150 DISCUSSION 



Now maximum shear per sq. in. = 2000 lbs. = x % x (ii). 



substituting for IF in (ii). 

 Maximum shear per sq. 

 2000 K>s. per sq. in. : and f -j = ^~ 



Maximum shear per sq. in. = # x l —^~ x I- 



x X = jy 



Therefore, in a rectangular beam of ironbark with an evenly dis- 

 tributed load, the resistance to shearing will be equal to the resist- 

 ance to direct stresses when the ratio of depth to span is one-ninth. 

 If the ratio be less than this the resistance to direct stresses wMl 

 be greater than the resistance to shearing, and vice versa. 



Walter Shellshear, Assoc m. inst. c.e. — The thanks not only 

 of this Society but of the public of the whole of the Australian 

 Colonies are due to Professor Warren for his instructive and 

 interesting papers on Australian Timbers and their uses, and in 

 this special line of investigation his labours may be considered 

 unique for by a study of his works on this subject the physical 

 properties of these valuable natural products are most clearly set 

 forth. Having had some little experience in timber structures of 

 various ages it may be of interest to point out a few things that 

 are necessary to be observed if this material is to be used to the 

 best advantage. In this colony we have two great enemies to 

 the life of timber structures, the first is white ant, and the second 

 dry rot, and in tracing the working of these damaging agents some 

 light may be brought out as to the best means of guarding against 

 them. White ants as a rule, attack bridge work at the abutment 

 end where the timber is in contact with the more or less moist 

 earth, and gradually work through the heart of the timber making 

 large pipes in the girders, thus weakening the girders and eventu- 

 ally completing their destruction. They also attack the piles at 

 about ground level, finding access through sun cracks, and once 

 inside the pile, usually confine their action to the heart of the timber, 

 extending their workings only a few feet below ground level but 

 rising upwards to the top of the pile and if left to themselves will 

 gradually eat the pile away until there is nothing but a mere shell. 

 They are also very liable to attack the deck of a bridge when 

 ballasted, the ballast protecting them from the light and affording 

 the necessary moisture for their existance. The white ants are 

 rarely found in timber that is isolated from moist earth, and good 

 ventilation is one of the greatest safeguards against them, conse- 

 quently for railway bridges what is known as the open topped bridge 

 has very great advantages over the close topped bridge and 

 ballasted road, and at the abutment end it would be advisable to 

 use small brick piers with the earth tipped round them instead of 

 the ordinary timber abutment. For protecting the piles it would 

 be advisable to slightly char them and give them a good coat of 



