166 THE NATUEAL HISTORY 



LETTER XXX. 



TO THE SAME 



Selborne, April 3, 1776. 

 Dear Sir, 



Monsieur Herissant, a French anatomist, seems persuaded that 

 he has discovered the reason why cuckoos do not hatch their 

 own eggs ; the impediment, he supposes, arises from the internal 



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 under- 

 standing 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 effec- 

 tually as if the heat were freely conducted away. A shallow pond on a hill-top 

 may in the course of a few hours become colder than the surrounding rocks and 

 earth, and act as an efficient condenser. 



Dew-ponds abound in Sussex, and are not uncommon elsewhere on the chalk- 

 hills of the south-eastern counties. Buttermere pond, near Inkpen Beacon (Berk- 

 shire), at a height of considerably over 900 feet, is never dry, though it waters a 

 large flock of sheep.— (T. W. Shore.) But in Hertfordshire, Lincolnshire and 

 Yorkshire few or none are to be found. The "meres" of upland farms in 

 Derbyshire have much in common with dew-ponds, but they are not characteristic 

 examples. Advantage is taken, whenever possible, of the wash from a road, and 

 of any surface-drainage, and the "meres" frequently dry up altogether. The 

 presence or absence of true dew-ponds in a particular district may depend upon the 

 distance from the sea in a north-east to a south-west line. The south-west 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. 



Mr. Clement Reid, F.R.S., of the Geological Survey, favours us with the follow- 

 ing additional information : — 



* ' The conditions that are required for a permanent dew-pond do not seem 

 generally to be understood ; failure or success appearing to be the result of chance 

 rather than of any clear comprehension of the principle on which the dew-pond 

 acts. On comparing, at the end of a long drought, the dried-up ponds with those 

 that still contain water, we find that, other things being equal, the best dew-pond 

 has the following characteristics : — 



" It is sheltered on the south-west side by an overhanging tree, often only a 

 stunted, ivy-covered thorn or oak, or by a bush of holly. Or else the hollow is 

 sufficiently deep for the south bank to cut off much of the sun. The depth or 

 shallowness of the water does not appear to make so great a difference as would 

 be expected. 



" When one of these ponds is examined in the middle of a hot summer day it 

 would appear that the few inches of water in it could only last a week. But in 

 early morning or towards evening, or whenever a sea-mist drifts in, there is a 

 continuous drip from the smooth leaves of the overhanging tree. There appears 

 also to be a considerable amount of condensation on the surface of the water itself, 

 though roads adjoining may be quite dry and dusty. In fact, whenever dew is on 



