6 BULLETIX 115, U. S. DEPAETMEXT OF AGBICULTUKE. 
the silt deposit above the others. Of course this same condition must 
be guarded against where other forms of shutter are used, but it is 
much easier to pull all vents, if it can be done at one operation 
for each opening, than to fish out one flashboard at a time, with per- 
haps 8 or 10 boards in each vent. 
This objection does not apply where boards are used to establish 
the crest of such a structure as a waste gate, designed for practically 
all of the water above the level of the crest to pass off and leave the 
canal. The water in the canal continually scours past the waste gate, 
keeping out the silt. and. as a rule, there are not many adjustments on 
such a gate throughout the irrigation season, there being a very ma- 
terial saving in the cost of gate lifts, which would be used only a 
few times in the season. 
If flashboards are made loose enough so that they will operate with 
comparative ease, then there also is danger that they will float if 
the slots are made vertical. If the slots are inclined, the friction 
against the upper side of the slot counteracts the floating action of 
the water. Inclined slots also place the shutter so as to carry the 
thrust of water downward through the floor of the structure rather 
than tend to overturn it. The inclined slot requires more material 
for a shutter of a given height than does a vertical slot. Flashboards 
in an inclined set of slots can be more easily made water-tight by 
packing with mud than those in vertical slots. 
SHEAR GATES. 
During the past few years the shear gate has come into use on 
irrigation systems. It consists of a round cast disk turning on a pin 
through a hole at the edge of the disk. At the side opposite the pin 
is attached an iron rod for a handle. TThen the gate is closed the disk 
covers the end of a tube which is cemented to a cast-iron orifice plate. 
By means of the rod the disk is turned on the pin until the desired 
amount of water is delivered through the opening made as the end 
of the pipe is uncovered. In closing, the disk drops down between 
wedging lugs which bind the disk tightly to the face plate, making a 
comparatively water-tight connection. This gate costs but a fraction 
of the amount necessary for a metal slide gate, and could have been 
used in many cases where a slide gate has been installed. 
To provide a locking device for this type of gate, the iron handle 
bar may be flattened and bored with slots to pass like a hasp over a 
staple set in the wall of the gate structure. The bar then may be 
slipped over the staple at the notch nearest the desired position and a 
padlock put through the staple. 
RADIAL GATES. 
The use of the radial gate is comparatively new in this country. 
It is essentially a gate raised and lowered by revolving on a horizontal 
