bubbles on rising from the deposited sludge strike the sloping lower sections of the inner partitions and are deflected to the portions of the tank next to the outside walls. A sludge pipe leads away from the bottom of the hopper-shaped sludge compartment, and at intervals of from one to four weeks the valve on this sludge pipe is opened for a short time and a small portion of the accumulated sludge is allowed to be forced out onto a sludge-drying bed by the weight of the sewage in the tank. The portion of the sludge thus removed has, of course, remained in the tank the longest time, generally five or six months, and has had the fullest opportunity to be reduced and rendered inodorous and easy to dispose of. This method of sedimentation was first experimented with about twelve years ago by Mr. H. W. Clark at the Lawrence Experiment Station of the Massachusetts State Board of Health, then partially developed by Dr. W. Owen Travis, of Hampton, England, and finally worked out by Dr. Ing. Carl Imhoff in connection with the disposal of sewage in the Emscher River district in Germany. The method has been extensively and successfully used in Germany, and similar tanks are now being installed in this country. While these tanks are probably more effective than septic tanks and the usual type of settling tanks in the removal of suspended matters in sewage, their chief value will undoubtedly be found in the rendering of the sludge less odorous and more easily handled. This form of settling tank is covered by patents, and a moderate royalty is charged on tanks of this type. A description of the Imhoff tank has been here included since it represents an important development in sewage disposal and helps to solve what has heretofore been one of the main difficulties of sewage disposal, especially for cities and villages, namely, the satisfactory and convenient disposition of sludge; but it is not considered that their construction is advisable or warranted where only a small quantity of sewage is to be treated, and settling tanks to treat sewage contributed by less than, say, two hundred persons would generally be constructed as previously described. CHAPTER III VALVES, SIPHONS, AND SIPHON CHAMBERS