20(5 THE BOOK OF THE LANDED ESTATE. 



or as chain-bridges. The best form adapted for general estate purposes 

 is to have the body of the bridge of iron and the abutments built up 

 with stone, or what is termed girder-bridges. The following design will 

 give an idea of this kind of bridge, fig. 77. 



FIG. 77 



Bridges of this description are very ornamental on an estate, and 

 especially where there are carriage-drives. The abutments should be 

 constructed of large dressed stones, first securing a firm and solid 

 foundation. The abutments being built up to any desired height, the 

 bridge is fixed on to them. This kind of wrought-iron girder-bridge 

 is manufactured by Messrs A. & J. Main of Glasgow. A bridge of 

 the description given in the foregoing figure, and suitable for foot- 

 passengers only, will cost about 30s. per foot of span. Considerable 

 expense is also sometimes incurred in securing foundations fot the 

 abutments on each side, as on this depends the stability of the 

 bridge. 



Wooden bridges can be made of any form for small spans on farm or 

 plantation roads. The cheapest way is to have the abutments con- 

 structed of good solid masonry, and then to throw heavy beams across 

 to support the roadway. If the span be too wide for one length of beam, 

 a pillar or pier can be erected of stone in the centre. 



Many bridges are made entirely of wood, by driving strong wooden 

 supports into the ground at each embankment, having two, four, or more 

 of these, as may be thought necessary. Where timber is plentiful and 

 well matured, this kind of bridge lasts a long time; but for general 

 utility I prefer abutments built of stone. Several bridges of this de- 

 scription were erected in forming new roads through the forests of 

 Duthill and Abernethy, in Strathspey. They were made of the old 

 matured Scots pine. 



In the piling of wooden bridges, the piles are cut to the length 

 required either round or square; and in every case they should be 

 coated with a sheeting of zinc one-eighth of an inch thick, up to the 



