USE OF CONCRETE PIPE IN IRRIGATION. 
29 
entrance to the pipe is commonly protected by iron bars set on an 
angle in front of the pipe entrance. The bars may be spaced 6 inches 
or more apart. Such protection will prevent boards and large tree 
branches entering the pipe. 
AIR VENTS. 
When air is drawn into a pipe carrying water, being lighter than 
the water, it tends to collect at the highest points of the line. Such 
accumulations of air lessen the water-carrying capacity of the pipe 
and may obstruct it altogether. To guard against occurrences of 
this kind means should be provided to allow the entrapped air to 
escape into the atmosphere. One of the safest and best means of 
Fig. 
-Showing long screen at junction of open ditch and pipe line. 
matic register at left of picture. 
Weir box and auto- 
doing this is by the insertion of standpipes at all points where air 
is liable to collect. These standpipes need not be large for the 
average sizes of pipe. They are not only automatic but continuous 
in their action. For lateral pipes a galvanized-iron pipe 1 inch 
in diameter and covered with a perforated cap and protected by a 
substantial post makes a cheap and serviceable air vent. It is also 
good practice to get rid of the air as soon as it enters the pipe by 
inserting standpipes near each intake, care being taken in all cases to 
extend the air pipe far enough above the pipe so that its top will 
be well above the hydraulic grade line, or in other words, above the 
head to which water will rise. 
Where standpipes can not be used the confined air may be released 
by the use of automatic air valves which can be inserted on pipes 
under medium and high pressures. One of the simplest of these is 
a spherical rubber ball which is pressed tightly against the circular 
