580 REPORT — 1900. 



south-eastern counties. It is generally held that moist winds throw down 

 their water most abundantly just beyond the summit of the first range of 

 hills which they meet. In the case of the downs of Hampshire and 

 Sussex the inland slope has the further advantage of being sheltered from 

 the sun and all the warmer winds, so that it is the fitter for the con- 

 densation of water-vapour. 



Mr. Clutterbuck believes that dew-ponds are ' not easily accounted for 

 by recognised physical causes.' It is plain that the water which collects 

 on the summit of a chalk-down is not drawn from springs, for the 

 saturation -level is hundreds of feet below. Nor is it due in any important 

 measure to surface-drainage. A small collecting area is furnished by the 

 margin of the pond, but this rarely equals the water-surface. A dew- 

 pond may occupy the summit of the ridge so precisely that there can be no 

 collecting ground worth speaking of. 



Hales's view (quoted by White) that more than twice as much dew is 

 deposited upon water as upon an equal surface of moist earth cannot be 

 accepted as it stands. He does not take into account circumstances 

 which may greatly affect the rate of cooling, and consequently the amount 

 of condensation, such as the depth of the water. It may often be observed, 

 for instance, that when a copious dew has been deposited upon the seats 

 of an open boat none is to be seen on the bottom. Contact with a large 

 body of insufficiently cooled water (as of a deep lake) has kept the bottom 

 of the boat at a temperature above the dew-point. 



Water is so bad a conductor of heat that some difficulty may be found 

 in understanding how a pond can cool sufficiently during a summer night 

 to act as an efficient condenser. But though water conducts heat very 

 badly, every surface layer, as it cools by radiation, becomes denser, and 

 sinks. Continual replacement of the surface layer by convection-currents 

 may thus cool down the water as effectually as if the heat were freely 

 conducted away. A shallow pond on a hill-top may in the course of a 

 few hours become cold enough to act as an efficient condenser. 



Water vapour, liquid water, and ice are all good absorbents of dark 

 heat-rays ; it may be inferred that they are good radiators of dark heat- 

 rays. This perhaps does not admit of experimental proof. The radiation 

 from water in a pond is complicated by so many circumstances, such as 

 the absorption of heat by the water vapour whicli the pond gives off, and 

 the sinking of the water as it cools, that no determination by direct 

 methods is possible. 



There is a good deal of testimony to the effect that a dew-pond 

 prospers best with a depth of about 4 feet. If it is deeper it shrinks to 

 such depth as will cool down during a short summer night, i.e. about 

 i feet ; if it is nmch shallower it dries up altogether in a drought. This 

 account, though probable in itself, needs to be scrutinised further. I 

 know as yet of no facts to the contrary, and several in its favour. 



Dew-ponds abound in Sussex and Hampshire, and are not uncommon 

 on the chalk hills of Berkshire and Wiltshire. But on the chalk hills of 

 Hertfordshire, Bedfordshire, Lincolnshire, and Yorkshire few or none are 

 to be found. This may be connected with the distance from the sea in a 

 N.E.-S.W. line. The S.W. winds, which bring the chief part of our 

 atmospheric moisture, can reach the South Downs almost direct from the 

 sea, while they can only reach the chalk hills of the Midlands and the 

 North of England after traversing a great extent of country and crossing 

 many ranges of hills. 



