The general arrangement of one of the variable head types is shown in Fig. 4. A constant head is maintained on the valve _V_ by a float and cock operating in a lead- or porcelain-lined tank. The circular tapered orifice _O_, cut in glass, is situated in the flanged end of the iron casting _C_ and the head, indicated on the gauge glass, is regulated by valve _V_. This arrangement is simple and reasonably accurate. The orifice may show slight incrustation after being in service for some time but it can be easily cleaned by means of a test-tube brush or a small swab moistened with acid; a wire or rod tends to break the edge of the conical orifice and should not be used. The volume of solution discharged by orifices of various dimensions is shown in Diagram XV, page 149. Diagram XVI, page 149, facilitates the calculation of the number of pounds of bleach required for any dosage. [Illustration: FIG. 4.--Variable Head Dosage Box.] The solution discharged from the orifice box is carried to the point of application either in galvanised iron pipes of generous dimension or in rubber hose. Pumps may be used for raising the solution to a higher elevation but unless special material is used in their construction they corrode rapidly and cannot be kept in service. Whenever possible, a water injector should be used as it does not corrode and assists in maintaining the delivery pipes free from sludge. All delivery pipes should be duplicated and blown out regularly by water under pressure; they should also be protected from frost. The adjustment of the hypochlorite dosage can be automatically regulated in plants where the flow of the water to be treated is measured by a Venturi meter or other suitable appliance. Various devices have been suggested and used but, in general, they are not so successful as automatic regulators for liquid chlorine on account of the presence of sludge particles which tend to diminish the area of the orifice. For small plants, barrels have often been used as solution and storage vessels with, in some instances, fairly successful results. The bleach process, however, cannot be recommended for small installations because the chemical control necessary for successful operation is usually not available. One drum of bleach may suffice for several months operation and as the powder gradually loses strength, the dosage constantly diminishes and may jeopardise the safety of the supply. Liquid chlorine machines are much more suitable than hypochlorite installations for supplies having no chemical control.