22 The Colorado Experiment Station. 
tration of a type more ingenious, however, than valuable, though 
it has less objection than some of the others. It would also 
require, like the Marseilles module, a considerable available fall. 
A satisfactory means of preserving a constant head from a 
lateral is known as the spill box, or excess weir, which has been 
used somewhat in this state. This is an attempt to maintain the 
head constant and thus to give a constant flow. Any variation in 
discharge is caused by varying the length of the opening. It 
has been used in this state for the discharge of water in terms 
of the statute inch, and thus requiring the statutory head. It, how¬ 
ever, could be used to deliver water by the weir method, and 
especially with the trapezoidal weir. The device is really a method 
of preserving a constant head. In the case of a small ditch, as 
Plans of the Spill-Box. 
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h m spilling m eo&a: m 
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shown in the figure, gates are located so as to force water into the 
spill box. On the side next to the lateral is a long sharp 
edged board whose crest is at the required height above the bottom 
of the opening. The length is such that the excess water spills back 
to the lateral. The sensitiveness depends upon the length of the 
box, and upon the regulation of the gates. It requires a sufficient 
available fall in the ditch so that the water will drop back into the 
ditch. 
As made in the San Luis valley, the spill box is about 16 
feet long. It generally requires a fall of about a foot. By making 
the crest longer, the device may be made more sensitive. As used 
in the San Luis valley, a hinged gate was used instead of the board 
and sometimes the boxes were used in pairs, as shown in Figure. 
This did not change the principle but saved expense in structures. 
The principle was first brought out by A. D. Foote, now of Grass 
Valley, Cal., and adapted to needs of canals by W. H. Graves, then 
of Monte Vista. 
THE WEIR MODULES. 
The form of module which is the most accurate is the weir or 
overfall. The knowledge of the flow through this, as through any 
other opening, depends primarily upon experiments previously made 
