xCIl. PERCY ALLAN. 
The iron hollow towers at the four corners of the lift span 
let 3’ into cylinders and surrounded for this depth with concrete, 
are connected at the top by transverse and longitudinal girders, 
thus preventing the tops of towers approaching one another and 
jambing lift span when being raised. Four short galvanised 
link chains attached to the top boom at each end of main 
girders, pass over chain wheels placed at the top of each tower, 
and are then secured to cast-iron balance boxes inside the towers 
adjusted with lead filling ; chains are attached to the bottom of 
balance weights, pass under sprocket wheels at the foot of towers 
and are secured to bottom corner of each main girder, thus making 
practically an endless chain and leaving bridge balanced in any 
position, the weight to be lifted being only that due to friction. 
The bridge was designed to be operated by two men each 
working a winch placed on a platform on the downstream side of 
each pier, driving a transverse shaft to which are keyed the two 
sprocket wheels in the bottom of each pair of towers, whilst 
uniform lifting of ends of span is ensured by connecting the chain 
wheel at top of tower with bevel gearing. The ratio of gearing 
is sixteen and a-half revolutions of handle to one of chain wheel ; 
permitting of bridge being lifted to its maximum height of 
19’ by two men in four and a-half minutes, or at the rate of 4:22! 
per minute. 
In 1895 the Bourke bridge was altered so as to permit of 
one man instead of two, working the lift span. Contracts 
being subsequently let for similar alterations at Brewarrina and 
Balranald. The alterations consist in the substitution of wire 
ropes for the four suspending chains, the arrangement of the 
ropes being designed by Mr. de Burgh, M. tnst.c.E., and may be 
shortly described as follows :— 
From each corner of lift span a wire rope passes over and 
around a rope wheel at top of tower, thence across the span and 
over the rope wheel on the opposite tower, the end being then 
connected to balance weight, the ropes from each corner of lift 
_ Span thus crossing one another at centre of span. The bridge is 
