166 PIPES AND NOZZLES.
with the corresponding angle irons on the next pipe, as
denoted
in Fig. 23, which shows the manner in which the pipes and elbows were
strapped together whenever the curve was sufficiently short to
require this precaution against an outward movement.
Air - Valves, Blow-offs.—On
a long line of pipe, or a siphon, " blow-offs " and air-valves are
provided to allow the escape of the air from the pipe while filling,
and especially to prevent a collapse of the pipe in case of a break.
The valves in use are of varied make. A simple construction is a piece
of leather loaded on the inside of the pipe, and arranged to cover an
opening from 1 inch to 4 inches in diameter. A bet-
