176 EVAPORATION— HUMIDITY OF ATMOSPHERE. 



the atmosphere which reigns dong the northern declivity of 

 the Himmalehs, and which augments at the same tune the 

 irradiation of the table:iand, and the transmission of the 

 xadiant heat which the table-land em_it^, may be considered 

 as the principal causes of the great difference of the heigh. 

 of the snow-line on the north and south sides of the central 

 mountain-ranges of India. , v,- u 



8 i:rflpara/iMi.— Evaporation is that process by which 

 the atmosphere is furnished with the moisture it contains. 

 Water assumes the vaporic form at all temperatures, how- 

 ever low. Thus vapour rises, not only from the plains ot 

 Bengal, but also from the icy and snowy mantles on tho 

 hif^hest rid<Tes and summits of the Himmalehs. Hitherto, 

 owin<T to the want of obsenations, naturahsts have not 

 been^able to trace out in a satisfactory manner the phe- 

 nomena of evaporation in different climates ; although we 

 know, from its general relations to heat, that it is most 

 powerful in the equatorial regions of the globe, and gradu- 

 ally diminishes towards the poles. The instruments neces- 

 sary for ascertaining the power of evaporation have never, 

 as far as we know, been used in India ; but the time is not 

 distant, we hope, when these will find a place in Indian me- 

 teorological observatories. . 



9. Humidxty of ihc Atmosphere.— The earth, as is well 

 known, is surrounded by an atmosphere of air and aqueous 

 vapour. These two matters are mechanically mixed, and 

 each is governed by its own peculiar laws. In order ta 

 supply the atmosphere with aqueous vapour, the process ot 

 evaporation is almost in constant activity ; and Nature has 

 set limits bej'ond which this vapour cannot pass, so as to 

 prevent excessive moisture and long-continued dryness. 

 This dependence of moisture on temperature enables us to 

 trace some of the phenomena of its distribution. There is, 

 as is well known, a gradation of heat from the equator to the 

 poles, and also from the surface of the earth upwards into the 

 hiaher regions of the air. Generally speaking, the lowest 

 stratum of the atmosphere, in whatever latitude it is found, 

 must contain the greatest quantity of aqueous vapour, on ac- 

 count of its being nearest the source whence that moisture is 

 supplied. If an equality of temperature existed, there- 

 fore, at the surface, the same quantity of air, in whatever 

 latitude it was taken, would contain when completely 



