Some Observations on Dew-Ponds: Discussion Author(S): Dr
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Some Observations on Dew-Ponds: Discussion Author(s): Dr. Strahan, Dr. Mill, George Hubbard, Sidney Skinner, Carle Salter, Edward A. Martin and Captain Wilson-Barker Source: The Geographical Journal, Vol. 34, No. 2 (Aug., 1909), pp. 191-195 Published by: geographicalj Stable URL: http://www.jstor.org/stable/1777823 Accessed: 28-06-2016 06:31 UTC Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://about.jstor.org/terms JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Wiley, The Royal Geographical Society (with the Institute of British Geographers) are collaborating with JSTOR to digitize, preserve and extend access to The Geographical Journal This content downloaded from 131.104.62.10 on Tue, 28 Jun 2016 06:31:59 UTC All use subject to http://about.jstor.org/terms SOME OBSERVATIONS ON DEW-PONDS-DISCUSSION. 191 be between 3 and 4 feet deep in the centre, with a further vertical rise of the same amount to the top of the surlrounding bank. A few clumps of grasses, and reeds or rushes, should be encouraged to glow in the water. A gorse-bush or two should be planted on the south-western side, so that its drip shall be into the pond. The shelving bank should be chalk-puddled, or clay-puddled and smoothed down and rammed hard, or even cemented, to obtain such drainage as is possible. It should be near the head of one of the dry valleys coming from the south or south-west. Then, in the winter months, such a pond will be recruited by rain, snow, mist, and dew; but in the summer and in times of drought the pond will be preserved from drying up by mist and dew. Grant that the word "dew " includes mist and fog, then in the summer, at any rate, some of the ponds will be entitled to be called " dew-ponds." On this account, therefore, I think the title by which they have come to be known might remain, without the various qualifications of the title which from time to time have been suggested. Dr. STRAIIAN (before the paper): The first paper this afternoon is one by Mr. Martin, giving his observations on dew-ponds. I will at once ask him to give us an abstract of his paper. Dr. STRAHAN (after the paper): We are indebted to Mr. Martin for what I hope is only a preliminary report on this investigation on dew-ponds which he is carrying out. There remains, evidently, much to be done, and I hope that we may get an account of his future researches at some future period. Dr. M{ILL: The paper, I am glad to see, emphasizes the fact that dew has but little to do with the formation of dew-ponds or with the replenishment and main- tenance of them. I think, however, it would be a pity if that picturesque name should be allowed to vanish, as it really does not matter whether the name is strictly descriptive of the origin of the pond or not. The observations that have been made, and especially those interesting photographs showing the general breadth of the margin round the ponds, go a long way, in my mind, to prove that rain, using it in the technical sense, is the principalsfactor in filling and maintaining dew-ponds. One of the first difficulties raised by this statement is the fact that dew-ponds at a high altitude do not get dry in summer, while ordinary ponds at lower altitudes do. It occurs to me to ask whether ponds at a low altitude are made water-tight as carefully as dew-ponds are ? Are they not much more liable to leakage ? The statement was made in the paper that dew-ponds occur only on chalk soil. There are very similar ponds found on the summit of many other ridges in the neighbourhood of London: along the ridges of Mill Hill and Totteridge there is a chain of ponds on the summit level which, I believe, never dry up, and they have certainly a very restricted drainage area to feed them, and must owe their replenishment to the same cause as the dew-ponds on the South Downs. With regard to the question of the total amount of rainfall at different elevations, there is no doubt that the gauge in the centre of the newly excavated pond would collect more rain than a gauge on the raised rim of it, but it is not because more rain is falling on the centre of the pond than on the rim, but simply because of the eddies set up by the wind on the more exposed rain-gauge, preventing the rain from entering it. It is one of our greatest difficulties in getting measurements of the rainfall on mountains and high moorlands that the eddies set up by the wind carry away a great deal of the rain, and I now recommend that such gauges should be surrounded by a fence to break the force of the wind. I think that the state- ment made in the paper as to the rainfall on the downs being lower than that in the adjacent low-lying region of the weald is inaccurate. I have brought here a rainfall map of the south-east of England which I have prepared on the scale of This content downloaded from 131.104.62.10 on Tue, 28 Jun 2016 06:31:59 UTC All use subject to http://about.jstor.org/terms 192 SOME OBSERVATIONS ON DEW-PONDS-DISCUSSION. 2 miles to an inch, and it shows that the rainfall in this district is closely related to the configuration of the ground; the highest rainfall occurring on the highest land, and the lowest rainfall on the lowest land. With regard to the relation of evaporation from a water-surface to rainfall, much information is published in British Raizn/all annually. Observations are being made at thirteen stations in the British Islands. Some of these have been going on for many years, and it has always been found that the evaporation from an open water surface sufficiently large to have a temperature corresponding to that of a reservoir or pond is much less than the rainfall. In a position like the South Downs, the average rainfall will vary from 35 to 40 inches on the summit, and the evaporation there, on account of the wind sweeping over, may be a3 high as 20 inches, so I should expect that from 15 to 20 inches of water per annum would accumulate in an open straight-sided pond with no drainage area from surrounding land. It is an experiment I am anxious to try; but it presents peculiar difficulties, and it has not yet been made in a satisfactory manner. That trees and shrubs condense or cause water to separate from fog and mist is, of course, well known, and this influence was worked out on Table Mountain by Dr. Marloth to show the influence of a bed of reeds in drawing water from the cloud " tablecloth." One particular make of rain-gauge, the Fleming, was doomed fifty years ago on account of inaccuracy due to this action of a rod. It had a float with a rod attached to it, which rose as the rain-gauge filled, and as it rose it caught so much moisture that the records of these gauges were totally mis- leading. There is no doubt that a row of trees leaning over a pond, especially on a place like the South Downs, would pour a large quantity of water into the pond, drawn from the mists. It is very difficult to distinguish between rain, mist, and dew in some cases. Condensation of moisture from the atmosphere on the surface of water does sometimes occur in excess of the condensation that would take place in a rain-gauge, and the measurements of evaporation at Camden Square in the winter time show that there is occasionally a slight excess of condensation over evaporation on the average of one or more of the winter months every year. The amount is, however, very trifling. I think that the result of those observations has been important in showing that the exaggerated idea of the enormous power of dew to fill a pond when there is no rain has no physical basis. One would like to see them continued for several years, so as to eliminate what we must call " accidental" variations and find out the real underlying facts. I think the new dew-pond Mr. Martin is making promises well, and I look forward with interest to observations which, to a large extent, should solve the question of what happens on setting down an empty tank and leaving it for a year or two exposed to rainfall and evaporation-only it would be necessary to keep the sheep and also the birds away. We owe Mr. Martin our gratitude for undertaking so interesting and so troublesome an investigation. Mr. GEORGE HUBBARD, F.S.A.: As a visitor here I hope that I am not intruding in offering a few words on this subject, which is one that has occupied a good deal of my attention for some years past. I cannot say that I entirely agree with Mr.