TIMBER-SLIDES. 



831 



Fig. i; 



Water is brought into the sHcle whenever it passes any stream 

 or spring. In the Salzkammergut, planks are used in a similar 

 way to the Tihri-Garhwal slide. In California, hundreds of 

 miles of wet timber- slides have been constructed as shown in 

 fig. 175. 



(d) Gradient. — The amount of gradient is a most important 

 consideration in constructing slides. Too small a gradient 

 renders a slide useless, 

 and with too great a 

 gradient the wood will 

 leave the slide, and 

 great danger arises to 

 any person who may 

 be near at hand. The 

 permissible limits are 

 5 per cent, and 35 to 

 40 per cent., but the 

 way in which the slide is used, and the size of pieces of wood to 

 be brought down, aftect the question. 



Thus there are dry slides, ice-slides, and wet slides. 



In the case of dry slides, a steep gradient is necessary, which 

 may go up to 40 per cent, and more. 



[If, however, the gradient be very steep, the slide should be fairly 

 straight, as, otherwise, the shocks to which it is subjected by wood 

 coming down causes too much wear and tear. There is also always a 

 danger of fire from friction in dry slides ^\ith excessive gra- 

 dients. — Tr.] 



As a rule, however, dry slides become slippery owing to the 

 moist air, or may even contain a certain amount of snow, so that 

 in such cases a lower gradient will suffice than if the slide is 

 used when quite dry, as may be the case in hot countries, with 

 scanty rainfall. 



In the case of ice-slides, water is introduced into the slide 

 during a frost, so that it becomes internally coated with ice, and 

 a very slight gradient is then required. 



In wet slides, a thin stream of water is necessary, and should 

 be deeper the steeper the gradient. 



Besides depending on the manner in which a slide is used, the 



