152 DISCUSSION. 



his paper, but with a small brick pier at the abutment end in place 

 of a timber abutment, the assumed depth of the piles below ©round 

 level is 15 feet. The water way in the case of the brick viaduct is 

 about 2,750 square feet, and in the case of the timber viaduct 

 3,750 square feet, giving an advantage of 1,000 feet extra water 

 way for the same height of rail. The following table gives the 

 relative first cost of these structures taking the prices at recent 

 contract rates : — - , , T 



Double Line. Single Line. 



Brick viaduct £6,800 ... £4,750 



Timber viaduct £2,225 ... £1,150 



For every additional foot in depth of foundations the cost of the 

 brick viaduct for double line will increase by about 10/-. per foot 

 run, and for a single line about 6/6 per foot run, and in the case 

 of the timber viaduct the cost for each additional foot of depth 

 for the piles will be 1/4 per foot run for a double line bridge, and 

 8d. per foot run for a single line bridge. Therefore in the case of 

 it being necessary for the foundations to be taken down to a con- 

 siderable depth the difference in cost between the brick and timber 

 structures w T ill be still more marked than in the above table. The 

 proportional first cost of the structures may be taken approxim- 

 ately as follows : — For a double line, timber 1, brick 3 ; for a 

 single line, timber 1, brick 4. Therefore considering this case in 

 the light of an investment at compound interest in the same way 

 as explained by Mr. J. A. McDonald m.i.c.e., &c, in his evidence 

 on the Cowra Bridge before the Public Works Committee we get 

 the following result : — Taking life of timber bridge at 25 years, 

 and brick bridge at infinity. Interest at 4 per cent, sinking fund 

 for timber bridge 2 - 4 per cent. For every £100 invested in 

 double line timber bridge we have £300 invested in double line 

 brick bridge, and for every £100 invested in single line timber 

 bridge we have £400 invested in single line brick bridge. There- 

 fore the annual charges are as follows : — double line timber bridge 

 per £100 invested, charges £6 8s., corresponding brick bridge 

 £300 invested, charges £12 ; annual saving for timber bridge £5 12. 

 Taking at end of 25 years at compound interest for timber bridge 

 per £100 invested £218 16s. Single line timber bridge per £100 

 invested £6 8s., corresponding brick bridge £400 invested, £16 ; 

 annual saving for timber bridge £9 12. Saving at end of 25 years 

 at compound interest for timber bridge per £100 invested £375 4s. 

 Or in the case of the bridges above referred to by the adoption 

 of timber a saving of no less than £4,868 6s. for the double line 

 bridge, and £4,314 16s. for the single line bridge would be effected 

 at the end of 25 years. For the class of viaduct under consider- 

 ation the cost of iron and brick are about the same, but for a given 

 height of rail the water way is considerably greater in the case of 

 iron superstructures, and the same care is not required with the 



