THE HEAT OF THE SUN 311 



On high downs these summer night-fogs help to feed the 

 dew-ponds, which are so valuable to the upland sheep farmer 

 in dry seasons, when water for general farm use may have 

 to be carted two or three miles. Dew-ponds are fed by rain, 

 when rain falls, like any other ponds ; and it is doubtful 

 whether dew as distinct from mist contributes much to their 

 store. They are not so miraculously inexhaustible as is 

 sometimes said, and they vary a good deal in their power of 

 withstanding drought according to height and position. But 

 considering their shallowness, small size, and usual direct 

 exposure to the sun's rays on the treeless down, it is very 

 remarkable how often and how long they hold water in the 

 fiercest summer droughts. The mist condenses on the surface 

 of the water and on the bare sides of the pool when the 

 water is low, and if a tree overhangs the supply is increased 

 by the drip from its boughs. There is no truth in the notion 

 that an overhanging tree prevents the collection of water 

 from the atmosphere ; but the presence of a tree at the edge 

 of a dew-pond is dangerous for another reason. Its roots 

 are likely to pierce the pan of puddled clay which holds the 

 water. In making a dew-pond this layer of clay or chalky 

 marl is the most important feature. A layer of straw is 

 often placed beneath it ; and though it is sometimes supposed 

 that the object of the layer of straw is to check the trans- 

 mission of the earth's heat, and so promote condensation, the 

 real object seems to be simply the careful packing of the 

 water-tight bottom. Besides the more copious condensation 

 of the mist, a reason for making dew-ponds on bare crests 

 of the down is in order to prevent surface drainage. If a 

 rivulet entered the pond in times of rain, before long it would 

 break up the clay pan, and the water would escape into the 

 chalk. In modern dew-ponds in the chalk valleys surface 

 water is occasionally utilised, and the inflow of the gutter 



