ai8 



THE IMPROVEMENT OF RIVERS. 



To lower the dam, the reverse operation is pursued, the needles being first 

 removed and the trestles lowered in order. 



Placing and Removing Needles. Small needles are usually placed and removed 

 by hand. To place them, the hi-ad is lu-M against the escape or support-bar, and the 

 end plunged up stream in the current which draws it round till it strikes the sill. In 



Itt II U IX 



ESCAPEMENT 



POIREE NEEDLE DAM 

 tCALi or re IT 



i i a . ; ) i t 



SECTION 

 GENERAL DRAWING OP AN EARLY POIRKE (NEEDLE) DAM. 



the Marne dams, where the needles are 4? inches square and weigh up to 100 Ibs., 

 the same method is used; but the down-stream sides are provided with hooks which 

 fit over round support-bars, thus holding the heads in the exact position desired. As 

 the length from the hook to the end exceeds the length from the support-bar to the 

 sill by j inch to f inch, the foot of the needle scrapes along the floor just as the needle 

 becomes upright. This takes away the shock from the support-bar almost com- 

 pletely, and assures the normal placing of the needle. 





