TIMBER BRIDGE CONSTRUCTION IN N.S.W. XIII. 
weather with a life approaching anything like, amongst others, 
the following :— a 
Bridge over Cox’s River, at Glenroy ............ 28 years old. 
Bridge over Bell River 32 years old. 
Bridge over Macquarie River, at Dubbo......... 32 years old. 
Bridge over Murrumbidgee River, at Wagga ...33 years old. 
Bridge over Duck River, at Parramatta ..... ...34 years old. 
Bridge over Cudgegong River, at Rylstone...... 34 years old. 
Bridge at Berrima 35 years old. 
Bridge over Murray River, at Albury............ 36 years old. 
The first metal bridge in the world, is said to have been erected 
over the Severn at Colebrook Dale in 1779, this bridge consists 
of a cast-iron arch of 100 feet span, the structure being thus 116 
years old. 
Wrought-iron and steel bridges are however of a much later date, 
and the fixing of the life of such structures, is to a great extent 
only a matter of conjecture; in connection with this matter 
Waddell says, “There is no reason why a well-designed iron 
highway bridge—when properly cared for—should , not last for 
ever.” ‘Under loads which are light and slowly moving com- 
. pared with those of railway bridges, the iron cannot possibly wear . 
out, and when properly protected from the weather cannot rust.” 
The author therefore to make everything favorable to the metal 
bridge in a comparison of its cost, with that of a timber bridge, 
has assumed the life of the former as infinity ; whilst the timber 
__ bridge has been taken at only 25 years—a life—it is only reason- 
_ able to expect will be much exceeded, with the better type of 
Structure, the more careful inspection of timber, and the greater 
care in erection that now obtains. 
L The whole of the timber for the Wagga Wagga bridge was 
3 brought from the Northern Rivers to Darling Harbour, about 
150 miles by sea, thence by railway to Wagga Wagga 310 miles, 
ra total carriage of 460 miles. 
_ The contract cost for one timber truss span at Wagga Wagga 
| is £1,300 and the estimated cost of one span of the iron lattice 
