The hydraulic gradient plays a very important part in the construction of a long pipe-line or conduit. If any part of the pipe should rise above the hydraulic gradient, the discharge would no longer be full below that point. It is necessary, therefore, always to lay the pipe or the closed conduit so that all parts of it shall be below the hydraulic gradient. Caution is obviously necessary to lay a pipe carrying water deep enough below the surface of the ground in cold climates to protect the water against freezing. At the same time if the pipe-line is a long one it must follow the surface of the ground approximately in order to save expensive cutting. There will, therefore, generally be summits in pipe-lines, and inasmuch as all potable water carries some air dissolved in it, that air is liable to accumulate at the high points or summits. If that accumulation goes on long enough, it will seriously trench upon the carrying capacity of the pipe and decrease its flow. It is therefore necessary to provide at summits what are called blow-off cocks to let the air escape. At the low points of the pipe-line, on the contrary, the solid matter, such as sand and dirt, carried by the water is liable to accumulate, and it is customary to arrange blow-offs also at such points, so as to enable some of the water to escape and carry with it the sand and dirt. [Illustration: IN LOOSE EARTH.] [Illustration: IN ROCK. Weston Aqueduct. Sections of Aqueduct and Embankment.] [Illustration: SECTION OF EMBANKMENT.] [Illustration: ON EMBANKMENT. Weston Aqueduct. Sections of Aqueduct and Embankment. Gradient, 1 in 5000.] =189. Flow of Water in Large Masonry Conduits.=—In order to apply Chezy’s formula first to the flow of the masonry aqueducts of the New York and Boston water-supplies, it is necessary to have the outlines of those conduits so that the wetted perimeter and hence the mean radius may be determined for any depth of water in them. [Illustration: OUTLINES OF AQUADUCTS. FIG. 3.] The figure shows the desired cross-sections drawn carefully to scale. Table XIV has been computed and arranged from data taken from various official sources so as to show the depth, mean velocity, discharge per second and per twenty-four hours, and the coefficient used in Chezy’s formula, together with the coefficient of roughness _n_ in Kutter’s formula for the conduits shown in the figure. This table exhibits in a concise and clear manner the use of Chezy’s formula in this class of hydraulic work. TABLE XIV.