165 



h. Carbonization (Ravazffs process). 



The apparatus used consists of a tube 26 feet long, 2 feet in diameter, lined with 

 fire bricks, through which a flame is drawn by a chimney built at one end. Sleepers are- 

 drawn through the tube by means of an endless chain ; they take fire and give part of the 

 heat required in the process. The speed of the chain is regulated to allow the 

 sleepers to come out thoroughly carbonized, which takes about 2 minutes. The burning 

 sleepers are extinguished by a jet of water when they come out. 



A modification of this process consists in plunging the sleepers that come out 

 of the tube into a tank of coal tar heated at 170 to 190. The tar penetrates readily 

 into the heated wood, and each sleeper absorbs about 4-4 Ibs. during the two hours it is 

 left immersed ; 900 to 1,200 sleepers may be treated in a day, and the results are said to 

 be uniformly favourable. 



3. RAILWAY SLEEPEKS. 



Railway sleepers are placed in conditions very unfavourable for durability : they are 

 subjected to constant alternations of dryness and wetness which induce decay ; and they 

 have, besides, to bear heavy pressures that destroy the cellular tissue of soft woods, eveii 

 with the use of chairs, and also lateral pressures tending to loosen the spikes. 



To resist these influences, the wood of sleepers should possess the following quali- 

 ties : 



a. Sufficient resistance to compression, since the wood has to bear pressures of 250 

 to 300 Ibs. per square inch, under the rail, though less when chairs are used. 



b. The fibres should be close and the tissue homogeneous. 



c. Durability when exposed to alternate damp and dryness ; either in the natural 

 state or after impregnation. 



d. A moderate density otherwise the sleepers become umvieldly and difficult to 

 pack ;' sufficient toughness to resist splitting by the spikes ; and sufficient elasticity to 

 resist rupture when the sleepers are badly packed. 



The timber should be of good quality and free from such defects as cracks, rotten 

 spots, and weak knots. Heartwood that has become discoloured is considered inferior 

 wood. 



The trees from which sleepers are cut, should not be less than 12 inches in diameter, 

 but they may be of any larger size. Branchwood is also frequently used. 



In order to obtain sleepers at a moderate cost, many Continental railway companies 

 admit several classes. In the Report of the V. S. Commissioner of Agriculture for 1884, 

 are given the following data relative to the sleepers used by the Compagnie du Nord 

 (France) : 



Beech sleepers for gauge 4*9 feet ; length 8'5 feet ; consisting of : 



(a) Square slepers : breadth 9-5 inches ; depth 5'5 inches. 



(b) Other shapes of sleepers : 



4-3 in. (at least). 



6-7 to 9-0 in. 



87 to 11-8 in. 



8-7 to 11-8 in. 



8-7 to 11-8 in. 



8-7 to 11-8 in. 



(Only four per cent, of the above kinds are accepted which measure so low as 8'7 to 

 9-0 inches in width.) 



10-2 to 12-6 in. 



10-2 to 12-6 in. 



