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THE theory of “The Dew-Ponds” is a subject which must be approached with a good deal of caution, for the further we carry our investigations the more difficult does the subject become, and grave doubts arise whether the many ancient ponds so ably described by Messrs. Hubbard in their recent work on “Neolithic Dew-Ponds and Cattle Ways,” particularly those in the immediate vicinity of Cissbury Ring and Chanctonbury Ring, and that on Maiden Castle, near Dorchester, and other so-called “dew-ponds” were really “dew-ponds,” in the strict sense of the word, but rather rain or mist-ponds.
In approaching the subject it is of course primarily necessary to consider how dew is formed, and as to this there is no unanimity of opinion. The old and popular theory, and that favoured by Mr. A. E. Martin, F.G.S., who has made the subject of “dew-ponds” a study and has given us the {23} benefit of his researches in those very interesting papers read before the Royal Geographical Society and printed in “The Geographical Journal,” August, 1909, and October, 1910, is that dew is formed by the precipitation of the aqueous vapour already existing, in the lower layers of the atmosphere, when the radiation of heat from the earth has caused its surface to be in the condition to chill, below the dew point, the layer of saturated air in contact with it. Precipitated moisture may appear in the form of dew, hoarfrost, mist, fog, or cloud, but in dew and hoar-frost (which may be described as white particles of frozen dew) there is precipitation without a cloudy intermediary.
A later theory propounded by Dr. J. Aitkens is that dew is really formed from the moisture which rises out of the soil with the radiation of heat, and that it is this which precipitates when the air into which it passes has been so reduced in temperature as to be unable to hold it as aqueous vapour. If this theory is the correct one it would at once dispose of the suggestion that dew-ponds are fed and filled by true dew, since the acquisition of dew could only then be obtained at the expense of itself by earlier evaporation.
Having shortly considered the formation and constituents of dew we will now turn our attention to the “dew-pond” itself. Messrs. Hubbard tell us that these ponds are formed by first hollowing out a place far in excess of the apparent requirements of the proposed pond; the whole of the hollow is then covered with a coating of dry straw. The straw in its turn is covered with a layer of well-chosen finely puddled clay, and the upper surface of the clay is then closely strewn or packed with chalk or stones or pitched with flints to prevent the cattle treading through the clay. The puddled clay is chilled by the process of evaporation, and the dry straw, being a non-conductor, prevents the heat of the earth from warming the clay. The result being that during the night the moisture of the comparatively warm air is condensed on the surface of the cold clay and the pond becomes gradually filled. Care has to be taken that the margin of {24} the straw is effectively protected by the clay, since if the straw becomes wet it will cease to attract the dew, as is ceases to act as a non-conductor of heat, and becomes of the same temperature as the surrounding earth. It is certain, however, that many alleged dew-ponds are not formed on this plan. “Dew-pond” has come to be a generic term, and, apart from its original conception, as applied to those ponds formed by the Neolithic People in or near their hill camps for the purpose of watering their flocks and herds, is now applied to the many kinds of ponds, made at far later dates by modern man on the hilltops or other elevated positions for the use of his cattle and sheep—such as rain and mist ponds, drip-ponds, catch ponds, &c.—each of which I propose to consider and give examples of later on.
But to return to our “dew-pond.” We have seen that Messrs. Hubbard use dry straw, then prepared clay, and then chalk or stones, or packed flints, and give their reasons for doing so.
Mr. G. G. Desmond, writing in the Nature Notes Column of the Daily News, gives a different arrangement for the basis of the dew-pond. He first forms his hollow, then lays down a bed of concrete, this is covered with dry straw, over which is placed another bed of concrete. I doubt very much if Neolithic man made his dew-ponds in this way. Mr. H. P. Slade, in a pamphlet on “dew-ponds,” written in 1877, discards the term “dew-pond” in favour of “rain-pond,” and says that “dew” had little or no part in filling his pond, which he lined first with a layer of clay 12 inches thick, mixed with lime to prevent the working of worms, second a coating of straw to keep the clay moist and prevent the sun from cracking the clay in the event of the pond becoming dry; and thirdly, a layer of loose rubble to prevent the hoofs of cattle trampling upon and perforating the clay and thus causing the pond to leak. This pond, which had a diameter of some 70 feet and was situate on the Thorpe Downs near Loughborough at a height of 450 feet above sea level, was seldom if ever known to be dry. {25}
In re-making two ponds for me on Stratton Down Mr. Lane, a well-known pond maker, of Fordington, used first a layer six inches thick of well-worked-up clay laid on the solid chalk, then a coating of gas tar, to prevent the worms working through, then another six inches of puddled clay, the whole being covered with a coating of flint pitching to prevent the cattle treading the clay, no straw being used. As rain or mist ponds these have been most successful.
Thus it will be seen there is a great divergence of opinion as to how these ponds should be formed, but I think it will be generally admitted that rain, mist, and fog are the chief elements which contribute to their supply.
I now propose to give you some account of my researches in this direction, on the downs around Dorchester during the late summer, and for the convenience of reference I propose to number my ponds.
Which, for the sake of further distinction, we will call the “Grimstone Down Dew-Pond,” I examined on the 3rd of August, 1911. This pond on plan is practically square, being 42 feet by 40 feet in measurement at the top of the bank, with runlets at the corner angles; it is 6ft. 6in. deep, including the bank, and as there is a silting up of about a foot, the original depth would have been 7ft. 6in. It is made on the north-west side of the hill, and would command a considerable gathering surface. It is placed some 580 feet above sea-level, as shown by the contour on the Ordnance Map. It is covered with vegetation, and has not been known to contain water within living memory. On cutting a trench through the centre I first came upon a layer, some 10 inches thick, of black alluvial soil, which had the appearance of a silting up, or might have been decayed vegetable matter. Then came a carefully laid layer of packed flints, followed by a layer of stiff red clay mixed with lime, which appeared to have been laid on in layers on the solid chalk, to a thickness {26} of seven or eight inches. I produce a specimen of this clay, which, at the time it was taken out, was quite damp and soft. I could find no trace of any layer of straw or other non-conducting material, either above or below the course of clay.
Pond I examined the same day; this is situate some 550 feet above sea level, to the north of the Grimstone clumps. It is of a somewhat similar shape to Pond No. 1, about 36 feet square and 6ft. 6in. deep, made with puddled clay, pitched with flint, no trace of straw either above or below the layer of clay being found. The bottom and sides are grown over with herbage, and there is a considerable growth of rushes near the bottom, which alone would account for its not holding water, being situate on the flat high ground; it had not the gathering surface of No. 1.
Both these ponds are placed without, though within the immediate vicinity of, the ancient British settlement on Grimstone Down.
Another pond of much the same character and shape as the two before mentioned is to be found on Maiden Castle, near the dividing scarp, within the rings. This pond measures 51ft. by 48ft., with a probable depth, including the embankment, of eight to ten feet.
Like No. 1, it is practically square on plan, and has a considerable rain-collecting area or watershed, some 15 to 18 feet wide, formed by its surrounding embankment, outside and above its flint-pitched or water-containing area.
Messrs. Hubbard describe this pond as a “dew-pond,” and refer to its unusual position—“within the rings”—a reason for which they say is to be found in the fact that outside the rings a supply of running water was near at hand, in this respect presenting a contrast to Chanctonbury-cum-Cissbury, where these ponds are found outside the rings, {27} and that, being near the dividing central scarp, it is conveniently placed to supply the needs both of the human community who occupied the Eastern Division, and of the animals to which the Western Division was devoted.
These three ponds are somewhat of a puzzle to me, the acknowledged shape of the Neolithic “dew pond” being circular, whilst these, as we have seen, are square, with runlets at the angles, so that it seems probable that these ponds have been re-formed and re-shaped in comparatively modern times, in fact we know that this was the case with the Maiden Castle Pond, which was re-made and enlarged by the late Mr. Henry Hawkins about 40 years ago.
Pond is on Stratton Down, and, like Nos. 1 and 2, is without, though within the immediate vicinity of, the British settlement on Grimstone Down. This pond is circular in form, about 30 feet in diameter, and was originally some 5 to 6 feet deep. On cutting a section, North and South, I came upon one foot of soil, mixed with flints. Then came a layer of clay, mixed with lime, resting on the solid chalk, but not so thick or well defined as that in the Grimstone Down Ponds. I have never known this pond to hold water.
Is a pond circular in form—also on Stratton Down—which was always dry. Some five or six years ago I had this pond re-formed and re-puddled by Lane, of Fordington, and the runlets cleared out, since when it has always contained a supply of water, even through the severe test furnished by the drought of last summer (1911).
There is a pond of great antiquity and interest, known as “Greenhill Pond,” situate on the elevated plateau on the {28} North side of Piddletown Heath, near the trackway leading from Higher Bockhampton to Piddletown, at a point where the trackway diverges towards Ilsington Wood.
This plateau overlooks, on the South, the three British Tumuli known as “rain-barrows” and the remains of the Beacon Keeper’s hut used in the war with Napoleon, and referred to by Mr. Thomas Hardy in “The Dynasts;” and on the North is a most extensive view of Piddletown and the Valley of the Piddle. This pond, which from its form and situation has every appearance of being a Neolithic dew-pond, measures about 90 feet in diameter, including its outer scarp—the water holding area being about 45 feet in diameter. The depth is considerable, but I had no means of ascertaining its exact measurement, and would, when full, have contained a large condensing area. It seems probable that this pond may have been formed from one of the “soaks” of which there are a large number upon the heath.
By the courtesy of Mr. Thomas Hardy I am permitted to give you, in his own words, an account of the following interesting adventure which happened to an ancestor of his (the author’s grandfather), beside this very pond, as nearly as possible a century ago. He was crossing the heath, one midnight in June, by the path which then, as now, skirts the pond, when he became aware that he was followed by two men whom he had noticed watching him when he left Piddletown. He had now little doubt that they were bent on attacking and robbing him, for times were more lawless then than they are at present. It had so happened that while crossing a green field called “Coomb” a little earlier in his journey, he had been struck by the great number of glow-worms that were shining in the grass, and being a young man he beguiled his walk by gathering several and placing them on the brim of his hat. As he was unarmed, and the men were gaining upon him, the only way of escape that occurred to him was by playing upon their superstitious feelings. He accordingly rolled a furze faggot into the path, and, sitting down upon it, took off his hat, placed it on his {29} knees, stuck two fir fronds on his head to represent horns, and pulled from his pocket a letter he chanced to have with him, and began reading it by the light of the glow-worms. The men approached, stopped suddenly, and then bolted at the top of their speed down the hill and disappeared. In a few days there was a rumour in the neighbourhood that the devil had been seen at midnight by “Greenhill Pond,” reading a list of his victims by glow-worm light. He tried afterwards to discover who the men were, but they never revealed their identity.
Another pond worth a visit is “Rushy Pond,” situate on the South side of Piddletown Heath near the trackway leading from Bhompston northward to Yellowham Bottom, at the point where the trackway intersects the old Roman Road from Dorchester, through Weatherby Castle to Badbury Rings. This pond is of a different form and character from that last described, and appears to be fed from the surface water collecting at this point from the trackway and the Roman Road, and I think should be classed as a “Catch-pond.” It has seldom, if ever, been known to be dry.
Through the courtesy and with the assistance of Mr. Arthur Symonds, of Wolfeton Manor, I examined in August last (1911) a very ancient pond on the Wolveton Estate, on what was formerly a part of the Charminster Down, near the Northern boundary, between that parish and Piddlehinton. This pond, which is seldom dry, is placed on the Down, swept by the thick sea mists from the South-west, some 500 feet above sea level. Its situation is at the foot and on the south side of a steep declivity—covered with thick brushwood and high gorse, with an ancient overhanging beech tree on the west. It is 32 feet in diameter and some 7 feet {30} deep. The theory of its never failing supply of water is that the moisture-laden currents of air from the South-west—and these sea mist drifts are heaviest in the early morning and towards evening—deposit their moisture on the branches and smooth leaves of the overhanging beech-tree and on the leaves of the brush-wood and gorse, and cause a continuous “drip” into the pond below, which is thus kept fully supplied with water; hence its designation of “drip-pond.”
There is a pond of similar form and construction on the adjoining down, part of Burton Farm, in the occupation of Mr. Cake. This pond is considerably larger than No. 8 and has overhanging trees of beech, ash, and Scotch fir on its North and West sides. These collect the rain and the mist from the South-west, giving a bountiful supply of water to the pond, which is seldom empty, even in the driest summer.
Of Catch-ponds there are numerous examples to be found. These are as a rule kept filled by the surface water from the roads and gullies close by. Two excellent examples of Catch-ponds may be seen at the bottom of Kingston Hill on the London Road, one on the right just inside the Park gate, fed by the rain water collecting on the Eastern quarter of the road, the other on the left, through the gate leading to Higher Kingston Farm, fed by that collecting on the Western quarter of the road, supplemented by the drip from overhanging beech trees; but these ponds hardly come within the purview of my paper.
Doubtless other ponds of ancient date placed on our Dorset Downs may be known to members of our club, but the above are all that I have had time to examine and describe. Possibly my investigations may be carried further at some future time. {31}
Messrs. Hubbard, in the 2nd Edition of their work, give a most interesting chapter on dew-pond experiments. These they carried out on rather a large scale. They obtained a suitable site and excavated the ground over a space of 100 feet square, thus obtaining a superficial area of 10,000 feet. The excavation was carried to a uniform depth of 1ft 6in., and a layer of 4in. of concrete was laid over the whole.
Upon this they put a coating of pitch or tar to stop any moisture from below from penetrating through the concrete to the layer of non-conducting material they intended to lay upon it, spreading dried sand over the tarred surface.
For this non-conducting layer they used mica, which is a well-known non-conducting medium; on this was laid a coating of asphalt to bind the whole upper surface together. This was successful to a certain point, namely, that their pond gathered a considerable quantity of water by night, though no rain fell, which was lost by evaporation by day. The rainy season having set in they were obliged to postpone their experiments until the following summer.
Unfortunately, through an accident, the asphalt got damaged in several places, thus admitting the water to the layer of mica, reducing it to a useless pulp, so that the experiment had to be abandoned.
Further experiments were conducted by Mrs. George Hubbard on ponds constructed on the same principle, but employing different materials, but though the results she obtained were encouraging, they failed to prove that water in any considerable quantity could be collected from dew, on the principle upon which they sought to prove that Neolithic man obtained his water supply for his camps situated in elevated positions on the tops of our downs, where they are usually to be found.
From some interesting experiments conducted by Mr. J. G. Cornish on the Berkshire Downs in January, 1910, it was found that after a night of fog the surface of his pond had (on January 18th) risen 1½ inches; the next day, following another heavy fog, it had risen no less than 2 inches, and on {32} January 24th an inch was measured. It is not recorded what was the principle on which the bottom of this pond was laid, or if there were any overhanging trees.
Attempts have been made from time to time to measure dew falls, and Mr. G. Dines, in a paper “On Dew, Mist, and Fog,” gave the average of dew fall during the year at 1.397 inches, or on the grass alone at somewhat less, namely, 1.022 inches. He says: “Making a liberal allowance for contingencies it may, I think, be fairly assumed that the average deposit of dew on the surface of the earth falls short of 1.5 inches.” This, if correct, completely overthrows the theory, advanced by some, of the “Neolithic Dew-Pond.”
From the foregoing I think it may be gathered that rain, dew, mist, and fog each contribute largely to the filling of ponds placed at high altitudes, and doubtless Neolithic man was fully aware of this, and so placed his ponds on these Dorset Downs near his camps, that they might get the full advantage of the rains, mist, and fog driven inland from the sea by the south-westerly breezes.
Be this as it may, the present generation have no longer the same need of these ponds. Modern machinery and modern plant have enabled us to carry water from the springs in our valleys to the top of our hills and on to our elevated plains, from whence it commands our pastures and waters our flocks and herds.
Within 20 yards of Dew-pond No. 2 on Grimstone Down, above described, now stands, some 550 feet above sea level, a cypress wood reservoir capable of containing 10,000 gallons of water, filled, by means of a pump driven by a wind engine, from a copious spring in the valley below, from whence the cattle and sheep on the three surrounding farms are plentifully supplied with water, run through galvanised iron pipes into drinking tanks, by gravitation, from the store reservoir, any surplus being made use of for irrigating purposes. A similar means of supply may be found on the adjoining estate of Godmanstone, on Clan Down, in the parish of Winterborne St. Martin, and on Maiden Castle, with galvanised {33} drinking troughs, within the very confines of the ancient earthwork itself, and not far distant from the Neolithic dew-pond described by the Messrs. Hubbard.
I fear I have not treated my subject quite as scientifically as I ought, or given it the same patient study that Mr. Martin and other writers on the subject seem to have done; but the result of my investigations has been to very much shake my belief in the theory of the Down-land “dew-pond,” and I have arrived at the conclusion that unless rain, mist, and fog may be comprised in the word “dew,” and if we are still to use the word “Dew-pond,” we must do so in the widest sense as including ponds filled by any form of condensation out of the atmosphere. Mr. Martin, in his paper above referred to, seems to hold this view and abandons the theory of the “dew-filled pond” with feelings of regret. He says: “The mystery surrounding the quite invisible formation of dew has a fascination for me, as for most people, but the result of greater knowledge must prevail, and the dew which waters our down-land grass and the corn on our dry flint-bestrewed down-land fields cannot longer be held to have any important bearing on the ‘mystery’ (which is no longer a mystery) of the Dew-pond.”
The subject is an extremely interesting one, and I hope the fact of my bringing it forward may lead to its being further discussed and enquired into by other members of the Club.