698 bepobt— 1878. 



oblique angle of 25 degrees and, though the canal is only 15 feet wide, the- 

 bridge carrying the railway requires to be nearly 40 feet long on the skew. 



The trains run over this bridge at about two feet over ordinary water level, and 

 whenever a boat is passing along the canal the bridge is lifted from 8 to 1& 

 feet, according to the _ height of the deck load, so as to permit the boat to pass 

 beneath. The bridge is formed of two strong single-plate girders of the usual type, 

 which lie underneath the rails, with cross girders and side brackets over which the 

 platform is laid. This bridge is lifted by means of a lever 40 feet long, formed of two- 

 plate girders braced together horizontally, and attached rigidly at right angles to the 

 centre of the bridge, and this lever is itself balanced at its centre on blunt steel 

 knife edges like the beam of a pair of scales. The weight of the bridge at one end 

 of the lever is counterpoised by an equal weight of metal attached to the other end, 

 so that the whole structure turns freely on the knife edges, which work in steel 

 pillow blocks on the top of metal standards, one on either side of the lever. The 

 opening and closing motions are regulated by a small crab-winch and chain worked 

 by hand ; the ends of this chain are attached to the lever at several feet on either 

 side of the knife edges, and its centre is wound on or off from the barrel of the 

 winch, which is itself bolted down to a mass of concrete extending beneath the- 

 metal standards. 



The man in charge works this arrangement with the greatest ease, and it is so- 

 regulated that the bridge is opened or closed in about one minute. It might be 

 moved much faster than this, as the friction is reduced to a mere trifle by the knife 

 edges, but it is not convenient to put so large a mass in rapid motion when there is 

 nothing to be gained by so doing. It was essential that the bridge should be 

 erected speedily and so as to interrupt the traffic as little as possible, and the first 

 engine passed over it in about twelve weeks after the contractors, Messrs. Courtney, 

 Stephens and Bailey, of Dublin, got instructions to proceed with the work and the 

 traffic was interrupted for only about one week during erection. The lever sloping 

 upwards has a somewhat singular appearance when the bridge is in position for 

 trains to pass over and, on the other hand, the bridge itself has a singular effect 

 when it is tilted up into the air for canal boats to pass beneath ; but the author has 

 successfully obtained what he aimed at — namely, simplicity of design, strength, 

 ease of working, little aptitude to go out of order and last, but not least, very 

 moderate cost. 



9. The Irish Siren Fog Signal. By J. R. Wigham— See Section A, p. 437. 

 10. A New Form of Mining Lamp. By Dr. Charles Ball. 



