LOCK GATES AND VALVES. 183 



warp it is best to allow more than this. Sometimes the pieces are planed to exact 

 size at the mill, but this plan has its drawbacks, as the timber usually shrinks and 

 becomes more or less scarred in transit and in handling, and if it gets in wind there is no 

 spare material for trimming it off. By using it ready planed some expense is saved, 

 but the work is rarely as satisfactory as can be obtained by dressing at the site. 



The beams should be finished so that when the gates are closed there will be no 

 space between their ends, or between them and the miter-sills. If they are too long 

 they will always leak, as the ends do not appreciably wear in use, and if too short, 

 recourse must be had to filler-blocks, which are always liable to be knocked off. When, 

 the gates are built up in place it is best to leave the toes rough, and to saw them off 

 to the exact length and bevel when the gates are finally swung, taking- the meas- 

 urement along each sill. These measurements should always be recorded on the 

 drawings, as they usually vary slightly, so that when the gates have to be replaced 

 the new ones can be cut to an exact fit. 



In new locks the gates are usually framed on shore and then built up in place 

 piece by piece, but in replacing gates in old locks they must be set up complete, so as 

 to delay navigation as little as possible. This is effected by building them on shore 

 on ways, and launching them when finished into the river. They are then supported 

 under a barge and taken into the chamber, when they are hoisted in place by crabs on 

 the wall or by other means. 



METAL GATES. 



In the last few years gates of metal have been coming into favor in the United 

 States, and several examples of this class of construction are to be found on different 

 rivers, while many others are proposed. They will doubtless be more widely applied, 

 since timber suitable for gates is becoming more expensive every year, and the quality 

 of that which can be obtained is steadily deteriorating. From a comparison of several 

 cases we find that at the present time metal gates cost from 30 to 60 per cent 

 more than wooden gates, depending on location and on size, the larger gates being 

 relatively the cheaper. As to duration, the consensus of opinion seems that the 

 former class will last about twice as long as the latter. It should be remembered, 

 however, that where the metal is constantly submerged it can never be properly 

 cleaned and painted. The effect of water on steel, which is the only metal obtainable 

 in this country from which plates and shapes are rolled, appears much more destruc- 

 tive than upon cast or wrought iron, and a few years' submersion will corrode and pit 

 the metal to a very serious extent, rendering it a practical impossibility to secure a 

 thorough cleaning. It might be possible to pump out the lock and thoroughly clean 

 the gate every year, but this would involve a stoppage of navigation, and even on 

 rivers of small traffic the loss and expense would scarcely justify the gain. In addi- 

 tion to this there would always be some recesses or portions of the construction, as for 



