66 



THE IMPROVEMENT OF RIVERS. 



Remarks. The Ix-ur-tnip is the only type of standard dam in regard to which 

 there is little experience available. Those built in recent years have not been always 

 successful, as the theory and the practice were alike undeveloped. At present, how- 

 ever, they are being more widely studied and adopted, two having been recently con- 

 structed with lengths of 120 feet and heights of 13 feet on the dams of the Ohio River, 

 for use as drift-chutes and regulating- weirs. 



The old bear-trap required a great width of foundation, which was a source of 

 exjvnse, but with the modified forms this objection has been reduced. The type 

 is not as simple, nor is it as certain in operation, as it has usually been designed, as the 



A 



LWAUKIC RIVER OUt BCAR TRAP 



\ 



MUSCLE HCML* CANAL KM THAT 



\ 



0.7 



NO BACKWATER 



FIG. 26. PARKER BEAR-TRAP CURVES. 



drum wicket, but it can be applied to greater lifts, since it requires no substructure below 

 it for a recess, as does the drum. It can therefore be used for passes where the latter 

 would be inapplicable because of the expense of the foundation. 



The head required to raise a gate should not be over 6 inches. On the Marne, 

 owing to faulty proportions, a head of 2 feet was required, which was obtained by a row 

 of Thcnard shutters just above the gate. Where the head is not obtainable from the 

 natural fall of the river it may be created by a pump and reservoir tank on shore. 



One of the difficulties met with in long gates has been a tendency to warp, as the 

 water has usually been introduced from one end only. This causes one end to rise 



