_n_ = 0.009 for well-planed timber; _n_ = 0.010 for neat cement; _n_ = 0.011 for cement with one third sand; _n_ = 0.012 for unplaned timber; _n_ = 0.013 for ashlar and brickwork; _n_ = 0.015 for unclean surfaces in sewers and conduits; _n_ = 0.017 for rubble masonry; _n_ = 0.020 for canals in very firm gravel; _n_ = 0.025 for canals and rivers free from stones and weeds; _n_ = 0.030 for canals and rivers with some stones and weeds; _n_ = 0.035 for canals and rivers in bad order. =188. Hydraulic Gradient.=—Before illustrating the use of Chezy’s formula in connection with masonry and metal conduits, of which mention has already been made, it is best to define another quantity constantly used in connection with closed iron or steel pipes. This quantity is called the “hydraulic gradient.” If a closed iron or steel pipe is running full of water and under pressure and if small vertical tubes be inserted in the top of the pipe with their lower ends bent so as to be at right angles to its axis, the water will rise to heights in the tubes depending upon the pressures of water in the pipe or conduit at the points of insertion. Such tubes with the water columns in them are called piezometers. They are constantly used in connection with water-pipes in order to show the pressures at the points where they are inserted. A number of such pipes being inserted along an iron pipe or conduit, a line may be imagined to be drawn through the upper surfaces of the columns of water, and that line is called the “hydraulic gradient.” It represents the upper surface of water in an open channel discharging with the same velocity existing in the closed pipe. In case Chezy’s formula is used to determine the velocity of discharge in a closed pipe running under pressure, the sine of inclination _s_ must be that of the hydraulic gradient and not the sine of inclination of the axis of the closed pipe. In the determination of this quantity _s_ by the use of piezometer tubes, if a straight pipe remains of constant section between any two points, it is only necessary to insert the tubes at those points and observe the difference in levels of the water columns in them. That difference of levels or elevations will represent the height which is to be divided by the length of pipe or conduit between the same two points in order to determine the sine _s_. [Illustration: Progress View of Construction of New Croton Dam.]