RESULTS OF DEW-POND INVESTIGATION.
At the Congress held at Hastings in 1908, I had the honour of reading a paper on the subject of Dew-Ponds. I then brought forward a number of theoretical considerations concerning their formation, leaving it a matter for experiment to decide whether or no there actually existed such a pond as a "dew-pond," and if so, to ascertain how best such a pond could be made. The point to be decided was whether it was possible for water to accumulate in a prepared upland basin, where it was exposed to the influences of the weather, and where no trees or verdure existed other than the grass of the downs around. The ponds, whose behaviour was to be under consideration must, with the exception of grass, be absolutely bare of vegetation. It was not denied that a tree overhanging a pond would condense a large amount of moisture out of the atmosphere, and that this would drip into the basin and so feed the pond. This might happen during nights of heavy dew-deposition, as well as when the downs around were covered with fog. But in neither case, as I hold, are we correct in regarding this replenishment as due to dew, although, at any rate as regards the former, it is merely a matter of nomenclature as to whether we regard our condensed vapour on overhanging trees as dew. In both cases a pond would probably be kept " alive " by these means, and they would make good the irregularities in the supply of rain. But what it was necessary to establish or refute was whether an open pond, with no overhanging trees whatever, was replenished by the dew which is seen so plentifully deposited on the grass around. One observer remarked that when there was dew on the grass, every pond must be receiving moisture, but no proof was given of so sweeping a statement. I have found no evidence in its favour. Another went so far as to say that the dew which was deposited on the grass must of necessity run down into the pond, oblivious of the fact that the margins of ponds are always more or less permeable, and remain so until saturated by a heavy rain ; and this being the case the small quantity of water resulting from dew-fall on a very dewy night would but moisten the ground around the roots of the grass, leaving none to run away down the slope of a pond. It is a very rare thing to come across a puddle of water, or even a muddy spot, amongst grass that has not received any other replenishment than dew ; if dew fall were so heavy as has been assumed, I fear many of our well-kept lawns would frequently suffer. As a matter of fact, the total annual deposit does not exceed F5 inch as estimated by Mr. Dines, and the result of an experiment which I made on a night of very heavy dew, gave an annual dew deposit of •77376 inch, or a little more than three-fourths of aninch. On the face of it, it seemed absolutely ridiculous to imagine that dew, such as we know it, could be in any way responsible for filling and maintaining a pond. It was, therefore, necessary to ascertain if it were likely that a pond by some speciality in its construction could be made to induce a far greater deposition on its own surface or on the margin around it, than on grass.
It should, perhaps, be stated in passing, that the margins of all the best ponds are kept scrupulously free from grass, so that even if dew which fell on the grass around did not percolate into the ground but ran down into the pond, the only grass which would collect this useful dew would be that which grew on the inner side of the bank. But sometimes there is no bank at all. A bank is not a necessity. Only when the pond is partly on a slope does it have to be banked up, and often a bank has been formed merely as a convenient place in which to deposit the excavated material of the basins.
Some downland ponds are chalk-puddled, some are clay-puddled. Some have straw used as layers in some portion or other of their construction. Some have not. It has been suggested that after night-fall there is a substantial lowering of the temperature of the puddled margin around a pond, and that this reduction frequently brings the temperature below dew-point. At the same time, the water parts with its heat, it is held, till dew-point is reached in this case also, and the pond receives an accession of moisture into itself. Again, it is held, straw is placed under the foundation of a pond, in order that the rising heat of the earth may be cut off from the pond, and thus may not interfere with the radiation of heat from the pond which is taking place.
In order to test this theory I made preparations for the construction of a pond, selecting materials for its foundations which were comparatively inexpensive, and such as would be at the command of a farmer who might wish to construct a pond, at a cost, say, of £30 or £40. Straw is of course a bad conductor of heat, but as this does not give a firm basis on which to place a bottom of puddled clay, I placed planks of wood between it and the clay. Immediately between the chalk and the straw I laid down a thick layer of wood-wool. I thus had three layers of inexpensive non-conducting material beneath the puddle. For the puddle I selected a fine red clay, this being more quickly worked into a fine puddle than chalk. I may say that these materials were selected after hundreds of experiments on their various heat-conducting qualities, and my note-books record these in full. Thermometers were placed upon, or in, or under, each in turn alone, and then upon, or in, or under, various combinations of the different materials. On nights of free radiation, there was no doubt that the straw, wood-wool, and wood, interfered considerably with the rising heat of the earth. They were however dried in each case before being used, as of course if wet they become fairly good conductors. The straw and wood-wool were kept loose in texture, so as to increase their non-conducting powers. Wood proved to be very useful on clear nights in keeping away from thermometers laid above it heat derived from terrestrial radiation. But the clear nights, when rapid radiation took place, were in the minority. Taking the nights of the three months from the 6th June to 8th September, when a pond required all the help it could receive from sources other than rain, on not more than 24 nights were there suitable atmospheric conditions to give any dew on the grass. The whole question is one which depends on the replenishment of a pond in those months. In other months its level varies only to a small extent, and there is little possibility of it becoming dried up. It is in the months when there are spells of dry weather that the qualities of a downland pond are tested.
At the commencement of a drought, unless there is at least a foot of water in the centre of a pond, a few weeks with little or no rain will see the pond completely dried up. Nothing is more striking amongst the numerous ponds I have examined than their shallowness. A pond measuring 40 feet across will not be more than 2 feet deep in the centre. This means that an enormous area is exposed to evaporation, and under a summer sun its temperature rises. Water is not so rapidly subject to extremes of temperature as the atmosphere, but the heat that it does receive it parts with more slowly. The rapid radiation which takes place from blades of grass so reduces the grass in temperature that it chills the air in contact with it to a degree below dew-point. Hence the grass quickly receives dew. If water in a pond is to receive dew in a similar manner, it must be also chilled by the radiation of its heat. Dew which forms on the grass around certainly never runs down into a pond. If the chalk or clay margin had similar radiating powers to grass, dew might form thereon, and, if the puddle did not allow of percolation, the water might run into the pond. But, firstly, the puddle scarcely ever gets chilled sufficiently to form dew ; secondly, the puddled margins have 29 times out of 30 been sufficiently porous to soak up all but heavy rains, and, thirdly, if the whole of the margin did receive dew, the quantity would be so small as to form no appreciable addition to the pond. Once grasp the principles on which dew is formed, and it becomes more and more difficult to see that dew has any share in the replenishment of a pond. Dew on the downs, although so apparent, must be put on one side. The filling of a pond must take place from causes acting within its own area, and on this area alone must attention be concentrated. Dew on the pond-margin is an impossibility so far as my observations go, and our enquiry practically concerns the water itself. Leaving for the moment a consideration of my own specially-prepared pond, I decided to test the temperature of the water of a pond of large size every hour during 24 hours. I was enabled to select a day and night when a clear sky lasted throughout the whole of the period.
We pitched our tent-myself and my son—on the bank at the side of a clear-looking dew-pond, 580 feet above the sea-level. The farm-house lay snugly sheltered in the valley of Standean below, and its occupant readily accorded us permission to camp out, the subject of downland ponds being one in which the farmers of South Sussex are taking a keen interest.
The unfrequented tracks over the downs are none too good for the transport of camping impedimenta, but with the aid of a friendly farmer and his more than friendly horse, the tent and cooking utensils were finally got in position, and the thermometers were safely bestowed around and within the pond. We were going to find out when and how dew reached the pond, if so be that it did so at all ; we were trying to catch it in the act, so to speak.
To the tune of the blows of the mallet as the pegs were driven home, and the crackling of the dry gorse as we prepared for tea, the bearings of the pond were taken, and the thermometers were given rope enough, so as to allow them to give their readings just where wanted and nowhere else. Fourteen thermometers in all were installed in position.
The sun had gone down in a blaze of glory after a hot day. It was one of those days in early August when the short summer had suddenly come upon us, and the London shade temperature had mounted to 86°F. The ground which had been panting and shimmering in the unusual heat, now commenced to give back the warmth which it had received, and rising with the beat came the moisture from beneath the downland grass. A mist came creeping up the valley from the sea five miles away, until it reached the head of Home Bottom, where it was unable to rise above the coombe (or cwm, as many would call it in Wales). So Ditchling Beacon stood out to the N.E. above the mist. Then the escape of heat into space from the ground and the grass all around rapidly chilled the stratum of air resting between, the dissipation of heat having little power to warm the atmospheric medium through which it passed. First tiny beads of dew collected on the grass, these ran together to form larger globules, and the blades were soon sparkling in the brilliant starlight. Now, surely, we should see the dew trickling down the hardened, chalky puddle around the pond. We watch, but it is yet too early. Dew-point is 62½°F., and the air is so humid that both dry and wet bulbs are the same. The grass temperature is 4° lower, so here is the explanation of the heavy dew deposit, and the thermometer itself is coated, both above and beneath, with dewy beads. Right away through the night until 6.30 a.m. the grass remains below dew-point, although the dry bulb ascends slightly, as showing quite naturally, too, that the air is losing its humidity.
But what of the water, is it collecting dew ? With the grass at 58½ °, and the dew-point at 62½° what shall we find the temperature of the water ? Gently we raise our thermometer from the middle of the pond, where it is resting on the bed. Will it show that, with its delightful coolness to the hands, it also has fallen below dew-point ? Alas, no ; it is at 75°. But perhaps nearer the surface it is cooler. Let us try it at 6 inches below the surface ; it may be cooler there. So it is, but only to the extent of a degree and a half. Well, let us try it at the surface itself. Here, if anywhere, dew ought to be falling. But it is little better. It is still 72°, so there is no dew-fall here. We must wait till later in the night, and give the water time to cool.
So we continue to watch the dew-pond. All is silent around. Nothing disturbs the absolute quietness of nature. The air is so warm as to prevent all discomfort in our tent. The grass may be cold outside, but inside, the canopy of canvas prevents radiation from the grass on which we recline. Every hour we read the temperatures as shown from our fourteen variously-placed thermometers. We anxiously watch one which is just fixed above the water, and another which is just fixed below the surface. As the first gradually, but very gradually, becomes colder, will the latter follow suit ? Will it become colder more quickly, so that it will chill the air above it, and make it disgorge its vapour ? At 7.30 p.m. the water is the warmer by 3° ; at 8.30 p.m. both have declined one degree, but at 10.30 p.m. there is only one degree difference. At 12.80 a.m. the water has dropped two degrees, and the air is the warmer by one degree. How long will this last ? At 1.30 a.m. they are together at 67°. Will the positions again be reversed, and the water become the cooler ? Not yet, at any rate. The bottom water is still 4° warmer than the surface water, and the warmer water must rise by convection currents until the whole becomes chilled. Will the night be long enough for this to happen ? We have no theory to maintain. We want to get at facts. At 2.30 a.m. it is becoming cold work. We don our great coats, for while the water has maintained its heat the air has become colder still, and so on, hour by hour. At 4.30 a.m. we begin to despair. The sun will be rising in a quarter of an hour, and the water at the surface is still aggravatingly warmer by 2° than the air immediately above it. Yes, we have reached the minimum for the night. An hour later, both have become warmer, although the heat has not yet reached the bottom water. Not once during the night has the water been anywhere near dew-point. Our night's vigil is over. If the pond must needs reach below dew-point to receive dew, this pond can have received no dew, on one of the most favourable nights for our purpose. The grass at 6.30 a.m. was shining in the sun with that peculiar greyness which it shows when it is wringing wet with dew. The mists were disappearing from the valleys, wreaths were passing upward, and becoming dissipated in the sun. The kine were beginning to low in the meadows. Here and there a solitary human being passed over a distant down to his work. And with the sun mounting higher and higher in the sky, another day was born. Men passed to their work. Our work was done.
We turned in to rest, and, pulling down the front of our tent, and rolling ourselves in our blankets, we soon bid fair to make up for our wakefulness of the night that had passed.
There will be no necessity here to give a detailed table showing the reading of the thermometers during the 24-hour observation. This and other tables will be printed elsewhere. Suffice it to say that only once (12.30 a.m.) did the water-surface become cooler than the air resting on it, and this was soon changed. More important than this, and more decisive, too, not once was the temperature of the water below dew-point. Not once was the temperature of the air resting on the water below dew-point, and the dew-point over the water was always higher than on the grass around the pond. As I have before stated the night was an exceptional one for the purpose, and there was left no room for belief that the pond received any dew whatever.
Exclusive of the observations made on the pond mentioned, observations were made on the experimental pond on 24 nights, selected on account of the meteorological conditions being such in the early evening as to promise dewfall on the grass of the downs. During those nights, dew-point over the water was determined on 71 occasions, both late at night and in early morning. On these occasions eleven thermometers were placed in different positions in, on, and around the pond. The tables showing the results are too lengthy to allow of their appearing in our Transactions, and the time now at my disposal forbids me from referring here to any but the principal results. An analysis, however, shows that the following points are worth notice. Out of 71 possible times, the water at its surface was but four times below dew-point. Of these four occasions three were between 7 a.m. and 7.30 a.m. The fourth was at 8p.m., but an hour later the temperature of the water had risen well above dew- point.
The temperature of the air resting on the water was on fifteen occasions below dew-point, and on three occasions at dew-point. The latter may be put out of consideration, as it is certain that immediately condensation commenced, the latent heat given out in the conversion would at once cause the dew-point to be raised. With regard to the former, although the air went below dew-point, I am assured by physicists that precipitation of dew would be impossible while the water upon which it rested was above dew-point. On twelve occasions the thermometer on the wet puddle at the edge of the pond passed below dew-point, and on nine occasions it touched dew-point.
On nineteen occasions the thermometer on the hard, dry puddle went below dew-point, and on five occasions it touched dew-point.
If these facts be considered, it will be seen that they lend but little countenance to the theory of dew-deposition in a pond sufficient to act as an efficient recruit to its supply. The four. occasions when the water was found to be below dew-point, may show that these conditions prevailed during a total of twelve hours in all, this being a very liberal estimate. The 71 occasions when tests were made, represent the 24 dewy nights during a period from June 6th to September 8th, that is three months. The number of dewy nights throughout the year would not be more than twice that number, and if the occasions during which surface water was below dew-point be doubled similarly, we should have 24 hours for the whole year when there may have been deposition of dew in the pond. Whether this method of calculation be correct or no, the fact remains that on four occasions only in three months was the water found to be below dew-point. From this it can, of course, be easily judged how infinitesimal is the quantity of dew, which thus reaches the pond. So far as the deposition of dew on the wet or the dry puddle is concerned, the proportion of dew-producing occasions is slightly greater, but it would be idle to suggest that the twelve or nineteen occasions respectively, when thermometers placed closely touching them went below dew-point, would allow of any appreciable quantity of dew being deposited. In the case of the dry puddle I observed one occasion when this was appreciably moistened by dew.
The experimental pond was completed in September, 1908, at the close of a number of experiments made with the object of choosing the best plan for the foundations. Owing to the lateness in the year, it rapidly filled with the coming of the autumn rains.
The appearance of a new pond on the downs gave rise to a good deal of curiosity on the part of pedestrians—so much so, that it was damaged in the early spring of 1909 by some individual who endeavoured and succeeded in pushing a stick through the foundation and draining the water away. The severe spells of weather in the winter of 1908-9 may, too, have injured the foundations. On two occasions the pond was a mass of ice, and the ice may have pierced its way through the edges of the pond, and so damaged the puddle. But herein is shown that the shallower the bed, and the more gradual the rise of the bank, the more likely would the pond be able to resist the thrust of the ice as it expanded in passing from the liquid to the solid form. As a matter of fact, all the ponds I have examined have a very gradual rise from the pond-area to the banks.
I have never been able to see how straw used in the foundation of a pond, and placed under puddled clay, could for any long period remain dry; as a matter of fact, it must be considerably moistened immediately the wet puddled clay is placed upon it. And owing to percolation in the margin of a pond, the straw, if carried right up to and under the bank, must become more moist as time elapses. It is not as though the whole area was at any time full of water. A margin always remains, and this margin either cracks under a hot sun, or becomes so dry as to allow rain to percolate into and through it when heavy autumn rains commence.
When I decided last spring to dig up the pond again, and remake it on slightly different lines, I particularly examined the straw as it was taken out, and found it was moist throughout, completely crushed, and very brittle. On the first occasion the foundations of the pond were (1) chalk bed, (2) wood-wool, (8) straw, (4) planks of wood, (5) clay-puddle. When I relaid the pond, they were as follows : (1) chalk bed, (2) planks, (3) straw, (4) clay puddle. In laying clay on the straw, the straw was, of course, immediately crushed down, and of necessity moistened to some extent. The thickness of the straw was such, however, as to allow of it being a fair test as to whether straw so laid would prevent the warming of the water at night by the upward radiation of the earth's heat. The results have been referred to, and I am bound to say that the theories which have received some amount of popularity in regard to the action and construction of dew-ponds, receive but scanty support from these results.
When we turn to consider the matter of rainfall, we find that the total annual fall would be sufficient to fill the pond, if the rain fell for the most part in the summer months. This of course is not the case. On the open down we had in 1909 a total rainfall of 39.18 inches. This was exceptionally high, made so by the phenomenal fall in October along the southern counties, amounting in my gauge to 9.39 in. for the single month. Dr. H. R. Mill gives the average annual rainfall along the highest ridge of the South Downs as 35 inches. A reliable estimate of annual net evaporation from water surfaces is 17 78 in., giving a net accretion of water of 17.22 in. To this must be added the rainfall which reaches the dry margins of the pond, which in part percolates, but which in part drains into the pond, and from these considerations it would seem that downland ponds should never dry up. That they do dry up at times is attested by the empty ones which meet one as one rambles over the downs, as well as by the evidence which I myself have seen. Rain is fairly regularly distributed over the year. But nine-tenths of the total annual evaporation take place in the summer six months. Small wonder then that they do occasionally dry up. That they do not more frequently can only be attributable, I think, to the fact that downland fogs come to the rescue, and, in the summer months, make good the unequal relations existing between rainfall and evaporation.
In my paper of 1908, I referred to some of the characteristics of the fogs of the downs, and my intimate experience of them assures me of the part they take in assisting in the replenishment of dew-ponds. Much fog is deposited mechanically in a hollow as it passes over, and there is every possibility also that the silent discharge of downland electricity from the rounded surfaces of the downs may result in the precipitation of fog. This has not, however, yet been experimentally manifested.
In my paper I have endeavoured to give you some of the results of observation. I did not anticipate that, when I again appeared before you, the tale which I had to unfold would be that dew must be thrown down from the pinnacle of all-importance on which it had been placed. I am really very sorry that I cannot speak better of its prowess. But if I still call these ponds " dew-ponds," the term must be understood as applying to a pond which receives all kinds of condensation out of the atmosphere, whether as rain, or mist, or dew.
I wish, in concluding, to acknowledge my indebtedness to the Royal Society, by whose kindness in awarding me a Government Grant I have been enabled to pursue my investigations into this subject.