t 4 THE IMPROVEMEXT OF RIVERS. 



Now c - , and 5 must be assumed as desired, say 1000 Ibs. per square inch, or more, 

 according to the timber to be used. Combining the equations for 7 and M, we find 



ioooXtt>if a loooXwjf* 6M 



Af- - - X7- - z - or J - 



12 *!' 6 ioooX< 



from which t can be found. 



For a head of water 18 feet, with the needle slightly inclined, t was found to be 

 nj inches, being i foot, and 5 about 1000 Ibs. 



The usual form of needles adopted in European practice is of square section, vary- 

 ing from i A inches by ij inches on the first clams, with a length of 8} feet and a weight 

 of 5 Ibs., to 4J inches by 4} inches, on the more recent ones, with a length of 16$ feet 

 and a weight of 104 Ibs. Small dimensions have been a desideratum with European 

 designers, as the operation of the dams has been by hand, and it was necessary to limit 

 the size of the needles to such as could be controlled by one man. 



The lap on the sill is made from 4 to 6 inches. 



Where there are appliances for handling them, needles of wide face are much 

 preferable to narrow ones, as there are fewer joints and less tendency to warping, thus 

 securing a minimum of leakage. With narrow needles, such as are generally employed, 

 it is necessary in a dry season to use weeds, straw, canvas, or other means, to close the 

 spaces between them. In any case, the thickness of the needle must not be greater 

 than its width, or the pieces will turn in the current when being placed. 



As regards the kind of timber, white pine appears to be the most satisfactory, as 

 it possesses the greatest strength, weight for weight, and does not splinter easily. 

 Oregon pine would probably make an excellent needle also. Georgia pine, while pos- 

 sessing less tendency to become water-soaked, splinters easily, and the section required 

 to support a given head of water will weigh more than the section which would be 

 required in white pine. Oak is not suitable, as it becomes water-soaked and is very 

 liable to warp, the last disadvantage belonging to yellow poplar also, unless sawed 

 from large trees. On the recently completed dams on the Moldau and the Elbe, in 

 Bohemia, larch-wood was employed, the largest needles being about 4^ inches square 

 and 13 feet long, and weighing when wet 72 Ibs. On other dams in Europe Riga fir 

 has been generally used. 



The section used for needles varies somewhat; some being square, some rectan- 

 gular, while another form has a uniform width, but is larger at the point of greatest 

 resistance than at either end. Hexagonal and semi-hexagonal needles, and needles 







with rubber up-stream facings overlapping their neighbors, have been proposed and 

 exi>erimented with, but none of these have come into general use. A hollow needle 

 made of four planks nailed together and banded with iron has also been proposed. 



General Remarks on Needle Dams. The needle dam offers many advantages for 

 rivers of small low-water discharge, as it can be designed to permit very little leakage, 

 and is inexpensive in construction and operation. For wide rivers, however, espe- 



