Amsterdam | 0.17 | 1.6 | 2.40 |Dune-sand. Rotterdam | 0.34 | 1.5 | 2.30 |River-sand; new. Liverpool, Rivington | 0.43 | 2.0 | 0.76 |Sand from bottom of | | | | filter. Liverpool, Rivington | 0.32 | 2.5 | 1.00 |New sand unwashed | | | | and unscreened. Liverpool, Rivington | 0.43 | 2.7 | 4.10 |Washed sand which | | | | has been in use | | | | 30 to 40 years. Liverpool, Oswestry | 0.30 | 2.6 | 9.40 |Dirty sand. Liverpool, Oswestry | 0.31 | 4.7 | 2.20 |Same, after washing. ---------------------+---------+--------+--------+-------------------- NOTE.—It is obvious that in case the sands used at any place are not always of the same character, as is shown to be the case by different samples from some of the works, the examination of such a limited number of samples as the above from each place is entirely inadequate to establish accurately the sizes of sand used at that particular place, or to allow close comparisons between the different works, and for this reason no such comparisons will be made. The object of these investigations was to determine the sizes of the sands commonly used in Europe, and, considering the number and character of the different works represented, it is believed that the results are ample for this purpose. The English and most of the German sands are washed, even when entirely new, before being used, to remove fine particles. At Breslau, however, sand dredged from the river Oder is used in its natural state, and new sand is used for replacing that removed by scraping. At Budapest, Danube sand is used in the same way, but with a very crude washing, and it is said that only new unwashed sand is used at Warsaw. In Holland, so far as I learned, no sand is washed, but new sand is always used for refilling. At most of the works visited dune-sand with an effective size of only 0.17 to 0.19 mm. is used, and this is the finest sand which I have ever found used for water filtration on a large scale. It should be said, however, that the waters filtered through these fine sands are fairly clear before filtration, and are not comparable to the turbid river-waters often filtered elsewhere, and their tendency to choke the filters is consequently much less. At Rotterdam and Schiedam, where the raw water is drawn from the Maas, as the principal stream of the Rhine is called in Holland, river-sand of much larger grain size is employed. It is obtained by dredging in the river and is never washed, new sand always being employed for refilling.