v.] EVAPORATION. 73 



artificial process of distillation. If it is required to distil 

 a liquid, the liquid is evaporated in a boiler, and the 

 vapour conducted to the condenser, where it becomes suffi- 

 ciently cooled to be deposited in drops. The natural 

 process is effected, not by boiling the water over a fire, but 

 by the heat of the sun, which quietly steals vapour from 

 every exposed piece of water, and the vapour thus raised 

 into the atmosphere is ultimately condensed as drops of rain. 

 In artificial distillation, any solid matter which happens to 

 be dissolved in the original liquid will be left behind in the 

 boiler, and the liquid consequently distils over in a state of 

 purity, excepting so far as it may be contaminated by the 

 presence of volatile matters. Just such a purification of 

 water is effected by the natural process of distillation. The 

 sea, which covers so large a proportion of the earth's surface, 

 offers a vast exposure of salt water to the heat of the sun ; 

 yet the salt is left entirely behind and nothing but pure water 

 evaporated. Fresh water is thus being constantly distilled 

 from the briny ocean. 



Thus, in seeking for the sources of the Thames, we are 

 led from the springs of the earth to the rain of the heavens ; 

 and from this to the watery vapour which forms part of the 

 atmosphere; and thence to the great caldron, the ocean, 

 whence the heat of the sun distils that vapour. The great 

 stream of fresh water which flows over Teddington Weir is 

 fed, in large measure, by vapour which has been raised far 

 away on the Atlantic. South and south-west winds sweep- 

 ing across that ocean become highly charged with watery 

 vapour; and these warm moist winds, striking the 

 Cotteswold Hills, deposit their freight of moisture in showers 

 of rain, much of which reaches the Thames basin. This 

 water is ultimately carried oui to sea by the flow of the 

 river, and mingles once more with its parent ocean, but 



