36-1/2 miles long.

The third, or gravity section, extends from the reservoir on the Corona
summit to the Rio Pecos at Mile 272, dropping from an elevation of 6,750
to 4,570 ft. in 80 miles. The pipe line extends to Pastura, 58-1/2 miles
from Corona, as shown on Plate V.

Where the pipe line passes a water tank on the railway, a 4-in. branch
pipe is carried to the bottom of the tank and up to the top, where it is
capped by an automatic valve. A gate-valve is placed in the branch pipe
at its junction with the pipe line.

There are regulating, relief, check, blow-off, and air-valves,
air-chambers, and open stand-pipes on the line, too numerous to mention
in detail. They are designed to keep the wood pipe full, regulate flow,
prevent accumulation of pressure and water-hammer, and remove sediment.

_Water Pipe_.--A study of the profile developed a system of hydraulic
grades, pipe diameters, and open stand-pipes limiting the pressure to
130 lb. per sq. in., except on 19 miles of the pump main between Coyote
and Corona where the estimated maximum pressure is 310 lb.

Investigation justified the assumption that wood pipe under a pressure
of 130 lb. would give satisfactory service for 25 years, on which basis
it would be less expensive than cast iron, and therefore it was used.
Cast iron was considered preferable to steel for pressures not exceeding
310 lb. on account of its greater durability.

_Wood Pipe_.--Machine-made, spirally-wound, wood-stave pipe, made in
sections from 8 to 12 ft. long, with the exterior surface covered with a