TIMBER USED ON, Oil IN, THE GROUND. 121 



sleepers have not proved durable, is due to their being taken 

 from the worst class of oak timber, which is frequently unsound. 

 As regards durability, much depends on the ballast, and the 

 nature of the soil and climate. If these factors are favourable, 

 they may allow an uninjected sleeper made of wood of otherwise 

 little durability to last for a long time. 



Attempts to replace wooden sleepers by stone ones were 

 quickly abandoned on account of the unsuitability of the latter. 

 Iron and steel sleepers are now, however, largely used, chiefly in 

 the trough-shaped form, the old "pot" form not having been 

 satisfactory. This substitution of iron for wood is due to the 

 large quantity of iron available and to its great durability. The 

 chief objection to the iron sleepers consists in the change in the 

 molecules of iron, due to the action of the traffic, which renders 

 the metal brittle. Another objection lies in the great cost of iron 

 sleepers. [In India the saline nature of the soil is often preju- 

 dicial to metal sleepers. — Tk.] On the whole, wooden sleepers 

 are preferable if they can be procured. It is, therefore, the duty 

 of the forester to produce as many oak sleepers as possible, and 

 to favour their impregnation, if the field is not to be abandoned 

 to iron. Attention in Germany should also be directed to im- 

 pregnating good young beechwood, of which only 1 per cent, 

 of the sleepers are at present made, although such sleepers are 

 largely used in France. 



6. Wood iised in Forts. 

 Pallisades in fortresses are made of all kinds of wood, chiefly 

 coniferous. Platforms for guns and other parts of forts are 

 made of all kinds of wood, chiefly oak and Scotch pine. 



7. Mining Timber. 

 In spite of the large use of iron in supporting mine-galleries, 

 large quantities of wood are also used for this purpose, as well as 

 for lining shafts in pumping-works, &c. Wood used in mines is 

 exposed to damp air, damp and frequently wet soil, and, in 

 the deeper mines, to a constant degree of comparatively 

 high temperature. Every circumstance therefore tends to 

 favour the decomposition of the wood, and it seldom lasts 



