Some Observations on Dew-Ponds Author(s): Edward A. Martin Source: The Geographical Journal, Vol. 34, No. 2 (Aug., 1909), pp. 174-191 Published by: geographicalj Stable URL: http://www.jstor.org/stable/1777822 Accessed: 26-06-2016 14:16 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 128.178.131.113 on Sun, 26 Jun 2016 14:16:47 UTC All use subject to http://about.jstor.org/terms ( 174 ) SOME OBSERVATIONS ON DEW-PONDS.* By EDWARD A. MARTIN, F.G.S. THE term " dew-pond " has been applied in the past to certain high-level ponds, the chief characteristic of which is that, as a general rule,they do not fail to give a supply of water when other ponds at lower levels have dried up. The source of the water by which the latter are supplied is apparent. They are fed in general by surface drainage, by brooks and ditches, or by the drainage from a road or high shelving ground around. The sources of supply in these cases are visible, and there is no mystery as to their origin or their replenishment. In the case, on the other hand, of ponds on high ground, where there are no brooks or ditches, and little, if any, drainage, certain of them have been found either never to have run dry within the memory of man, or only very occasionally. It has appeared, therefore, to those who have used such ponds, that there must be some means of replenishment which is not known in the case of ponds at lower levels, and, as there has been no visible source of replenishment at the higher levels which is not present at lower ones, it has been judged that moisture is received at the former in a mysterious invisible manner, and as the high grass ground around such ponds has been observed in the hottest weather to be thickly covered by dew after nightfall, it has been assumed, although not proved, that similarly the ponds received a deposit of dew, and so, with the customary habit of assuming where proof is absent, such ponds, about which the mystery hangs, have come to be termed dew-ponds. All ponds on high ground are not called dew- ponds, but there seems to be no agreement as to what constitutes a dew-ponlld, even between those who use the term for some ponds whilst denying the title to others. Mr. Slade has endeavoured to show that there is no such thing as a dew-pond, but I think there are reasonable grounds for believing that certain high-level ponds do owe their maintenance to some other source than the rainfall, although the rainfall is a considerable factor in filling all ponds. The point we have to remember is that rain-fed ponds at low levels do run dry, as a rule, more rapidly than those on high ground, and this statement has been attested in the past by very many observers, from Gilbert White onward. If this were not a fact, we may assume that the term "dew-pond" would never have arisen, since, as I hold, it was invented to describe those ponds whose replenishment was a mystery, and for whose filling rainfall alone was seen to be insufficient. We start, then, with the postulate that certain high- level ponds maintain their supply in the driest, hottest weather, long after those in the plains below have more or less completely dried up. It seems to be generally acknowledged by farmers on and near the Sussex downs that the word "dew" must be held to include mist and fog, and the word "(mist- pond " is, as a matter of fact, more in constant use in some parts than the word " dew- pond." Any inquiry into the source of the supply must therefore, I think, comprise in its scope the possibility of both dew and fog being deposited on the surface of a pond. One sees occasionally on a damp morning, or sometimes even late in the day, a deposit of water on the walls of a house, inside as well as outside: the walls "sweat," as it is called. There has been no fog, and one cannot call it dew. It is condensation out of the atmosphere, perhaps taking the form it does because of a lack of dust-particles in the atmosphere for the vapour to settle upon. Similarly we must consider the possibility of a condensation of this nature in a pond. Such might happen when a moving mass of heated and saturated air came in contact with a cooler water surface. Again, on many a damp morning, but with no fog, tall trees * Research Department, April 22, 1909. This content downloaded from 128.178.131.113 on Sun, 26 Jun 2016 14:16:47 UTC All use subject to http://about.jstor.org/terms SOME OBSERVATIONS ON DEW-PONDS. 175 will be heard dripping their burden of aqueous condensation on to the road or paths beneath. This cannot be termed dew, properly speaking, although frequently referred to as such. So the fall of condensed vapour of this nature from vegetation must come within the scope of an inquiry into the subject of " dew-ponds." The "dew," therefore, which goes to fill our dew-ponds must be regarded as including mist or fog and any possible form of condensation out of the atmosphere other than rain. Dew-ponds are always found on high ground. The question arises as to whether the maintenance of their supply is due to this position, or whether there is something in the manner of their construction which encourages an extra amount of precipitation over them. First, as to their position: this is usually the highest ground in the neighbourhood, sometimes at the very summit of the dowvns, and in Sussex, at any rate, without a single tree near which could in any way influence the deposition of moisture. In Berkshire and Surrey there is frequently a large overhanging tree. Wherever dew-ponds are found, it is a necessity, as most of the evidence goes to show, that they should either be upon chalk, or, if the subsoil is not chalk, a chalk foundation must be laid down. This would seem to show that chalk is a factor in the matter. The lower temperature found at higher levels may cause a greater precipitation, as dew or mist. Temperatures taken by Mr. Sidney Skinner in proceeding to the top of St. Boniface down (787 feet) showed a reduction as follows:- Bottom. Top. Diff. December 26, 1907 ... 1 42? F. 36? 8 F 5'2 F. ,, 28, , .... 37? F. 32? 2 F. 4o?8 F. ,, 29, , ... 38?03 F. 33?'5 F. 4?'8 F. With the aid of a Royal Society Government grant, I have lately had under my special observation a tract of downland in Sussex, bounded on the north by the escarp- ment of the South downs from the Devil's dyke to Plumpton, and in this area are included some excellent ponds. My headquarters have been one of the disused mills on the Clayton downs, close to whose base is the Ordnance Survey bench-mark of 581 feet. A long spell of dry weather happened while I was there, and, being on the spot from dewy eve till early morn, I was able to make some useful and interesting observations. There are many downland ponds in the area to which I have referred; but two ponds stand out conspicuously amongst the rest as excellent examples of dew- ponds. The first is one which stands high up above the waterworks in the Waterhall valley, near Patcham, and is about 700 feet above sea-level. When I last visited it, on September 13 of last year, the water-edge had a circumference of 260 feet. Between the edge of the water and the top of the ridge surrounding it, there was a shelving bank, measuring 15 feet across, the ridge itself being 347? feet round. The centre had a quantity of green growth below the surface on the eastern side, the place of this being taken by potanmoyeton on the western side; but in neither case did this approach to within 3 feet of the edge of the pond, nor did it grow above the surface of the water. All around this centre of green was a band of this width of clean white chalky puddle with no dark impurity at all. Around the edge of the water there was, on the northern side, a peculiar little ridge, like cliffs in miniature, marking the boundary of the pond. On the shelving bank thistles and hawkweeds and other plants were growing, and the pond would not appear to have exceeded for a long time its present limits. The pond is but slightly below the summit of the down, and had no drainage except such as it received from the banks. The second This content downloaded from 128.178.131.113 on Sun, 26 Jun 2016 14:16:47 UTC All use subject to http://about.jstor.org/terms 176 SO)IE OBSERVATIONS ON DEW-PONDS.