Sometimes the face of the country requires flumes; they may even be hung along the face of a cliff. In shattered ground and where water is scarce flumes are better than ditches. The grade for a flume is usually 25 to 35 feet per mile and its capacity is smaller than that of a ditch. Pine planking 2-1/2 inches by 12 to 24 inches, and 12 feet long, is the dimension stuff generally preferred. A flume 2 feet 6 inches square requires posts, caps, and sills of 3×4 inch; stringers 4×6 inch. Great care is needed at curves to avoid slack water and splashing. The boxes must be shortened and the outer side wedged up until the water flows as evenly as in the straight stretches. Should anchor ice form the water must be shut off at once. The life of a flume seldom exceeds a dozen years, whereas at the end of a similar period a ditch would be carrying 10 per cent more water than at first, owing to the sides and bottom having become consolidated. Wrought iron pipes are employed largely in California to replace ditches and flumes. When the pipe crosses a ravine it is known as an inverted siphon. Piping is also used to convey water from the "pressure box" to the "gates" and "nozzle." Wrought iron pipes have to stand pressure varying from 34 pounds to 800 pounds to the square inch. Air valves or blow-offs must be provided at intervals to allow the escape of air from the pipe while filling, and to prevent a collapse of the pipe after a break. A covering of coal-tar should be given the pipe both inside and out. Cost varies from one dollar to two dollars a running foot. The pressure box ends the ditch and from it the water passes into the supply pipe. The head of water is measured from this point. A box to catch sand and gravel, with a side opening and sunk below the level of the ditch, is called the "sand box."