Hence the variety of artificial substitutes of contact beds, costing from 5000_l._ to 12,000_l._ per acre, which have been proposed of late years, with the object of purifying a large volume of sewage on a small area. [Sidenote: Leeds experiments.] Mr. Dibdin first startled the world with the formula 1,000,000 gallons per acre, but that has long been cut down to 200,000 gallons, and the life of the contact bed has become the subject of serious concern, as shown in the annexed table of experiment at Leeds. Others have sought to increase the proportion of sewage to area by arranging for _continuous_ instead of _intermittent_ application; but the difficulty of sprinkling so that every part of a bed may be kept just moist, in order that aeration may be continuous as well as the dropping sewage, is very great, and increases with every gallon and foot from the scale of a laboratory experiment to that of a practical working for a town’s sewage. There was an article published a few years ago in the Journal Royal Agricultural Society (England) on “The Making of the Land,” showing how nearly all the value of agricultural land in England has been stored up in it by the exertions of our forefathers, through a process of successive improvements from, in many cases, worthless sand and clay, to a condition of the greatest fertility; and I often think that the 12,000_l._ spent at Birmingham or elsewhere on an acre of contact bed could be expended to better purpose in preparing 100 acres of the worst land to deal, for any number of years, with as much sewage as the contact bed may do for a few years. In the one case we know no limit to the life of the purifier, and that it must be a very short one in the other case. TABLE SHOWING THE VARIATIONS IN CAPACITY OF CONTACT BEDS.