240 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



The gates are fitted with forged-steel rollers carried in u 

 frame, having a bearing under the lower timbers, and supported 

 behind by a spear terminating with a screw adjustment. Each 

 roller carries a scraper fitting close down to the path. 



The roller paths are laid to a radius of 43 feet, and are con- 

 structed of wrought-iron plates on edge with a steel face on 

 the top. 



To prevent the gates from floating, a cast-steel holding-down 

 bracket is fitted and fastened to the top of each heel-post. 



The San Fernando Dock, Buenos Ayres. 1 These gates, designed 

 by Mr. D. Macalister, A.M.I.C.E., have some novel and interesting 

 features of detail, i.e. in the material used in the sill, and the 

 method adopted in securing a water-tight joint at the mitre- 

 posts. 



The width of the entrance is 64 feet, and the depth from 

 coping to sill 18 feet 2 inches ; the normal depth of water on 

 the sill is 13 feet, but the rise and fall is very variable, being 

 more dependent on the winds than the tides. 



The conditions to be met were the gates to be capable of 

 excluding the water to coping level, and of working under these 

 conditions, and also with only 5 feet of water on the sill, the 

 ordinary low-water level. Owing to the absence of suitable 

 stone and the expense of skilled labour, and also to the rapid 

 decay of timber, it was decided to make the meeting-face of the 

 dock sill of cast iron (Figs. 231, 232). 



The gates (Figs. 230 to 232) were constructed of wrought 

 iron with timber meeting-faces and mitre-posts. The lower 

 portions consist of tanks of such capacity that when the water 

 is at its normal level the gates are almost in a state of flotation, 

 and they can then be opened and shut with ease. But as the 

 gates have to be opened with only 5 feet of water on the sill 

 the displacement is then considerably less, and consequently 

 rollers had to be provided to take the excess weight. Each gate 

 consists of four horizontal girders connected by the vertical box- 

 girders or manhole trunks (or perhaps the water-tight tanks 

 may be regarded as one girder), and the horizontal strains are 

 transmitted from the four adjustable thrust-blocks to the thrust- 

 blocks built in the masonry. 



Any one who has had to do with gates knows the difficulty 

 of getting a water-tight joint at the hollow quoins, not to 

 1 Engineering, vol. xliv. p. 510. 



