There is plenty of water if only people would store it when the rain comes, but it is, of course, irregularly distributed during the course of the year. With the facilities of modern times in getting water through the tap, old-fashioned ponds have gone out of use. Scores of dew-ponds have been neglected, and it might be as well to bring them into action again, as they are easily constructed and last a long time. They are virtually open-air reservoirs and large sums of money need not be spent on them. I use the name “dew-ponds” as being a popular name, but all that is necessary is that they should be on the highest ground available, and they could be drawn on only in times of shortage. Of course dew has nothing to do with their replenishment.—Mr. Edward A. Martin, F.G.S., 14, High View Close, Norwood, S.E.19.
Letters to the Editor, The Times, 8 January 1934, p. 18, col. E
TO THE EDITOR OF THE TIMES
Sir,—Mr. E. A. Martin thinks that dew has nothing to do with the replenishment of dew ponds, but the observations which have been taken are not in accord with that view.
In 1901, at the request of Dr. J. Cohen and Professor Miall, Shepherd Elliot, of Letcombe Regis, Berks, took measurements of the rise of water in the large dew pond by “The Castle,” as the ancient camp is called. Between January 18 and July 7 he noted the rise on 12 nights of fog, or with heavy dew, and the total rise was 14in., the largest being 2in. In 1902, at the request of my brother, the late C. J. Cornish, Elliot made a larger series of records from February 14 to October 18, 22 nights in all, when the total of the rises came to 32in. Shepherds and farmers on the downs had told me that the ponds rose more from fog than from rain, but I doubted that until my old friend Elliot had proved it.
J. G. CORNISH.
Salcombe House, Sidmouth.
Letters to the Editor, The Times, 22 January 1934, p. 8, col. B
TO THE EDITOR OF THE TIMES
Sir,—The letter on dew ponds, written by the Rev. J. G. Cornish and printed in The Times of January 8, is not only interesting, but also important, because it furnishes the best and most direct evidence of measured increase in the water-level of dew ponds on a misty night.
The scientific construction of dew ponds and their modus operandi are no less interesting. The object aimed at in the construction of a dew pond is to create a surface of water that at all times shall be colder than its surroundings. The deposition of dew will then take place upon it whenever the humidity of the air is sufficient. This object it attained as follows. A saucer-shaped pond, the larger the better, is constructed, with a depth of about 5ft. This concave surface is then covered with the best available non-conductor of heat. The non-conductor has to be kept quite dry, and therefore, in its turn, has to be covered with a waterproof layer. In order to avoid accumulation of heat, this waterproof layer must not be black in colour. The pond is then filled with snow or water.
The non-conductor prevents heat from below from reaching the water. Simultaneously, the water above it will tend to radiate its own heat into space. The result is that a relatively cold surface is exposed, upon which the moisture of a moist and comparatively warm air will be deposited. This will take place most rapidly when radiation is not prevented by the presence of clouds.
A shepherd on the Sussex downs has told us that in the early morning he has occasionally observed a column of mist over a dew pond at a time when the air was otherwise clear, and that he had noticed that the surface of the water was agitated by the rapidity with which the deposition of dew was going on.
ARTHUR JOHN HUBBARD, M.D.
GEORGE HUBBARD, F.S.A.
27, West Park, S.E.9.
Points from Letters, The Times, 24 January 1934, p. 6, col. D
I am not going into the question of the scientific construction of dew ponds raised by Messrs. Hubbard in The Times of January 22, nor of the atmospheric action which takes place. But I am going to challenge the statement that the majority of dew ponds were actually made in that way. During my farming days I had one new pond constructed and two repuddled, besides observing many on adjoining farms. They were all made in the same way: First a layer of clay and lime mixed in the approved manner and well rammed, then another layer of the same, and perhaps another, until a thickness of about 3in. of well-consolidated puddle was attained. A good bed of tightly packed straw about 2ft. thick was placed on the puddle, while that was covered with a layer of chalk rubble to the depth of about 15in. The puddle was built up from the bottom, and was covered every night with the straw as a protection from frost in the winter and from sun in the summer.—Mr. Edward Coward, Southgate House, Devizes.
Letters to the Editor, The Times, 31 January 1934, p. 8, col. B
Sir,—Dew ponds are sometimes more correctly called “cloud ponds.” On a summer’s night the downs are generally covered with dense mist drifting up from the sea. The particles of mist carry small charges of electricity, and the consequent large charge of the cloud induces a charge of opposite sign on the ground underneath. This latter is largely concentrated on the ridge of the downs, and especially on the high points to which the particles of mist are consequently attracted. A dew pond is made by excavating a depression at one of these high points and lining it with puddled clay, through which the deposited water cannot percolate into the pervious chalk. Over 30 years ago I carried out an experiment in which two basins were exposed during a summer night near Chanctonbury Ring, on the South Downs. Each basin had a sheet of copper erected vertically over it, so that moisture collecting on the sheet would drain into the basin. In one case the sheet was connected electrically with the earth, in the other it was insulated. Distinctly more water was deposited on the earthed sheet, although the insulation of the other one was not very good. This confirmed the theory that the action of the dew pond is due to difference of electrical potential between the hill top and the cloud of mist.
The object of the layer of straw that is generally used in the construction or the ponds is apparently to prevent the moisture of the puddling from being absorbed by the underlying chalk; otherwise there would be danger of the clay cracking before the pond had time to fill with water. By the time that the pond is well established the straw must be decomposed to a considerable extent and compressed, so that it would be quite ineffective in causing differences of temperature in the pond. as some of your correspondents have suggested, but its real purpose is served only in the early stages of the pond’s life.
3, Hillsleigh Road, Campden Hill, W.8.
Points from Letters, The Times, 28 June 1934, p. 8, col. D
As there is still much controversy on the subject of dew-ponds, the source from which they receive their supply, the best way to construct them, and whether they can exist without rainfall, it would be interesting to hear how dew-ponds under observation are faring in this prolonged drought. Mists, on which they are said to depend, seem to have been frequent enough. I saw what appeared to be a little one in the Chilterns this week-end; it was quite dried up: but it had evidently been cemented, which is said to be not a good thing. This must be a severe testing time for the best of them.—Mr. W. W. Little, 35, Boundary Road, N.W.8.