SOME DEW-PONDS IN DORSET. 23 



benefit of his researches in those very interesting papers 

 read before the Royal Geographical Society and printed in 

 " The Geographical Journal," August, 1909, and October, 

 1910, is that dew is formed by the precipitation of the 

 aqueous vapour already existing, in the lower layers of the 

 atmosphere, when the radiation of heat from the earth has 

 caused its surface to be in the condition to chill, below che 

 dew point, the layer of saturated air in contact with it. 

 Precipitated moisture may appear in the form of dew, hoar- 

 frost, mist, fog, or cloud, but in dew and hoar-frost (which 

 may be described as white particles of frozen dew) there is 

 precipitation without a cloudy intermediary. 



A later theory propounded by Dr. J. Aitkens is that dew 

 is really formed from the moisture which rises out of the 

 soil with the radiation of heat, and that it is this which 

 precipitates when the air into which it passes has been so 

 reduced in temperature as to be unable to hold it as aqueous 

 vapour. If this theory is the correct one it would at once 

 dispose of the suggestion that dew-ponds are fed and filled 

 by true dew, since the acquisition of dew could only then 

 be obtained at the expense of itself by earlier evaporation. 



Having shortly considered the formation and constituents 

 of dew we will now turn our attention to the " dew-pond " 

 itself. Messrs. Hubbard tell us that these ponds are formed 

 by first hollowing out a place far in excess of the apparent 

 requirements of the proposed pond ; the whole of the hollow 

 is then covered with a coating of dry straw. The straw in its 

 turn is covered with a layer of well-chosen finely puddled 

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

 or packed with chalk or stones or pitched with flints to prevent 

 the cattle treading through the clay. The puddled clay is 

 chilled by the process of evaporation, and the dry straw, 

 being a non-conductor, prevents the heat of the earth from 

 warming the clay. The result being that during the night 

 the moisture of the comparatively warm air is condensed 

 on the surface of the cold clay and the pond becomes 

 gradually filled. Care has to be taken that the margin of 



