MECHANICS AND USEFUL ARTS. 71 



In the fiist place, she is a diver, which cuts into the sea and rises 

 with difficulty from a bath, which covers her decks Avith water as 

 she pitches from sea to sea. Repeated immersions of this kind are 

 exceedhigly uncomfortable to those on board, and cause the shij^ 

 to lift some tons of water before her buoyancy is restored to meet 

 the next and every other succeeding wave into which she plunges 

 in a rolling sea. I think it is the duty of every ship-builder to 

 ajjproximate as closely as possible to the lines of least resistance, 

 which, in my opinion, ought to be carried to the utmost limits in 

 smooth water, but in smooth water only. It may not be out of 

 place to suggest that all passenger and emigrant ships should be 

 modified in their construction, so as to give increased displace- 

 ment at the bows and stern, but more particularly at the bows, 

 where they require bvioyancy, having to encounter the force of a 

 large body of water rushing over them and scouring the decks 

 from stem to stei-n. For several years I'have endeavored to im- 

 press upon the minds of naval architects the necessity of increased 

 strength on the upper deck of sea-going vessels, in order to bal- 

 ance the forces of tension and compression, and the double bottoms 

 on the cellular principle of construction. The ultimate strength 

 of a vessel is the resistance of its weakest part, and this being the 

 case, it is evident that it is of little or no value to have a strong 

 double bottom if the deck is liable to be torn asunder by the alter- 

 nate strains of a vessel pitching at sea. That these strains, often 

 repeated, lead to fracture does not admit of a doubt, and it has 

 been pi-oved by exi^eriment, that, under these circumstances, time 

 is the only element in the endurance of the structure ; and this 

 varies according to the intensity with which the strains are pro- 

 duced. I am convinced that heretofore the decks have been the 

 weakest parts, and that several iron vessels have broken right in 

 two from the ccMistant working of alternate strains at midships 

 along the line of the decks." 



HYDRAULIC-LIFT GRAVING-DOCK. 



Mr. Edwin Clark has described to the Institute of Ciyil Engi- 

 neers the plans adopted by him at the Yictoria (London) Graving- 

 Docks. The principle of these docks is to provide a single lifting 

 pit, out of which the vessels may be raised bodily on pontoons, 

 which afterward float them in shallow water to a convenient berth 

 for graving purposes. The shijos are raised by hydraulic presses, 

 the idea of which appears to have Iseen derived from the presses 

 employed in raising the Britannia and Conway Tubular Bridges, 

 designed under Mr. Clark's superintendence. At the Victoria 

 Docks, the depth of water in the lift-pit is twenty-seven feet ; that 

 over the rest of the dock is only six feet. In raising a vessel, one 

 of the pontoons is brought over the lift-pit, filled with water, and 

 sunk. The vessel is then floated in over the pontoon, and the 

 pontoon and .A^essel raised together by the hydraulic presses. 

 When at a sufficient altitude, the water is drawn oil from the pon- 

 toon, whic-h then, floats tlie vessel to its bertli in tlie shallow water. 

 The whole operation of lifting occupies only about half an hour. 



