156 DISCUSSION. 



accordingly suggested the idea, without going into a calculation 

 such as the Professor has gone into now. I am by no means a 

 defender of it. I a^ree with him it ousjht to be ironbark. 



Professor Warren — I am glad Mr. Purge has explained the 

 statement made by him at the last meeting. The Union Bridge 

 Company, who designed and constructed the bridge, told me that 

 if they had known the value of our ironbark timber, they certainly 

 would have used it in preference to Oregon timber. The remarks 

 made by Mr. Trevor Jones imply that the shrinkage of the 

 wedges in a compound beam would reduce its strength and 

 stiffness, but the design illustrated in Plate 7 clearly anticipates 

 this shrinkage, and provides for the wedges being driven in to 

 fit tightly in the notches cut in the main beams to receive them. 

 It is only necessary for these wedges to fit tightly in the direction 

 of the length of the beams, as they have to resist the horizontal 

 shearing stresses only ; the vertical shearing stresses, which are 

 of equal intensity, will be resisted by the bolts. In consequence 

 of the shrinkage of the timber, there is always considerable 

 maintenance in timber bridges, more especially when unseasoned 

 timber has been used in their construction. This maintenance is 

 greater during the first 12 months than afterwards, but there is 

 obviously no more difficulty in driving in the wedges from time 

 to time than in screwing up the bolts, consequently the design 

 illustrated in Plate 7 would not be less efficient from the shrinkage 

 referred to. With regard to Mr. Jones' remarks on the strength 

 of circular beams, I explained fully how the strength of a beam 

 of any section might be determined, but the beams used in 

 Plate 2 are prepared from round logs of timber by adzing at top 

 and bottom, and may be calculated from the formulae given in 

 the paper by merely substituting the mean breadth for b. Mr. 

 Haycroft appears (as far as I can judge from his diagrams and 

 calculations) to have considered the stresses in the Cowra Bridge 

 just as if the diagonal members were rigidly connected to the 

 booms and were consequently capable of transmitting tensile as 

 well as compressive stresses. Of course it is not possible, in 

 listening to a description such as that given, to say precisely how 

 Mr. Haycroft has considered the question, but I understand that 

 he has dealt with the stresses just as he would have done in an 

 iron bridge with the diagonals rigidly attached by means of 

 rivets, and thus capable of transmitting tensile as well as com- 

 pressive stresses. Thus the diagonal marked Y s in Plate 13 is in 

 compression if the joints to the left are loaded, while it is in 

 tension if the joints to the right are loaded, and if the diagonals 

 were attached by means of rivets as in an iron bridge the 

 resultant stress on it would be tensile, as the tension produced 

 by loads to right would more than balance the compression pro- 



