The cost of washing new sand might be somewhat less. The other costs of cleaning filters, scraping, transporting, and replacing the sand are much greater than the washing itself. Lindley states that at Warsaw 29 days’ labor of 10 hours for one man are required to scrape an acre of filter surface, and four times as much for the annual deep scraping, digging up, and replacing the sand. The first expense occurs in general monthly, and the second only once a year. At other places where I have secured corresponding data the figures range from 19 to 40 days’ labor to scrape one acre, and average about the same as Lindley estimates. Under some conditions sand-washing does not pay, and in still others it is almost impossible. No apparatus has yet been devised which will wash the dirt out of the fine dune-sands used in Holland without washing a large part of the sand itself away, and in these works fresh sand, which is available in unlimited quantities and close to the works, is always used. At Breslau the dirty sand is sold for building purposes for one third of the price paid for new sand dredged from the river, delivered at the works, and no sand is ever washed. Budapest, Warsaw, and Rotterdam also use fresh river-sand without washing, except a very crude washing to remove clay at Budapest. CHAPTER VI. THEORY AND EFFICIENCY OF CONTINUOUS FILTRATION. The first filters for a public water-supply were built by James Simpson, engineer of the Chelsea Water Company at London in 1829. They were apparently intended to remove dirt from the water in imitation of natural processes, and without any very clear conception of either the exact extent of purification or the way in which it was to be accomplished. The removal of turbidity was the most obvious result, and a clear effluent was the single test of the efficiency of filtration, as it remains the legal criterion of the work of the London filters even to-day, notwithstanding the discovery and use of other and more delicate tests.