TIMBER-SLIDES. 335 



here and there aloiif^ the edges of the sHde to prevent the logs 

 from leaving it. 



In the Black Forest wet sloping meadows are used for this 

 purpose, the line of the slide being bounded by logs. In the 

 Alps the method of sliding along the ground often alternates 

 with timber-chutes. Ground-slides are used for the transport of 

 logs only. 



A ground-slide serves its purpose only when its base and walls 

 are sufficiently firm and smooth ; all stones, roots, &c., must 

 therefore be removed, intervening rocks blasted, the way 

 improved here and there by laying down transverse pieces of 

 wood, and in the more difficult places which have to be traversed 

 short wooden slides constructed to complete the work. 



It is evident that ground-slides cannot be maintained in work- 

 able condition for any prolonged length of time. If they have 

 no rocky subsoil they are soon torn-up by drainage water, and 

 may become buried in silt, gravel, and other debris. 



Sometimes a wire rope is fastened to the logs whilst they 

 descend a ground-slide. A rope is coiled round a windlass at 

 the top of the slide so that as a log goes down attached to one 

 end of the rope, the other end being wound round the windlass 

 ready to be fixed to another log as soon as the former has 

 reached its destination : three or more logs are often fastened 

 one behind the other, and go down together. The windlass 

 works with a simple break arrangement. The logs may also be 

 placed in trucks and these let down a tramway by the rope. 



Although ground-slides should possess steep gradients, yet if 

 they are used when covered with snow or frozen, the gradient 

 need not exceed 20 to 25 per cent., especially when they are well 

 constructed, and bounded by logs placed laterally, for in such 

 cases descending logs soon attain a very high velocity. 



3. 1 load-slides. 



In some valleys leading from the Black Forest, especially those 

 of the Wolf and Kinzig, regularly constructed roadways are used 

 for sliding logs and sledging, as shown in fig. 177. 



It has been already laid-down on p. 316 that roads when 

 used as slides should have gradients of 9 to 18 per cent., and 



