190 THE IMPROVEMENT OF RIVERS. 



than 40 years, and some of those in the docks at Liverpool, built of oak, are said to have 

 had 70 years of sen-ice. The comparatively short life of American gates is doubtless 

 due to severer climatic conditions. 



The life of iron gates used in canals in Germany has been from 28 to 48 years, 

 with an average of 40 years, and in Holland similar gates have lasted for 60 years. 

 Galvanized iron gates used in sea locks in France are estimated to last from 35 to 40 

 years.* Major Hodges, in his work on- Lock Gates before alluded to, after quoting exam- 

 ples of iron gates constructed between 1850 and 1870, and still in use in 1892, gives 

 the average duration of a metal leaf as forty years, when properly cared for and repaired. 



The greater durability of wooden gates in Europe, as compared with those in 

 America, is a factor largely in favor of their use, and metal gates, where exposed to salt 

 water, do not appear to last much longer than those of wood. For such positions 

 greenheart timber is frequently used, as it withstands the ravages of the teredo better 

 than any other kind, although it is said to be subject to their attacks in tropical waters. 



As the lock gates are so vital a point in a system of navigation, it is unwise to allow 

 them to deteriorate until they become weak. We are acquainted with a case, the river in 

 question being at the time under the control of a private company, where a set of lock 

 gates were allowed to go without repairs until a portion of one of the lower leaves gave 

 way. A raft was passing out of the lock at the time, and was caught in the result- 

 ing current, and before it could be checked struck the lower gates. The timbers were so 

 decayed that the shock broke several more of them, increasing the current till in a few 

 moments all the four leaves were carried out, and the river was pouring through the 

 chamber. As it was the time of low water, the damage was not as serious as might 

 have been anticipated, but it was many weeks before navigation could be resumed. 



With fixed dams of high lift it is often best to open the lower gates just before 

 a flood drowns the lock, and to fasten them securely in that position to the walls. 

 This may cause some deposit of sediment in the chamber, but if the gates are left 

 closed, the reaction from the dam may force them open and injure them seriously by 

 hammering against each other and against the walls. This has happened in several 

 cases. In one of them it had been the practice for several years to close the gates and 

 tie them together with a i|-inch iron rod at the top. During one rise some unusual 

 combination of forces occurred which tore the rod in two, thus setting loose the gates 

 and injuring them greatly. 



While this procedure will not always ward off damage, since examples are known 

 where lower gates, fastened open, have been torn down by extremely violent rises, it 

 appears to be a safer practice than that of leaving the gates closed. 



Care must always be taken to see that wooden gates are not too buoyant when 

 the water rises over them or they will float up at the toe and be liable to injure the top 

 masonry of the quoin, and possibly displace the pintle. Cast-iron weights of preferably 

 about 30 Ibs. each are usually employed to counteract this tendency, being placed on the 



* Papers read before the International Congress of Navigation, DUsseldorf, 1902. 



