186 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



intermediate longitudinal girders, which were joined up to the 

 transverse girders. The top plates were carried by rolled-iron 

 joists, the whole being tied to the girders and side walls by 

 brackets and gusset-plates. 



The sinking of the caisson after it had been brought into posi- 

 tion was effected by the masonry of the dock, which was executed 

 in such a way as to distribute the weight as equally as possible 

 over the whole surface, and also so as to disturb as little as 

 possible the balance between the load and the pressure of the 

 water. 



In order to obviate any difficulties arising from expansion 

 and contraction, the sides and bottom were enclosed with 

 masonry as quickly as possible. 



In order to detect and provide against any signs of weakness, 

 arrangements were made to observe and record the behaviour of 

 the caisson as the work progressed. 



When the masonry was so far completed that an immersion 

 of 60 feet was reached, and the bottom of the caisson near the 

 ground, the lower chambers were filled with compressed air, and 

 from these chambers the bed which had been partially prepared 

 by dredging was completed. 



To displace the water, a set of twelve air compressors were 

 provided, each being capable of compressing about 22,000 cubic 

 feet of air per hour to a pressure equivalent to a 60-feet head of 

 water. The semi-liquid mud and debris was removed by means 

 of syphons and a small amount of compressed air, whilst stones 

 and solid material were passed through the air locks. 



After the bed for the caisson had been properly levelled, the 

 lower chambers were filled with concrete, which was passed 

 through two special air locks into each chamber. The concrete 

 was packed from the ends towards the centre for the whole 

 depth, the portion immediately in the centre was filled in direct 

 from the locks, which were then removed, leaving the shafts to 

 be filled up with concrete to the level of the masonry of the 

 invert. 



Timber Docks. 1 In the United States of America, timber has 

 been extensively used in the construction of dry docks, and it 

 appears to be much preferred to masonry, on account of the 

 nature of the North- American climate. 



The advantages claimed for this method of construction are 



1 Engineering, and Scientific American^ vul. Ixi. p. 341. 



