May 30, 1878] 



NATURE 



121 



Dear Sir,- 



between its valves, and was secured in the act of being 

 transported, seems to me well worth recording. 



Charles Darwin 

 -The following case will, I think, prove of 

 interest to you, as it corroborates your 

 belief that freshwater shells are some- 

 times transplanted by the agency of aquatic 

 birds. 



In the sketch I have endeavoured to give 

 you a correct idea of the way in which the 

 shell was attached to the duck's foot. 



It was given to me by Mr. H. L. New- 

 comb, who shot the bird, which was a blue- 

 winged teal (Querquedula discors), while 

 flying, near the Artichoke river at West 

 Newbury, Mass., September 6, 1877. The 

 shell, the common mussel, or clam {Unto 

 complanatus), is a very abundant species, 

 being found in nearly all the rivers and 

 ponds of the Atlantic slope. How long 

 the shell had been attached is only a matter 

 of conjecture, but it had abraded the skin 

 of the bird's toe, and left quite an im- 

 pression. It was living when the bkd was 

 shot. 



It would have undoubtedly been trans- 

 planted to some pond or river, perhaps miles 

 from its original home, had the bird not 

 been shot, and might then have propagated 

 its kind. Arthur H. Gray 



DaviVersport, Mass., May 8 

 To C. Darwin, Esq. 



THE NATIONAL WATER SUPPLY 



THE;JCongress convened by the Society of Arts, at the 

 suggestion of His Royal Highness the Prince of 

 Wales, their President — and which has been presided 

 over by Sir Henry Cole, K.C.B., and numerously 

 attended by Mayors of Provincial Towns, Chairmen of 

 Local Sanitary Authorities, Medical Officers of Health, 

 Members of the Thames Conservancy Board, Engineers, 

 and men of science — may fairly be considered a suffi- 

 ciently representative body to discuss with some amount 

 of authority a national question. 



The papers prepared at the request of the Council of 

 the Society, and discussed by the Congress, may con- 

 veniently be divided into three groups : — the quantity of 

 rainfall available for water-supply; the necessity of 

 improved legislation to give it quickly and cheaply to the 

 people ; the necessity of compulsory powers being given 

 to a government department, to carry out the amended 

 law. 



The first head, quantity available, was appropriately 

 opened by a paper on the rainfall, by Mr. G. J. Symons, 

 the indefatigable head of the 2,000 unpaid observers, 

 whose results leave Httle to add to our knowledge upon 

 this matter. "No part of the British Isles has, on the 

 average, less than 20 inches of rain per annum," and 

 *'the bulk of the supply falls upon elevated mountain 

 tracts, where it ranges from 50 to more than 100 inches 

 per annum.' ' 



Mr, Symons did not give the results of his expe- 

 rience on the probable amount of the rainfall evapo- 

 rated on different soils, under different atmospheric 

 conditions, and in different .parts of the country ; this 

 figure must ever be an important factor in estimating 

 the quantity of water available in a district. Mr. 

 John Evans, F.R.S., however, informed the congress 

 that while on bare hard rocks nearly the whole of 

 the rainfall is carried off by the surface strea?ns, on 

 some porous rocks, such as chalk, an average quantity 

 of not more than six or eight inches per annum finds its 

 way to a depth of three feet from the surface, the re- 

 mainder being carried off by evapo7-ation, and vegetation, 

 and he adds " that for the supply of the population in 



districts of different geological character, different means 

 must be adopted." 



Numerous speakers insisted at some length on the in- 

 fluence of the varying degree of permeability of the rocks, 

 in determining whether the rainfall is thrown off in 

 floods, which should be collected and stored in reservoirs, 

 or whether it is absorbed into the ground, where it can 

 only be reached by wells, or by carefully preserving lines 

 of springs. Mr. Chadwick, C.B., pointed out that the 

 Map of the Geological Survey of the United Kingdom, 

 and the other publications of this Department, formed 

 an admirable basis for any inquiry into the water-bea,ring 

 facilities of the British rocks. Mr. Whitaker's Memoir 

 on the London Basin, which contains the particulars of 

 more than 500 wells sunk in and around the Metropolis, 

 is a good illustration of this, and useful as showing the 

 facilities already possessed by a Government Department, 

 which is capable of greater extension in this direction, 

 and whose officers now constantly work in concert with 

 those of the Local Government Board. 



Mr. Lucas exhibited a useful map, showing the under- 

 ground contours of the surface of the water in the chalk, 

 of some 800 square miles of the Thames and Hamp- 

 shire basins, so that the level of the water in regard to 

 the Ordnance datum line can be seen at a glance. He 

 has also in some cases indicated the underground 

 level of underlying impermeable strata, a method which 

 has been long used in the Geological Maps of Paris ; and 

 it is a matter of surprise that a map of London which 

 should answer the purpose of a section in all directions 

 was not published before. However useful such a 

 map may be amongst the permeable rocks of the green- 

 sand, chalk, oolites, and new red sandstones, which are 

 penetrated by deep wells, — in the more ancient formations, 

 consisting almost entirely of impermeable rocks, it would 

 be impossible to construct such a map, and the ordinary- 

 Geological Map is all that is required. 



These porous secondary strata occupy an area of 

 26,000 square miles in England and Wales, and in Scotland 

 and Ireland are practically absent, and wells of any depth 

 are rare ; while the more shallow wells, penetrating the over- 

 lying drift, are in all districts, as pointed out by the Rivers 

 Pollution Commission, dangerous sources of supply, 

 though in some cases, as Prof. Prestwich, F.R.S., 

 pointed out to the Congress, the gradual removal of cess- 

 pools, and improvement of house-drainage, has caused 

 the shallower well-waters to again improve. He, how- 

 ever, gave a remarkable instance of a retrograde charac- 

 ter, that of a deep " dry well," being carried through the 

 London clay, to drain a cemetery near London, into the 

 underlying Thanet Sands, which still give an important 

 quota to the Metropolitan potable waters. 



Next to the quantity of water available, there is no 

 question so important as the quality and purity ; and on 

 this point Dr. Frankland gave important and reassuring 

 evidence; for though he tells us that the increasing pollu- 

 tion of rivers and streams " renders the supply of whole- 

 some water from them more and more difficult," yet "two 

 sources of wholesome water" still remain in England, 

 viz., "upland surface water and subterranean water." 

 The tables accompanying a paper laid before the 

 Congress by Mr. De Ranee, show that the formations 

 yielding water of these two characters occupy the 

 following areas in England : — 



FORMATIONS YIELDING : — 



Subterranean Waters. 



Sq. Ms. 

 Permian and Trias . . 8,645 



Oolites 6,671 



Hastings Sands,^ 

 Green Sands, > . . 11,371 

 and Chalk ) 



26,687 



Moorland Waters. 



Sq. Ms. 

 Granite, Metamorphic 

 Rocks, Cambrian, 

 Silurian, & Devonian 11,455 

 Carboniferous Rocks, 

 (without the Carb. 

 Limestone) .... 10,080 



21,535 



