HISTORY OF CIVIL ENGINEERING IN NEW SOUTH WALES. 613 



position. There are three internal cylinders or shafts to each caisson, 

 eight feet in diameter, splayed out at botttom to meet the outer 

 shell and tlie cutting edges of the cross walls. These shafts are 

 used to excavate the materials from the inside by means of grab 

 dredges, and the spaces between them and the outer shell are used 

 to load the cutting edges. The depth of No. 1 pier is one hundred 

 and lifty-one feet below base of rail ; No. 2 pier, one hundred 

 and ninety-nine feet six inches ; No. 3 pier, one hundred and 

 eighty-nine feet six inches ; No. 4 pier, one hundred and ninety- 

 four feet six inches; No. 5 pier, one hundred and ninety-six feet; 

 No. 6 piei', two hundred and fifteen feet six inches. 



This important bridge represents in its design the combined 

 experience of the most eminent engineers both in England and 

 America. The steel was manufactured by Messrs. William Arrol, of 

 Glasgow, and the Steel Company of Scotland. All the riveted work 

 has been manufactured in Scotland, the eye-bars only by the Union 

 Bridge Company out of blanks prepared in Scotland. The steel 

 is specified to stand a tensile strength between thirty and thirty- 

 three tons per square inch, with an ultimate elongation of twenty 

 per cent, in a length of eight inches. An eye-bar tested to 

 destruction gave the following results: — 66,44.5 pounds ultimate 

 tensile strength per square inch ; contraction of ai-ea at fracture, 

 51.05 per cent. ; elastic limit, 36.063 pounds per square inch ; 

 elongation in 12 inches, 27| per cent. ; elongation in 8 feet 20.76 

 per cent. 



There are three tunnels between the Hawkesbury Bridge 

 and the junction with the line from Newcastle, the longest of 

 which occurs at forty-one miles fifty chains from Sydney, and is 

 one thousand eight hundred and forty-eight yards long. The 

 remaining tunnels are one hundred and ten and one hundred and 

 seventy-six yards long respectively. They are similar in design 

 and construction to those already referred to in this line of rail- 

 way. 



There are five wrought-iron plate web girder bridges over 

 the Orimbah, Wyong, Woy Woy, and Wallara Creeks and the 

 Broadwatex'. Each bridge consists of three openings sixty feet in 

 the clear, formed with two main girders one hundred and ninety- 

 eight feet long over all and seven feet deep, spaced twenty-five 

 feet six inches centre to centre ; each girder continuous over two 

 piers. The top flanges consist of horizontal plates eighteen inches 

 wide by nine-sixteenths of an inch thick, with two angle-iron 

 stirteners each four inches by four inches by half an inch. The 

 bottom flanges consist of horizontal plates eighteen inches wide by 

 five-eighths of an inch thick. The angle irons uniting the flanges 

 to the web are four inches by four inches by half an inch. The 

 cross girders are two feet nine inches deep. The piers consist of 

 cylinders six feet in diameter. The bridge over Cockle Creek 



