458 



NA TURE 



[October 14, 1909 



men of science. In particular they may_ expect 

 sympathy from the astronomical world, which will 

 soon be faced by an allied problem. The_ question 

 must, before many years, come up for decision as to 

 when a repetition of the chart of the heavens, which 

 is slowly nearing completion, will be justified by the 

 conclusions to be drawn from it. 



SCIENTIFIC STUDIES OF DEW-PONDS. 



AN endeavour to solve the so-called mystery of the 

 dew-pond has recently been made by Mr. E. A. 

 Martin, and the results of some of his observations 

 are shown in a paper which appears in the Geograph- 

 ical Joiinml for August. The paper was read before 

 the Research Department of the Royal Geoaraphical 

 Society on April 22. Attempts were made by direct 

 experiment to ascertain how the replenishment of such 

 ponds takes place. During- the autumn of 190S, Mr. 

 Martin spent many nights and days on the Clayton 

 downs, in Sussex, and thus was on the spot during the 

 hours when, according to theory, the ponds should be 

 receiving dew. The result of a large number of ther- 

 mometrical observations went to show that very rarely 

 does the temperature of the water of the ponds_ sink 

 below that of the air above it, or below dew-point. 



The term " dew " is widely used to mean any kind 

 of condensation which does not fall as rain, hence 

 " dew-ponds," " mist-ponds," and " cloud-ponds " are 

 terms which are used for one and the same kind of 

 pond. On the Sussex Downs no overhanging tree to 

 condense moisture out of the air is found, as a rule. 

 The bare down is all around, whilst in the water 

 there is, as a rule, pond-weed, or reeds, sometimes pro- 

 jecting above the surface of the water. Where this 

 happens, dew is undoubtedly precipitated on the reeds, 

 and this helps to replenish the pond. But many ponds 

 have no projecting vegetation, and yet do not suffer 

 greatly in times of drought. It is pointed out that 

 the measurements of some ponds and their surround- 

 ing basins give a receiving area sometimes double 

 the area of the water. In one case the pond-area was 

 4120 square feet, whereas the shelving margin gave 

 an area of 5795 square feet. Other similar examples 

 are given, and it is this width of margin which has 

 caused many observers to conclude that rainfall is 

 the chief factor in filling the ponds; but not the onlv 

 factor, as Mr. Martin points out, otherwise there would 

 be little reason why the lowland ponds should dry up 

 in times of drought, and leave the upland ponds fairly 

 full. 



Thermometrical observations show that the depth 

 of a pond at the commencement of a drought has 

 much to do with its continuance. A shallow pond 

 was found rapidly to dry up by evaporation, the high 

 temperature gained during the day being well main- 

 tained during the night. On the other hand, a deep 

 pond will but slowly be heated, and may well be 

 saved excessive evaporation until a break in the 

 weather comes, and normal conditions again prevail. 

 One pond which was but a foot deep was found so 

 late as 8.20 p.m. in July to show no differences of 

 temperature at i inch, 6 inches, and 9 inches, the 

 thermometer registering 67'5° F., whilst that on the 

 bank showed a reduction to 585° F. The water lost 

 heat but slowly, and no doubt evaporation went on 

 well into the night. Three weeks later it was dry. 

 Another pond, 3 feet deep, showed, at 6 p.m., 76° F. 

 at I inch, 74° F. at 6 inches, and 71° F. at 9 inches, 

 and two hours later the i-inch temperature had been 

 reduced to 7oJ° F., whilst the 6-inch and 9-inch 

 temperatures were uniform at 71° F., the surface 



NO. 2085, VOL. 81] 



temperature showing a considerable loss. There was 

 in this pond a large quantity of rushes, and the loss 

 by evaporation was almost compensated for by the 

 deposition of dew upon their exposed surfaces. This 

 pond did not dry up. 



Attention was given to the alleged chilling of the 

 water below dew-point, but it was found that although 

 such a circumstance rarely happened, it sometimes 

 was seen that the temperature of the air resting on 

 the water was below dew-point. Further observ- 

 ations in this direction are to be made. Numerous 

 experiments were made to determine whether straw, 

 wood, and woodwool were likely to effect a chilling 

 of the water of a pond resting on a foundation of 

 these materials, and the evidence pointed to these 

 acting in the desired direction. A series of experi- 

 ments showed that both " downward " and " up- 

 ward " dew would be found on different nights 

 according to certain atmospheric conditions, and it is 

 pointed out that if a pond were to depend on the 

 latter only for its replenishment, it would simply 

 receive what it had previously lost by evaporation. 

 The chilling effect of grass on the moisture-sodden 

 lowest stratum of the atmosphere results in dew on 

 the grass, but there is no such chilling of the air by 

 the pond-water, and if dew is there deposited there 

 must be some other cause at work. 



It is found that out of seven localities quoted where 

 straw has been used in the foundations of dew-ponds, 

 in no case has it been used with the idea of inducing 

 dew-deposition in the pond. Sections of dew-ponds 

 are given in the paper, constructed according to 

 various authorities. The most remarkable case seems 

 to be that in Wiltshire, where foundations are laid in 

 the form of six layers of straw and clay alternately, 

 but here again the reason given is that the straw 

 prevents the clay from cracking. Incidentally, Mr. 

 Martin refers to the danger to clay-puddled ponds from 

 the small red-worm, swarms of which were met with 

 in some ponds. An estimate of dew-fall on grass was 

 made, giving o'77376 inch per annum. 



So far as rainfall is concerned, it was found that 

 in thirty-two days the amount measured on the down- 

 land was 2'57 inches, but a gauge placed in a hollow 

 dug for an experimental pond measured 3'5i inches. 

 This seems to show that a pond-depression on the 

 downs would draw into it, by setting up currents and 

 eddies of the wind, a greater quantity of rainfall. By 

 experimenting with a gauge in the rim of which had 

 been placed some straw and grass, in imitation of 

 conditions which obtain in some ponds, it was found 

 that when o'37 inch was measured on the down, o'54 

 inch was measured in the gauge ; when the former 

 showed o'32 inch, the latter showed 069; when the 

 former showed o'46 inch, the latter showed o'So inch. 

 The gauge with the straw and grass was placed in 

 the hollow. 



In order to determine whether the chemical 

 composition of pond-waters would give any clue 

 to their origin, a number of analyses of such 

 waters was made at the South-Western Potytechnic, 

 and the results are given in the paper. These seem 

 to show that there is too much sodium chloride con- 

 tained in the ponds to have come from rain-water, and 

 in normal conditions dew certainly contains no 

 common salt. The sea-mists may reasonably be 

 held to be responsible for the saline qualities of the 

 waters. 



So far as the antiquity of the name and the idea of 

 the dew-pond is concerned, Mr. Martin seems to think 

 that puddling by cattle-trampling by accident may 

 have caused artificial ponds first to have been made, 

 and although proof must be lacking, it is possible that 



I 



