An able report on supplying the cities of Washington and Georgetown, with an unfailing supply of pure water, has been made by Lieut. Montgomery C. Meigs, of the corps of engineers, who has made the necessary surveys, and estimates. It has been published in the National Intelligencer. The following extract from it, respecting the purifying of water, should circulate far and wide :— A beautiful application of a mechanical principle, used in laboratories for the sorting of chemical precipitates, into different degrees of fineness, has been suggested by J. Price Wetherill, for many years chairman of the Watering Committee of Philadelphia. In his White Lead Works, as in establishments for the pulverization and sorting of emery, a stream of water thickened with white lead, mixed with it b- brisk agitation, is allowed to flow through a trough, divided by cross partitions. The stream is slow ; the heavy material tends to sink to the bottom, and in passing across the first division the coarser particles descend below the level of the partition, and are retained ; the next arrests those which are finer, and thus, by multiplying the partitions, the water, which entered thick and turbid, can be made to leave the trough clear and bright, retaining only those impurities which, being dissolved, no mechanical means will separate. He proposed, I have been told, to build walls across the reservoirs of Faiimount, rising to near the water surface, and thus apply on the great scale what has so well fulfilled the same purpose on the small one. I see no difficulty in this application, and I believe that here is the best mode of purifying the water of rivers when used for cities. The substances which discolor water are mechanically diffused. No city will supply itself from a source much contaminated by chemically dissolved impurities. The waters of the Potomac and Rock Creek, according to Professor Torreys analysis, contain of salts soluble in pure water not more than one grain, and of carbonate ol lime and magnesia, insoluble in water alone, but held in solution by excess of carbonic acid, from three to four grains to the gallon. [With respect to the use ol water for drinking purposes, a very interesting paper was recently read before the Institute of British Architects, by G. R. Burnell, C. E., on the Spanish Province of Orenese Gallicia, in which we find much that is instructive relating to the quality of water—that which is healthful and that which is not. We will endeavor to present the information, giving the authors views although not his words—for the sake of brevity. He had noticed that, in different parts of England and Scotland, invariable softness of water was prejudicial to animal life. The Board of Health, in London, state that soft waters were the most wholesome, but this was against good authority and contrary to his observations in various countries. In Spain, in the granitic regions of Plymouth, in England, Cherbourg, in France, the millstone grit of Yorkshire, and some parts of Scotland, glandular affections were very prevalent. In Gallicia, it is notorious that no cavalry regiments can be kept, because the hay is not of a quality that is able to support the horses. The hay obtained from fields irrigated with soft water at Bagshot sands, England, was found, by the cavalry stationed at Guildford, Eng., to be unfit for their horses—it was positively injurious to them, and was finally excluded from the barracks. Some valuable plants, such as water cress, trefoil, &c, are entirely wanting in soft water regions. In the millstone grit ragions of Yorkshire, Eng., horses cannot obtain the materials necessary for the secretion of the elements of their external skeleton (bones are mostly composed oi the phosphate of lime). Upon human beings the effects produced by an absence of calcareous matter in the waters, are characterized by a low tone of the system. These observations are in unison with the circumstances which, in some localities, pro duce goitre or scrofulous enlargement of the glands oi the neck. In Alpine regions, where this terrible disease is so prevalent, it is attributed to a habitual use of snow water, which [Sk is but slightly charged with mineral matter of any kind. The geology of those regions being primitive, the waters are deficient in lime, though not of other earths, such as magnesia and alumina neither of which can replace lime in the animal economy. Scrofula, in some of its many forms, will be found in all districts consisting exclusively of a primitive formation, such as the Channel Islands. These views are well worthy of attention, but if the requisite quantity of lime cannot be obtained in water, it can be obtained from some kinds of food. The views here presented, however, embrace the idea that scrofulous diseases will always prevail in districts deficient in lime.