36 THE BOOK OF THE OPEN AIR 



by mists and fogs. Dew, if we inter- wayfarer who rises betimes could tell 



pret the word strictly, is of less impor- a somewhat similar tale, 

 tance, hence ;the objection to the When we come to ask why such a 



name dew-pond. The pond of course large amount of moisture should be 



receives a little rain at times, accord- concentrated on the small area of the 



ing to the area of the cavity, and the mist-pond there is a hot dispute, 



angle of the sloping sides, but this Some writers airily dismiss the problem 



amount forms a small proportion as very simple, though their own 



of the total. Ponds are ofttimes explanations are by no means of that 



intentionally constructed on the nature. Others assume the action 



shoulder of a hill, or near the junction of electricity, others again invoke the 



of two spurs, in which case much rain aid of the dust particles floating in the 



is directed to the hollow, which be- atmosphere. The question has been 



conies a "rain and mist-pond." discussed at a meeting of the British 



That fog and rain are the prime Association, though even there unani- 



agents in the supply of a real mist-pond mity was absent. Amid the babel of 



is admitted. The Rev. J. G. Cornish voices a general principle or two may 



made experiments on the Berkshire be heard which may give the clue. 



Downs by means of notched sticks The upper surface of the water in the 



which he thrust into the pond and pond is cooled by radiation, and 



examined night and morning. The convection currents are thus set up. 



water level was found to rise as much In other words the colder layers of 



as an inch or two in a single night, the surface descend, and the warmer 



though no rain whatever had fallen ; water from below rises to the top. 



the increase was due to dense fog. This goes on until the contents of the 



When there was a heavy dew, but no pond are colder than the surrounding 



fog, the increase was less. Observa- rocks and soil, when condensation 



tions like these set the matter beyond of the aqueous vapour, whencesoever 



cavil. No one who is acquainted with arising, goes on at a rapid rate, 

 the chalk downs would care to dispute Mr. Clement Reid and other com- 



the intensity of the mists. Shepherds petent observers state that a tree 



and those whose business calls them beech or oak, or preferably a stunted 



forth in the early hours of the morning bush of holly or hawthorn, will facili- 



often return soaked to the skin. The tate the deposition, especially if it 



