OF SELBORNE 165 



his Vegetable Statics, advances, from experiment, that "the 

 " moister the earth is the more dew falls on it in a night : and 

 " more than a double quantity of dew falls on a surface of water 

 " than there does on an equal surface of moist earth ". Hence 

 we see that water, by it's coolness, is enabled to assimilate to 

 itself a large quantity of moisture nightly by condensation ; and 

 that the air, when loaded with fogs and vapours, and even with 

 copious dews, can alone advance a considerable and never-failing 

 resource. Persons that are much abroad, and travel early and 

 late, such as shepherds, fishermen, &c. can tell what prodigious 

 fogs prevail in the night on elevated downs, even in the hottest 

 parts of summer ; and how much the surfaces of things are 

 drenched by those swimming vapours, though, to the senses, all 

 the while, little moisture seems to fall.i 



I am, &c. 



• [White's account of the dew-ponds of Hampshire and Sussex can be largely 

 confirmed by recent observations. A good modern description of such ponds is 

 to be found in a prize essay on " Water Supply," by the Rev. J. Clutterbuck (Journ. 

 Roy. Agric. Soc, 2nd ser., vol. i., 1865, pp. 271-87). He says that the tops of chalk- 

 hills, where no surface-water or springs can furnish a supply, are often chosen as 

 the sites of dew-ponds. They "are constructed by persons of experience and 

 skill. At the spot selected an excavation is made in the surface of the chalk, either 

 round or rectangular, from 30 to 40 feet or more in diameter, from 4 to 6 feet deep. 

 The bottom, of a basin shape, is covered in portions with clay carefully tempered, 

 mixed with a considerable quantity of lime to prevent the working of the earth- 

 worms. As the portions are finished they are protected from the action of the sun 

 and atmosphere by a covering of straw ; when the whole bottom of the pond is so 

 covered with an efficient and impermeable coating or puddle, a layer of broken 

 chalk is placed upon it to prevent its injury by cattle or other means. Their cost 

 varies from £'y> to ;^SO. When all is finished, water is introduced by artificial 

 means. If there is a fall of snow, this is collected and piled up in the pond, as the 

 readiest and least expensive method of accomplishing the object. . . . Ponds so 

 constructed and filled have been known for periods of twenty or thirty years never 

 to become dry ; the summer of 1864 was a notable exception " (1887 was another). 



Mr. Clutterbuck believes that dew-ponds are "not easily accounted for by re- 

 cognised physical causes". Nevertheless, an explanation may be attempted. It 

 is plain that the water in such ponds is not drawn from springs, nor from surface- 

 drainage, nor wholly from rain. Part of the supply, and probably a large part, 

 comes from the invisible moisture of the atmosphere. Moisture-laden winds, rising 

 from lower levels, become chilled by expansion, and throw down their water, either 

 in the form of clouds, such as often gather round a mountain-top, or dew. Such 

 condensation takes place on a very large scale on the Alps, where the great excess 

 of condensation over evaporation is very clearly and simply shown by the large and 

 rapid rivers which flow out of Switzerland. 



Hales' 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. He does 

 not take into account circumstances which may greatly affect the rate of cooling 

 and consequently the amount of condensation. Thus it may often be observed 

 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 



