226 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



point it passes over another pulley, and finally is secured to tin- 

 masonry at the place where, in ordinary practice, the roller-box 

 is fixed. The chain for closing follows a similar course on the 

 other side of the gate, and in like manner is made fast to the 

 masonry on the opposite side of the entrance. 



The advantages of this arrangement of gate machinery and 

 chains are, that two engines with double crabs do the work of 

 four with single crabs ; the usual chainways in the masonry 

 of the side walls, with the numerous roller-boxes for guiding 

 the chains, are entirely dispensed with ; and that each leaf is 

 worked both ways from the same side, so that in the case of 

 sea gates, which require to be held back against the action 

 of waves, the closing can be effected by one man, instead of two, 

 at each side of the opening. 



An objection to the chains crossing the entrance, as in the 

 above methods of manipulating the gates, is that the effective 



depth of water over the 

 sill is reduced. Further, 

 it is necessary to pay out 

 a large amount of slack 

 in the closing chains, so as 

 not only to allow vessels 

 to pass between the gates, 

 but also to admit of the 

 chains being pushed clear, 

 in the event of a vessel 

 fouling them. 



At the Barry Docks, 

 the gates are moved 

 by direct-acting hydraulic 

 rams (Fig. 213), the fol- 

 lowing description of 

 which is taken from Mr. 

 J. Robinson's paper on " The Barry Dock Works." l 



"In the case of the entrance-gate machinery, the water is 

 admitted to a direct-acting hydraulic cylinder, having a piston 

 2 feet 5| inches in diameter, and a 18-inch ram, with a stroke 

 25 feet 9 inches, attached to the gate. 



"The cast-steel cylinder is made in three parts, and t 

 to withstand a pressure of 3000 Ibs. on the square inch. 

 1 M.P.I.C.E., vol. ci. p. 14!). 



FIG. 213. 



