Before entering upon the consideration. of the technical questions :armee-tad with the subject of a water supply, and before quitting the past history of thaw operations, it may be desirable again to dwell open a very generally received opinion with respect to the unburned — - — — ignorance, by the ancients, of the principle of the inverted syphon ; or of the principle by which water will find the same level at the two untie of a contiuuoue pipe, unless prevented by some external force. This opinion has been repeated in so many popular works upon science, that it is almost universally believed ; yet, not only does the aqueduct of Patera entirely refute it, but the great syphon upon the course of the aqueduct of Lyou proves that the ancients were as well acquainted with the principle as the moderns themselves are. It is the more extraordinary that this singular opinion should have obtained such universal credence as it has done, because Vitruvius discusses in a very practical spirit the mode of executing the descending and ascending pipes, in chap. vii lib. 8 ; and he points out the precautions to be taken to prevent the rupture of the pipes by the compression of the air in the lowest parts of the ignition.
The first inquiry at the present day, when it may bo desired to execute a system of water supply to a town, after having ascertained present and the probable future extent of the coneumption (which is usually reckoned to take place st the rate of forty gallons per head of the population per day>, must of course be, as to where a permanent supply of water of a proper quality is to be obtained. Much discussion has of late years taken place with respect to the question as to what constitutes the proper quality of such a water, and the formerly received opinions of physiologists and eugineers have beet, boldly challenged by men who have had little claim to either of those titles. The discussion has principally turned on the subject of the hardness and of the softness of waters, consequent upon the preseuce in them of the salts of lime, in the form of the bicarbonates of that base ; the former authorities upon such matters contending that a certain pro portion of lime is necessary in a potable water, whilst the new lights contend that absolute purity should be aimed at. Tho truth, in this case, seems to be that there is still too considerable an amount of uncertainty with respect to the real action of the salts in question upon the human frame to allow of the formation of any very decided opinion on the subject, or of the establishment of any absolute law for the adoption, or rejection, of a particular source of supply, when its departure from the standard of absolute purity is very small. Local
considerations of economy must, in the present state of knowledge, exercise great influence upon the choice of the source of supply ; and it is only when the quantities, or some peculiar properties, of the extra neous impurities, are very objectionable, that it becomes desirable to reject a source which is close at hand. The qualities which are indis. reusable in a water designed to be-distributed in a town, are, to quote the words of the `Annuaire des Eaux de France,' that "it should be fresh, limpid, and without smell; that its flavour should be hardly perceptible, and that it. be neither disagreeable, tat, brackish, nor sweet ; that it should contain little foreign matter, but a sufficient quantity of air in dissolution ; that it should dissolve soap without leaving curds, and should cook vegetables easily." A small portion of carbonic acid gas increases the wholesomeness of a water, by its influence upon the digestive organs; but it enables the water to take up an additional quantity of the salts of lime (which in their turn increase the hardness of the water), and to develope some forum of disease (such as gout, calculi, /he.) in the populations using it habitually. Waters containing salts of magnesia, or of sulphate of lime, are con sidered, on the other hand, to produce the loathsome disease. the goitre ; and the very purest waters obtained from the melting of ice, or of snow, are deficient in some elenteuta necessary to maintain the heelthi. nese of the human frame. The waters which hold in solution notable proportions of organic matters, arc, however, those which are the most objectionable ; and diarrhoea, dysenteries, and other acute and chronic diseases may be traced to the use of the teeter obtained from ponds, marshes, or wells, containing excessive proportions of altered organic matters, either in suspension, or in solution. It was considered, by he authors of the • Annuaire des Eaux,' that the small quantity of the chloride of sodium to be found in ordinary river or spring water was rather advantageous than otherwise; and they further remark, that the chlorides in water are almost always associated with the iodides and bromides, which unquestionably increase its salubrity. It may be added, that when the proportion of the bicarbonate of lime in a potable water exceeds 1 in '200, it becomes punitively injurious; that the bicarbonate of lime is thrown down by boiling; but that neither the salts of magnesia, nor the sulphate of lime, are thus expelled.