14 
BULLETIN 831, U. S. DEPARTMENT OF AGRICULTURE. 
more of these have been installed on the more recently constructed 
spillway regulators than any of the other devices. Their popularity 
is based on the possibility of using them where long span is essential. 
The rolling dam is a patented type and consists of a large hollow 
cylinder, the diameter of which is usually made to correspond to the 
height to which it is desired to raise the pond level above the crest 
upon which the cylinder rests. Its length is made equal to the width 
&\-6a/e Hanger P/h. 
rYhep yya/er/n/?iver gets 
h/gh fh/s Gore turns dorrs?, 
\ &nd Counter rye /<?/?/ ra/ses 
\ up-W0/#/ng3earr?r/pf>Jngt 
\ \ according//- 
'A'': a ..-£ 
?$f- % ' Concrete /?oJ/>ray&T$c7fe. 
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Fig. 3. — Concrete clam 17 feet high with flood gates, Nashua, Iowa. This structure on 
the Cedar River is equipped with automatic flood gates 7 feet high hy 46 feet long. 
of the opening into which it is fitted. As a general thing the material 
of which it is made is boiler plate riveted together and braced to 
withstand the strain to which the dam is to be subjected. The me- 
chanical operation is provided for by having a gear engage a rack 
on an inclined abutment and by means of a sprocket chain wrapped 
around one end of the cylinder and connecting with the operating 
mechanism, the dam is rolled up or down the abutment as desired. 
The power is generally furnished by an electric motor or by winches. 
Various forms have been installed at Grand Junction, Colo., on one 
