HYDRAULIC PJRESSES AND LIFTS.

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The ironworlc of fclie Britannia and Conway tubulär bridgeswas put together on the shore, and each tube was floated out onpontoons to a point which was vertically underneath its finalposition, when it was raised by means of hydraulic presses tothe top of the pier and abutment upon which it was to rest.After several proposals had beeil discussed, it was finally decidedthat each tube of the Conway bridge should be raised by a singlepress at each end, the weight to be raised being 630 tons. Thepresses were placed at the top of the abutment upon cast irongirders, which spanned recesses that were left in the masonryfor the ends of the tube to pass through. Upon the top of eachpress was a cast iron crosshead from which two Suspensionchains hung, the lower ends of which were made fast to theend of the tube. The internal diameter of the press was 20inches, the thickness was 8f inches, and the stroke was 6 feetThe diameter of the ran: was 18 inches, and it was cast hollow.It was guided vertically by two 6-inch wrought iron guide-rodsfitted in a socket at the top of the press, and keyed above intoa cast iron girder built in the masonry. The piston of the force-pump was 1 T V inches in diameter, and its stroke was 16 inches.The pumps were worked by a steam-engine, having a 17-inchcylinder with 16 inches stroke. To lift the press 6 feet, theengine made 1018 strokes, or 34 per minute.

The weight of one of the tubes of the Britannia Bridge wasconsiderably greater than that for the Conway Bridge, thetotal weight to be lifted being 2065 tons. As the originalpresses were not powerful enough for this load, another presswas made. The two original presses were placed at one end ofthe tube to lift 921 tons, and the new one was placed at theother end of the tube to lift 1144 tons. This last press had aram 20 inches in diameter, the cylinder having an internaldiameter of 2L inches, and a thickness of metal of 6 inches.The weight of this press was 13 tons.

Considerable difficulty was experienced in casting thesepresses, and in rendering them watertight under the pressure.When a press was found to leak, the iron was well hammered,