, in the state of vapour; 2nd, as 

 rain and dew ; 3rd, in its subsequent distribution on the earth. 



I. The different parts of the globe, according to their 

 structure and investments, furnish sources for the production 

 of aqueous vapour ; from the proponderance of the ocean 

 over the dry land, and the situation of its deep gulfs and 

 bays along the coasts of the large continents, it is undoubtedly 

 the most fruitful source, and must be always regarded as the 

 chief origin, of the insensible vapour suspended in the atmo- 

 sphere. A vast quantity is daily absorbed when the tempera- 

 ture is moderately warm, for a surface with a diameter of eight 

 inches, exposed on a summer's day, has been found to lose 

 as much as six ounces in twenty -four hours ; and when the 

 surface becomes much increased the accumulated amount is 

 truly surprising. After the ocean, tracts of country covered 

 wdth forests yield the greatest quantity, for trees are continually 

 taking up and giving out moistm'c, and the amount they con- 

 tribute will be in proportion to the luxuriance of the vegeta- 

 tion, the temperature being the same. When the condition of 

 a territory is such as to yield little or no vegetation, the vapour 

 it contributes to the atmosphere is very trifling, and in some 

 of the herbless tracts and deserts it would be a difficult task 

 to appreciate the very small portion resigned. The excessive 

 aridity of the air over the African deserts has been a source of 

 great annoyance to travellers, who complain of the drjniess and 

 roughness of the skin occasioned by it, and also of a very 

 sensibly increased thirst from the rapid transpiration in an 

 atmosphere greedy of moisture. 



Owing to evaporation, the extremes of temperature are 

 modified to favourable conditions ; great heats are kept under 

 by the quantity of caloric becoming latent in the transition 

 from the sensible to the insensible state ; and lest such an 

 enormous evaporation should take place to disturb the pro- 

 per equilibnum in nature, it has been so ordered, that in pro- 

 portion as the air becomes loaded with vapour, vaporization 

 proceeds with less energy. In the extremes of low tempera- 

 ture the former circumstances become reversed, and are thus 



