Lateral Stability of the Victoria-street Bridge. 285 



ceed that of the bridge in question. On the Goulbura 

 Valley railway, at Toolamba, is a very large timber bridge 

 crossing the Goulburn. The highest pier of this structure is 

 69 feet high, and 27 feet wide at the base. It consists of 

 redgum piles, driven through 7 feet of soft material, and 

 then resting on the bed rock. At first sight the Toolamba 

 pier seems much more stable than that at Victoria-street. 

 But calculation tells a different tale. The former structure 

 is composed of timber, a material which loses its weight en- 

 tirely when immersed in water, while the latter is composed 

 of iron and concrete, and will not lose more than one-third 

 of its weight under similar conditions. Allowing for this 

 circumstance, we find by a calculation, the details of which 

 need not be given, that the moment of stability of the Too- 

 lamba bridge when the river is at high flood is barely half that 

 of Victoria- street under similar conditions. As the Goulburn is 

 a larger, deeper, and swifter river than the Yarra, and as 

 the Toolamba bridge has already endured uninjured one very 

 heavy flood, in which the floating timber formed a complete 

 dam across the river, it follows that there are no grounds of 

 apprehension at Victoria-street; and even if there were, 

 additional cylinders on the down-stream side only would in- 

 crease the resistance threefoW, and render the bridge more 

 than double as strong as the somewhat similar structures at 

 Johnston-street, Collingwood, and Swan-street, Richmond. 

 Thus the proposed alteration is seen to be as unnecessary 

 from the flood as from the wind point of view. 



The Victoria-street bridge question derives its importance 

 from the fact that it is a point of contact and of conflict 

 between two opposing schools of thought on engineering sub- 

 jects. Those who belong to the old, or empirical, school, who 

 hold that mathematical investigation is "mere theory," and 

 that practice is the only guide, unanimously condemn it 

 because it departs from the proportions to which they have 

 been accustomed. Those who belong to the new and scien- 

 tific school, who hold that the principles of statics are really 

 and universally true, and constitute the essential basis of all 

 sound engineering practice, approve of it because they find 

 that its proportions throughout agree with the requirements 

 of exact mathematical calculation ; and the question before 

 us to-night is, which of these opposing views is correct. If 

 the recommendations of the two engineers who condemn the 

 bridge are well founded, it will then follow that the principles 

 of statics as laid down by all the authorities, and as taught 



