3io NOTES ON DOCKS AND DOCK CONSTRUCTION. 



the centre, although the load is distributed by the pontoon 

 and the wide base used for the blocks. The water is forced 

 into the presses immediately beneath the collars at the top, 

 this being an accessible position. 



The grouping of the presses was an important consideration. 

 If each press were worked entirely independent of its neigh- 

 bours, it is evident that precisely the same quantity of water 

 must be thrown into each press to avoid unequal strain. Again, 

 if the whole number were supplied from a common head, the 

 slightest excess of weight at any part of the platform or gridiron 

 would lower that part, the water passing back through the 

 pipes to the presses, where less pressure existed. The same 

 difficulty would be experienced with two groups, however 

 arranged. Stability is, however, secured by arranging the 

 presses in three groups, the whole number occupying both sides 

 of one half the length of the lift from one group consisting 

 of sixteen presses. The remaining eight presses on one side 

 form a second group, and the opposite eight form the third 

 group. 



" The presses in each group are all connected, so that perfect 

 uniformity of pressure is secured in each as regards the indi- 

 vidual presses, while the three groups are so arranged that their 

 centres of action form a tripod, and may be raised or lowered 

 without regard to the other two, by the most simple manipula- 

 tion. The pontoon can be either maintained perfectly level, or 

 any inclination can be given to it that may be desired. 



" Any pair of presses may be instantly cut off in the valve- 

 room by means of a plug, during the operation of lifting, without 

 interrupting the process. No delay, therefore, arises from the 

 failure of a collar or pipe, and even should a press burst, which 

 appears an impossible contingency, the water can only escape 

 slowly through the half- inch pipe which feeds it, and by opening 

 the escape-valves in the other groups, vessels partially raised 

 descend slowly and steadily into the water. 



"The force-pumps are 1J inch in diameter. There are twelve 

 pumps worked by direct action by a 50 horse-power engine; 

 six of these pumps are used for the large group, and three 

 pumps for each of the smaller groups. The power, when re- 

 quired, is increased by cutting off one or more of the pumps. 



' The raising of a vessel occupies about twenty-five minutes, 

 During the operation, the engine continues to pump, and the 



