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The Dew-Ponds and Their Secret


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We have no waters to delight
Our broad and brookless vales
Only the dew-pond on the height
Unfed, that never fails.

Mr. Kipling's lines are well known - they are, of course, from his "Five Nations," and "our broad and brookless vales" are the vales of Sussex. Poetically, they contain a truth, and they crystallise a memory; the lonely "dew-pond" on the chalk heights of Sussex surrounded by a ring of South Down sheep slaking their thirst in the heat of summer drought is a picture not easily forgotten. Nor should it decrease by one iota our admiration for the graceful attractiveness of the poet's verse to learn that even the dew-pond fails on occasion.

But speaking in a very wide sense, the dew-pond "never fails." There would be no mystery nor a problem to solve did they become dry as other ponds and pools become dry under the heat of the summer sun. One pond, not far from Chanctonbury, at least holds an honourable record of many years "life." But it is as well to dispose of - at the outset - any notion of the miraculous; the widow's cruse of oil is one thing, and a dew-pond is another thing. During the severe drought of 1911 a very large proportion of the ponds on the Sussex Downs dried up; on the other hand, here and there a pond stood the test remarkably well. The same facts apply to the ponds on Salisbury Plain: while the well-known dew-pond on Martinsell, the great hill near Marl-borough and the highest in Wiltshire, dried up, although it had not done so for seventeen years. On the other hand, a pond at the foot of Martinsell, on the road to Wooton Rivers, was full of water on September 18, 1911; but it should be noticed that in this case the pond was surrounded by trees, some of which overhung the pond, and the application of this fact to one side of the question will be appreciated as we proceed.

The literary history of the dew-pond is short but not without interest. The Sussex shepherd is no doubt accustomed by this time to the term "dew-pond," but in the old days "sheep-pond" was good enough for him - significant of the practical side of the question. We shall search the poet Thomson - he of "The Seasons" - in vain, although he failed not to draw a pleasing picture of flocks and herds watering:

A various group the flocks and herds compose;
Some ruminating lie; while others stand
Half in the flood, and, often bending, sip
The circling surface...

but when we consider that a plausible theory takes the history of some of these ponds back to Neolithic - perhaps Palaeolithic - times, it is strange that reference to them is so scanty.

It is in 1813 that we first meet with the term "dew-pond." In Surrey, Kent and Wiltshire the term "mist-pond" has been recorded, and in Hampshire, "cloud-pond," while at Hampstead the word "fog-pond" has been used for at least half a century. In the nineteenth century the question was investigated, more or less scientifically, and the evidence goes to show that dew-ponds are really "artificial rain-ponds." Here, obviously, we touch their practical importance.

Interesting though it may be, we must pass by the speculations of archaeologists as to the age of some of these ponds and consider the main problem. First, as to their structure, it seems that in making such ponds an excavation was made in the chalk on the top of the hills, from thirty to forty feet or more in the diameter, and from four to six feet deep. The bottom was covered with clay mixed with a large quantity of lime. (This was supposed to prevent the earthworms from working their way through, and thus rendering the pond pervious. It is, however, doubtful whether the purpose of the lime was to prevent the working of earthworms; they cannot live beneath a pond.) This bottom was protected from the sun by straw and a layer of broken chalk.

As amatter of fact, the changes have been rung on such an arrangement of materials. Counting from the bottom we find such schemes as straw, clay, stones; or concrete, straw, concrete; or, again, lime-tempered clay, straw, rubble; or perhaps, tempered clay, straw, and broken chalk. Wandering gangs of men were reputed to make dew-ponds for a consideration. In the summer of 1906, a pond-maker, who seemed to be well known, was said to be at Alfriston. But the ponds he made, whether on high or low ground, consisted of an excavated hollow, with a carefully concreted bottom.

It is obvious, from the evidence, that there are great differences of opinion as regards their construction. At the British Association Meeting at Bradford in 1908 one expert said that he did not believe that dew-ponds were made in anything like the scientific way pretended by their makers.

There are three dew-ponds near the famous Chanctonbury Hill in Sussex. Two of them have been made in recent years by the owner of the land and the occupier of the farm jointly. The clay which occurs in these parts in pockets in the chalk was mixed with lime and used for puddling. A barrel and a half or so of water was poured in, and then the ponds were filled. Even in the long, hot summer of 1893, when the ponds in the Weald were dried up, these dew-ponds of Chanctonbury survived. The largest, which may be very old, lies due south of the Ring, between it and Cissbury. There are records of its being cleaned but none of its having been made.

The object of the layer of straw was, theoretically, to prevent the rise of heat from the earth after nightfall. Thus the water in the pond would be kept cool, straw being a bad conductor of heat, whilst at the same time the water lost its heat by radiation. Then, the water having cooled down to a temperature below dew-point, dew becomes deposited on its surface, and thus the pond is replenished. Of this the expert, Edward A. Martin, observed: "It is a pretty theory, but no explanation is given as to how the clay puddle was to be made to behave properly upon a loose bed of moving straw; nor how the straw was to be prevented from being pressed down into a hard layer under the heavy puddle and becoming practically of no use as a non-conductor of heat. Then, according to the accepted theory... dew is deposited on (the pond's) surface... It is entirely overlooked that when the surface layer of a pond commences to cool it immediately sinks, and that during the process of cooling of a pond convection currents are being set up until the pond has been cooled as a whole, that the short nights of summer are all that are available for the process, and that very rarely could the whole of a pond be reduced in a few hours to a temperature below dew-point." He found, as a matter of fact, by direct experiment on existing as well as on specially constructed ponds, that "rarely did the water touch or go below the dew-point."

Further, one experimental pond, on being dug up, was found to have a layer of straw which had been laid down under the puddle so crushed as to form but a very poor non-conductor of heat. "I am strongly inclined to think that the use of straw may have a good deal to do with the attraction of moisture to a pond," says Martin, adding that straw is used in India to produce a low temperature, and so obtain ice in the open air at night-time. In 1909 the pond was relaid, and was rapidly filled by rain. There were occasions when the water surface rose slightly, although there had been no rain, or very little rain, and on reference to the log-book it was found that these rises corresponded with the existence of night fogs or early morning mists. The conclusion he arrived at was that the so-called dew-ponds were mist-ponds, and that dew was practically negligible.

Rain, as a dominant factor, is ruled out of court on at least two counts: first, that dew-ponds do not dry up when ordinary ponds are evaporated, and second, that dew-ponds cannot possibly be filled by rain. As to the matter of rainfall, the total annual fall would be sufficient to fill the experimental pond that Martin made if the rain fell for the most part in the summer months. But it doesn't. Rain is fairly evenly distributed in England over all the year. On the other hand, nine-tenths of the total evaporation occurs in the six months of summer.

As to dew (and apart from certain conflicting theories as to its origin or cause), the annual deposit is only about 1.5 inches, and so it is absurd to think that dew, by itself, could be responsible for filling and maintaining a so-called "dew-pond." At the commencement of a drought, unless there is at least a foot of water in the centre of the pond, a few weeks with little or no rain will see the pond completely dried up. All the dew-ponds, or practically all, are very shallow. A pond measuring forty feet across will probably be not more than two feet deep in the centre, though some ponds are three or four feet deep. Thus an enormous area is exposed to evaporation, and under a summer sun the temperature of the pond rises.

The margins of the ponds are nearly always wide, with a gentle slope, and "rainfall may have a great deal to do with the filling of such ponds. Yet not everything, as has been held by some, and this has been, I consider, sufficiently shown by those ponds which have but slightly suffered during drought, although drawn on by hundreds of sheep day after day during the same period."

The consensus of modern opinion leans to the side of "mist-pond" as opposed to "dew-pond" fairly decisively. The ponds are often on the very ridge of a down, and by choice on the northern slope in most of the south-eastern areas, and it is generally held that the moist winds throw down their water most abundantly just beyond the summit of the first range of hills they meet. In Sussex and Hampshire this is the side sheltered from the sun, and dew-ponds abound in Sussex and Hampshire. They are not uncommon in Berks and Wilts, but are scarce in the chalk hills of Bedfordshire, Hertfordshire, Lincolnshire and Yorkshire; the few in the Midlands depend upon surface drainage.

It was found on comparing, at the end of a long drought, a dried-up pond with one still containing water, that the latter (other things being equal) was sheltered on the south-west side by an overhanging tree or trees; often only an ivy-covered stunted thorn or oak; or else the bank was high enough to cut off the sun's rays wholly or largely. Further, that mists and dews were much heavier than one would expect on open downs in the midst of summer, and that the mist hung for hours in the evening and early morning. If trees were present, then the dripping was copious and continuous. During summers of drought it was found that nearly all the newly-made ponds dried up, but the older ones not. Not that the older were better; it was a case of the survival of the fittest; once a pond was dried up a new one had to be made. The farmers were constantly making new ponds, and by accident they hit on a favourable site. There was far more of the haphazard than the scientific in the business.

Finally, as the result of analysis of the water of ten ponds - one in Wiltshire, one at Shankiin, but the majority being Sussex ponds - the conclusion was reached that little could be said on the "mist" or "dew" question, but the evidence showed that whether brought as rain, mist or dew, the waterbearing vapour came from the sea.


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Pictures for The Dew-Ponds and Their Secret

THE DEW-PONDS FOR SHEEP AND CATTLE OF THE SUSSEX DOWNS
THE DEW-PONDS FOR SHEEP AND CATTLE OF THE SUSSEX DOWNS >>>>

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