LIV. T. W. SEAVER. 



on to next, and when it has reached a certain fixed 

 distance, the " drop boards " may be put in and the full 

 depth of water conserved. 



In cases, in which it is advisable to have the means of 

 removing the whole structure, and so give a clear water- 

 way, some form of falling gates must be used. The oldest 

 of these weirs, that known as the Bear Trap was first 

 erected by Josiah White in 1818 across the Lehigh River 

 in Pennsylvannia, and consisted of two gates, the lower of 

 which acted as a strut for the upper, being raised by 

 admitting water beneath it by means of a valve. 



In 1831 weirs formed of long timbers, known as "needles," 

 resting at their lower ends against a sill, and at their upper 

 ends against an iron framework were first erected by M.M. 

 Poiree and Ohanoine. This weir was afterwards improved 

 by M. Oamere who substituted for the vertical needles 

 horizontal boards hinged together, and which could be 

 rolled up like a curtain. 



Tiie next important invention in connection with these 

 works was that of movable shutters by M. Thenard in 1837. 

 This form of weir, which has been used to a considerable 

 extent in India, consists of two gates close together and 

 hinged to the floor. The upper gate falls up stream and is 

 raised by the force of the water, being held back by chains 

 when it is up, the lower gate is raised and kept in position 

 by means of a strut. This later gate now takes all the 

 water pressure, when the upper gate can be lowered to its 

 first position. This weir was defective, in that the great 

 shock received by the sudden raising of the upper gate, 

 often caused the chains to break, or the floor to be pulled 

 up ; to remedy this Lieut. Fernacres made use of an 

 ingenious telescopic strut to be used instead of the holding 

 back chains. As the gate is raised the piston of this strut 

 is pushed into a cylinder, the water in which is forced out 



