498 



Dew-ponds. 



[NOV., 



The " dew-pond," of which some ancient examples still 

 exist, is a pond constructed in such a way that it obtains 

 water by condensation from the atmos- 

 Dew-Ponds. phere and is altogether independent of 

 springs or rainfall. Attention was called 

 to this method of obtaining water in dry situations by Messrs. 

 A. J. and G. Hubbard in their book "Neolithic Dew-ponds and 

 Cattle-ways," and a description of the method of construction 

 was given in this Journal, June, 1906, p. 181. In a second 

 edition of their book Messrs. Hubbard give an account of some 

 experiments the}^ have conducted in this connection and point 

 out that some scientific modification of this device might be 

 the means of making habitable large tracts of country which 

 are now desolate owing to the absence of water supplies. 



The method of making a dew-pond as still practised in 

 some parts of England is to hollow out a space in excess of 

 the requirements of the proposed pond and then thickly cover 

 the whole of the hollow with a coating of dry straw. The 

 straw in its turn is covered by a layer of well-chosen, finely- 

 puddled clay, and the upper surface of the clay is then closely 

 strewn with stones. Care has to be taken that the margin 

 of the straw is effectively protected by clay. The pond will 

 gradually become filled with water the more rapidly the 

 larger it is, even though no rain may fall. 



The explanation given by Messrs. Hubbard of this fact 

 is that if such a structure is situated on the summit of a 

 down, the earth during the warmth of a summer day will 

 have stored a considerable amount of heat, while the pond 

 protected from this heat by the non-conductivity of the straw, 

 is at the same time chilled by the process of evaporation from 

 the puddled clay. The consequence is that during the night 

 the moisture of the comparatively warm air is condensed on 

 the surface of the cold clay. As the condensation during the 

 night is in excess of the evaporation during the day, the pond 

 becomes, night by night, gradually filled. 



The general interest that was shown in the above theory 

 induced Messrs. Hubbard to attempt the construction of a 

 large dew-pond on scientific principles. Land could not be 

 obtained at an elevation and the experiment was made on a 

 low-lying site. A space 100 feet square was excavated to a 



