>J4 THE IMPROVEMENT OF RIVERS. 



Girard Shutter. To overcome the difficulty of raising the Thenard shutter M. 

 Girard proposed to do away with the counter-shutters and to substitute for the props 

 hydraulic jacks, which would raise or lower the main shutters at will. Seven of this 

 type were constructed in 1870 at Auxerre on the Yonne, and gave excellent satisfac- 

 tion. The shutters were 1 1 J feet wide and 6J feet high, the cylinder of the jack being 

 12 inches in diameter. On the introduction or withdrawal of the power the piston- 

 rod moved out or in, thus raising or lowering the shutter. 



The cost of the dam per lineal foot was $60 for the fixed parts and $119 for the 

 movable parts, or a total of $179. 



Owing to the untimely death of its inventor no further examples of this type 

 were constructed. 



Remarks. The shutter dam possesses the advantages of simplicity, and of not 

 being easily injured by drift or ice, since there are few parts in which these can 

 be caught. The difficulty of raising it, however, and the expense of the counter- 

 shutters, proved a serious drawback, and after the introduction of the Chanoine 

 wicket the type became practically obsolete. 



A-FRAME DAMS. 



This design relies wholly upon trestles for the retention of the water, as well as 

 for the support of its pressure. The trestles may be raised and lowered precisely as 

 are other trestles, or with a special arrangement similar to that used on the needle dam 

 on the Big Sandy River at Louisa, Ky. 



Description. The dam consists of a number of A-shaped trestles set up adjoining 

 each other across a stream, the up-stream faces or legs of which, in connection with a 

 sill (which also protects the trestles when down), hold back the water. The two legs 

 of each trestle are connected at the top, by pktes forming a walk, and at the bottom 

 they terminate in eyes which are connected by pins to journal-boxes and sills attached 

 to the masonry. 



The up-stream member of each trestle is a frame of channels suitably arranged 

 and covered with plates riveted on. The edges of these plates touch each other on 

 adjacent trestles when standing, and may extend slightly over the channels, or the 

 latter may be set flush with flanges toward each other. The latter construction gives 

 a thicker wall through which the water must pass in leaking. Wood may be inserted 

 along the channels when exceptional tightness is desired. The down-stream post may 

 be similar to the up-stream one, or it may be latticed, or even consist of a single mem- 

 l>er like the prop of a wicket. At the bottom of each post a piece with an eye, bent so 

 as to have the part containing the eye stand vertical, is riveted to the frame, and these 

 are connected by pins to journal-boxes on the floor. These boxes have their eyes cen- 

 tered at a greater distance from the floor than one-half the width of the trestle face, 

 so that the trestles may turn without binding. The upper box is imbedded in the 





