ANNIVERSARY ADDRESS. 15 



that a good sound rock bottom was obtainable round Darling 

 Island, at depths of from twenty-eight to thirty feet, it was 

 decided to use concrete. The first cost will be higher, but when 

 maintenance and reconstruction of a timber wharf in twenty-five 

 or thirty years are taken into account, a large saving will be 

 ultimately effected, besides which the design will admit of the use 

 of heavy travelling cranes along the frontage. The blocks are all 

 moulded on the island near the water's edge, the concrete being 

 mixed in the proportion of four cement, nine sand, twenty-four 

 broken sandstone, besides which large pieces of sandstone up to 

 five and six cwt. are embedded in the concrete. The size of the 

 blocks runs from seven feet by six feet by six feet to twelve and 

 a quarter feet by six feet by six feet, and they weigh from fifteen 

 to twenty-eight tons. Two holes for introducing lifting bars are 

 moulded in each block. A steam derrick crane placed on a punt, 

 capable of lifting about forty tons, is then used for handling the 

 blocks. 



At Jervis Bay a new lighthouse and full set of keeper's quarters 

 are now being built at Point Perpendicular, the material used 

 throughout for the walls being concrete moulded into blocks each 

 to the exact shape required, and afterwards set in position. 



Road Bridges. — Among the numerous contracts let during 1897 

 by the Bridges Branch, under charge of Mr. Darley, the most 

 important was that for the timber-truss bridge over the Macleay 

 River at Kempsey. This consists of four one hundred and fifty 

 feet timber trussed spans on cylinder piers, with timber approaches, 

 the contract cost being £18,300. The deck is twenty-two feet 

 six inches wide between kerbs. This bridge is interesting for 

 having the largest spans and being altogether by far the largest 

 timber bridge in the Australian colonies. 



During the year, the strength of the tension joint now employed 

 in railway and road timber truss bridges in this colony has been 

 tested at Cockatoo Dock, in a machine specially designed for this 

 work by Mr. 0. W. Darley, capable of exerting a pull of two 

 hundred and twenty tons. The results of the tests made of full 



