TIMBER-SLIDES. 



325 



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 he the case in hot countries 

 with scanty rainfall. 



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

 ing a frost, so that it becomes coated internally with ice ; 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 gradient will be 

 affected also by the si/<> of the pieces brought down, so that 

 there are slides for firewood, logs, or scantling such us railway - 

 sleepers, and in the Alps, for billets two to three meters long 

 used for charcoal. 



Slides intended for bringing down logs and butts must have 

 lower gradients than those used for firewood, as the former 

 pieces attain a much greater velocity than the lighter pieces of 

 firewood. 



The following gradients are usual: 



Fiir. 203. California!! wet slide. 



In the case of dry slides, as already stated, the degree of 

 dampness of the air, and the nature of the atmospheric 



