6 BULLETIN 115, TJ. S. DEPARTMENT OP AGRICULTURE. 



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. When 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 



