DISCUSSION. 151 



tar before driving. The best safeguard against dry rot is ventil- 

 ation and the open topped bridge may be considered nearly proof 

 against this trouble. As to the question of preserving the timber 

 by paint, tar or other agents : from observation it appears advis- 

 able to slightly char and tar the piles below ground level and for 

 a foot or two above, but on any part of the timber work exposed 

 to the action of the sun, tar has proved very detrimental to the 

 life of timber as it has the effect of destroying the nature of the 

 wood or opening out the fibre. The exposed ends of all timber 

 should be well painted with white lead as it prevents the timber 

 opening out ; and it might be advantageous to give the timber a 

 good coat of paint after the bridge had been erected about twelve 

 months, but the advantages of painting are not very great as it is 

 not the surface of the timber which deteriorates but the heart. 

 As to the life of Australian timber in bridges, it is difficult to say 

 how lonsr the best classes of timber would last under favourable 

 circumstances. The original timber bridges on the Western line 

 between Parramatta and Penrith which were opened for traffic in 

 1860-62, were renewed in iron in 1886. When the old structures 

 were removed more than half the timber (which was mostly iron 

 bark) was found to be in a very good state of preservation. The 

 original bridges between Campbelltown and Menangle which were 

 opened for traffic in 1862 are still in use and are in a fairly good 

 state of preservation, the only repairs and renewals to these struc- 

 tures have been a few new girders, a few deck timbers and piles, 

 the total renewals not amounting to more than about 10 per cent. 

 of the total amount of timber in the structures up to date. From 

 the above facts it may be safely calculated that the life of a timber 

 railway bridge built of the best Australian timber is at least 25 

 years. A question of very great importance in connection with 

 railway construction in these colonies is the commercial and 

 economical advantages of timber bridges of small spans as com- 

 pared with structures of brick and iron. On almost all lines it is 

 necessary to cross a large number of small creeks which in a dry 

 season have little or no water in them, but in wet weather may 

 extend to a width of many yards, necessitating the construction 

 of a large number of long shallow waterways ; the soil upon which 

 these waterways have to be constructed being often many feet in 

 depth before anything like good foundation ground is reached. To 

 illustrate this matter, the attached plan shows designs for an arched 

 bridge for double and single line, also a timber bridge for double 

 and single line over the same creek. In the case of the arches the 

 design is that in common use in the elevated portions of the 

 London Metropolitan Railways and the foundations are taken at 

 an average depth of 7' 0" from ground level. The timber viaduct is 

 of 12 feet spans similar to that proposed by Professor Warren in 



