CURTAIN DAMS. 445 



high, weighing respectively 4000, 3000, and 1 760 Ibs. They are constructed of channel 

 iron, with two uprights front and back, joined by cross-bars and braced by a St. 

 Andrew's cross. A continuous chain is used to raise and lower the trestles, operated 

 by a windlass on the shore, and they can be maneuvered six at a time. 



In the right arm of the river, i. e., in the elevated pass, Camere curtains are used 

 for closing the dam. The middle channel or weir is closed with Boule gates, and the 

 left arm or navigable pass is closed with gates and curtains alternately. The last 

 method of arrangement has proved the best, for the reason that the curtains when 

 rolled up do not always preserve a cylindrical form. This fact is likely to cause adajcent 

 curtains to jam. 



In time of flood the top rows of gates are first removed, and then the curtains 

 are rolled up. If the flood continues, the gates are all removed and the trestles in the 

 navigable pass are lowered. Thus the whole pass is opened to navigation with a depth 

 of 10 feet on the sill. The time required to either raise or lower the trestles of the 

 pass (fifty-seven in number) is three hours. As it involves a considerable delay to 

 navigation to pass boats through the lock, the trestles are always bedded as soon as 

 there is sufficient water on the sill for the purpose of navigation. If the river falls, 

 the trestles are immediately replaced in order to preserve a proper stage of water. In 

 the other passes, which are but little used for navigation, the trestles are not lowered 

 until almost submerged. The curtains are removed by means of a special car carrying 

 a small windlass, and are carried to a platform on shore and hung upon a special frame 

 in the same position as the dam. The operation is laborious, as each curtain and 

 frame weighs 1600 Ibs. As they are rolled up, d6bris of various kinds catches 

 between their sticks, and they have to be unrolled and cleaned. 



The method of removing gates is to pile them on a truck rolling on the bridge, 

 when they are taken to the bank and stacked there. 



A new lock, 525 feet long and 56 feet wide, was placed between the left bank and 

 the old lock. The latter was rebuilt, and below it a lift- wall was placed. This was con- 

 tinued by a new lock 165 feet long. 



The cost of the dam is given as $632 per lineal foot, all included. 



Calculations for Trestles for Gates or Curtains. Trestles for use with the Boule 

 or Camere types of dams have to support in addition to the direct strains, the bending 

 from the water-loads on the up-stream leg. 



To determine the dimensions required, first find by moments or by graphics the 

 strains in the various pieces AB, AG, etc. (Fig. 21) induced by the water-loads P,, 

 Pj, etc., considered as concentrated at the panel points A, B, etc. This will give the max- 

 imum strains in the web and down -stream members, but the up-stream member A BCD 

 has to support in addition, between each panel point, the bending from the gates or 

 curtains, which rest directly on it. In all ordinary cases economical manufacture will 

 require ABCD to have the same section from top to bottom, as, for instance, two chan- 



