a4 
CVI. DISCUSSION. 
however, as this question of the relative economy of Australian 
hardwood and metal structures had been so exhaustively dealt 
with by him in his paper on ‘Timber Bridge Construction in 
New South Wales,” he (Mr. Allan) considered it hardly necessary 
to again touch upon the question in connection with the side spans 
of these two structures. 
In reply to Mr. Haycroft, he said that several bascule or end lift 
bridges had been erected in the Colony, of a cheap character. 
The leaf and towers were of timber, the necessarily varying 
counterweights being provided for by successively dropping 
sections of the weights on stops secured within the hollow 
towers, which were of the same height above top of pier as span 
of opening; the great height of tower with the pull on the top 
thereof, consequent upon the raising or lowering of leaf had 
resulted in the towers “canting” in spite of the tie rods anc 
ing the top of towers back to the side spans, clearly showing 
the necessity of a large based tower with this class of bridge, oF 
expense of which, taken in conjunction with the required rigid 
and costly pier foundations, would be less economical in construc- 
tion than the straight lift with its additional counterweight, but 
shorter and lighter towers carrying only vertical loads. 
Whilst he was fully alive to the advantages of the bascule for 
waters carrying masted vessels with small beam, yet Mr. Allan 
from his experience of opening bridges, was firmly wedded to 
the straight lift, Swan Hill type, with a certain and direct motion, 
for rivers where only a limited headway was required. 
Mr. Haycroft had referred to the Halstead Street Chicago Lift 
Bridge, the contract for which was placed in 1893, but it was not 
until the middle of 1895 that the number of the Proceedings of 
the American Society of Civil Engineers, containing the account 
of this bridge, first reached this Colony. Previously to this, pro 
visional protection had been granted to Mr. de Burgh for the 
arrangement of ropes for the Bourke Bridge, which although 
Somewhat similar, differed from the Chicago Bridge in that the 
Supporting ropes for the Bourke lift span passed from one corner 
