EXPERIMENTS WITH DEW-PONDS.

AN addition to the literature which has collected round the subject of dew-ponds suggests an excursion into interesting byways of archaeology, and doubtless the paper, entitled "Some Observations on Dewponds," which is con- tributed by Mr. E. A. Martin to the Geographical .Tournal for August will have many readers. Mr. Martin in September

last year had a valuable opportunity of making observations over a considerable tract of downland in Sussex, extending from the Devil's Dyke to Plumpton. Part of the use he made of his opportunity was to carry out a number of tests of temperature of the water in the downland ponds as compared with the air above the ponds, and these experiments appear to have led him to a very remarkable conclusion. We do not think that he makes out his case ; indeed, his data seem to us to point to a conclusion precisely opposite to that at which be arrives; but the case, at all events, is worth stating.

The most generally accepted theory in regard to "the

dew-pond on the height, Unfed, that never fails," is that, owing to some not wholly understood principle in its construction, the pond is able to collect from the air a larger supply of moisture than other ponds. The name of the pond varies. In parts of Wiltshire, Surrey, and Kent the term is "mist- pond," and in some districts of Hampshire it is "cloud-pond." These names are worth noting, as indicating a local belief as to the origin of the water in them which, it is to be presumed, has been banded down orally from father to son since the ponds were first made. As to the principle of construction which enables dew-ponds, or mist-ponds, to collect more moisture from the air than other ponds, the explanation which has been generally received is that the pond is insulated from the temperature of the surrounding earth by a layer of straw placed under a clay lining. New dew-ponds continue to be made upon this old principle to-day. The straw is a non-conductor of heat, and the pond, therefore, radiating its heat after sundown, rapidly cools to a tem- perature lower than the air, and so distils on its surface all the moisture in the air with which it comes in contact. If we understand Mr. Martin, it is this theory which he rejects. His contention seems to be the exact opposite, and he apparently bases it on an experiment which he made in testing the temperature of one particular pond. Its name is One Tree Pond, and in the drought of July and August last year it dried up. The circumference of the pond when Mr. Martin observed it on July 25th was a hundred and two feet, but there was very little water in it, no more than nine inches. The pond became rapidly heated every sunny day, and though it cooled off a little in the night, it did not lose sufficient heat to be cooler than the air above it. Mr. Martin took its temperature on several nights in succession, and on each occasion, he writes, "the atmosphere was colder than the water, and hence there could be no possibility of the water chilling the air sufficient to compel the latter to dis- gorge its aqueous vapour. If vapour were actually given up

by the air there must have been some other cause for it." What, then, was the other cause ? Mr. Martin takes a par- ticular night, July 28th, and argues from what be observed as follows At 9.50 p.m. the water was, everywhere in the pond, at 61°. The air immediately above it was 56°. The water was no doubt losing heat, but the air was losing it more rapidly, and the temperature of the air would easily fall below dew-point some hours before that of the water did the same. In that case, the point of saturation being reached, the stratum of air must give out some of its moisture, and this would be rapidly assimilated into the pond. If evaporation of the pond were proceeding at the same time, this would tend to cause the point of saturation of the air to be reached the earlier. So I am inclined to think "—this is Mr. Martin's conclusion—" that dew may yet be received by a pond on its surface even while the pond-water itself has not yet fallen to dew-point. On the other hand," he continues, "if it can only receive dew by chilling, as it is often said to do, the air above it, I cannot see how pond-water can ever reach a temperature at night-time lower than the air above it, which would be necessary if there is anything in this alleged chilling, so long as there is no change in the weather."

Surely that is a very remarkable conclusion. In the first place, what evidence is there that the warm pond-water received from the cooler air above it any discharge of moisture at all P There would be a reasonable supposition that it did so if the water had kept at anything like the same level, but as a matter of fact the pond dried up. Next, Mr. Martin "cannot see how pond-water," if it can only receive dew by chilling the air above it, "can ever reach a temperature at night-time lower than [the temperature in] the air above it."

But on his very next page 3fr. Martin sets down a fact which exactly demonstrates what he fails to understand. He refers to "the process by which ice is obtained in India at night- time, while the temperature of the air above it may be a good deal above 32°. Shallow earthenware pans, with water con- tained in them, are placed over about a foot of straw, and the straw prevents, by reason of its non-conductivity, the earth's heat from rising and warming the water, whilst the heat of the water is lost by radiation. Thus, although the tempera- ture is above 32°, films of ice appear in the pans." Now, on the face of it, does not that experiment seem to suggest that there is, after all, a sound, definite reason for placing a layer of straw under the clay lining of the dew-pond ? It may be perfectly true, as Mr. Martin remarks, that "not one of the builders of such ponds seems to hold that the straw has any- thing whatever to do with the filling of the ponds." It is conceivable that the straw may be placed under the clay, or between layers of clay (as it is sometimes found) merely to give the clay-layer elasticity, and to prevent it from cracking.

But, as against that theory, we have three very remarkable facts to consider. In the first place, even if the builders of the ponds cannot say why the straw is put under the lining, the fact remains that it is put there. So far as we can ascertain, the custom of placing the straw under the lining appears to have descended from generation to generation from time immemorial. In the second place, the custom is wide- spread. Wherever dew-ponds are found, in Sussex, Hamp- shire, Wiltshire, Essex, Yorkshire, straw underlies clay. In the third place, these ponds so made do, as a fact, continue to bold water when other ponds not so constructed dry up.

Mr. Martin's One Tree Pond dried up. The interesting point would be to ascertain whether or not it had a clay lining with a layer of straw under the clay.

Many of 3fr. Martin's readers will wish that he bad carried his experiments as to water temperature a little further. For instance, what were the temperatures by day and by night of the ponds he had under view which did not dry up P Again, during the period when he was making the experiments we gather from his paper that there were occasional fogs or mists.

What was the effect of the mist upon the different ponds ? Did the level of water rise ? Previous experiments, notably a series made by the Rev. J. G. Cornish at Lockinge, on the Berkshire Downs, would seem to show that mist adds at least as much as dew to the quantity of water in a dew-pond. Mr. Cornish'a brother, the late C. J. Cornish, in "The Naturalist on the Thames," gives the figures :— "Whenever he [the shepherd employed by Mr. Cornish] thought that a heavy dew or fog was to be expected (and the shepherds are rarely wrong as weather prophets) he notched a stick, and drove

it into the pond overnight, so that the notch was level with the surface. Next morning he pulled it up, marked how high the water had risen above the notch, and nicked it again for measure- ment. On January 18th [1901], after a night of fog, the water rose 11 in. ; on the next day, after another fog, 2m. ' • and on January 24th, 1 in. Five nights of winter fog gave a total rise of 8 in.—a vast weight of water even in a pond of moderate area. Five days of heavy spring dew in April and May, with no fog,

gave a total rise in the same pond of 31 in., the dews, though one was very heavy, giving less water than the fogs, one of which even in May caused the water to rise n Perhaps an experiment made by the author of "The Naturalist on the Thames" himself might also serve as a

basis for further experiments. Trees, as is well known, under certain conditions of atmosphere act as alembics, and distil water, which drips from them ; and many shepherjs believe that a properly constructed dew-pond ought to have a tree hanging over it. Mr. Cornish wished to test the distilling powers of trees, and placed two vessels out of doors on an evening when heavy dew was falling. One was placed on

some stone flags, the other under a cherry-tree in full leaf. In the morning both vessels held a considerable quantity of water, but the vessel taken from under the cherry-tree held

twice as much as the other. Might not experiments with neighbouring dew-ponds, one overhung by a tree and the other bare to the sky, decide to what extent in a given period the tree benefited its pond ?

Of course other considerations must be taken into account in estimating this or that difference of water-levels in dew- ponds. For instance, most dew-ponds serve as drinking. ponds for cattle or sheep. No one has yet been able to calculate exactly the amount of water which a flock of sheep takes from a pond in a day or a week; and clearly, if any

experiments are to be made contrasting the capacities of one dew-pond with those of another, sheep and cattle must first

be fenced off from the ponds. One might begin, indeed, if drinking animals were fenced oft from the ponds, with the very simplest experiment possible dealing with rainfall and

evaporation. It has been calculated that the average rainfall on the tops of the South Downs is thirty-fire to forty inches

during the year, whereas the annual evaporation is not more than twenty inches. From rain alone, then, in a water- tight downland pond, there should be an annual gain in depth of fifteen to twenty inches. Is this really so ? If beasts were fenced off from the pond, the point could easily be verified, and some very interesting problems in rain storage by means of large cisterns on hilltops would be opened out. The problems which await solution in connexion with dew-ponds and storage of water are indeed almost endless; but we must content ourselves with two more questions only. First, why are dew-ponds found almost invariably on chalk downs ? Is it because on other grounds such as limestone rock, there are pretty nearly always springs breaking out at high altitudes, whereas springs do not rise from high levels in chalk ? Or have dew-ponds ever been constructed on high ground where the soil is rocky or

gravelly ? This is of course a question of fact. The second is a practical question suggested by a sentence in Mr. Martin's paper in the Geographical Journal. He states that in the tract of downland which came under his observation there are "numerous depressions which appear once to have been ponds, but neglect has caused them to leak and run dry." Have any of these depressions been examined or excavated? and if so, were there any traces of the pond having been lined under the chalk puddling with layers of clay or straw ?