490 AUXILIARY FOREST INDUSTRIES. 



9. Hate of Motion of the Carriage. 



The rate at which the butt-carriage moves towards the saw 

 must correspond with the rate of the saw and the depth of the 

 cut. The butt must not be too forward for the action of the 

 teeth ; in order, therefore, not to overtask the teeth, the butt 

 must advance less than the slope of the saw and size of the 

 teeth apparently permit. In most old sawmills the depth of 

 the cut is between 6 and 12 mm. ; in new ones between 30 

 and 36 mm. Instead of the old arrangement of the rack 

 and pinion feed, rollers are used, by means of which the work- 

 man has a far better control over the rate of progression of 

 the carriage. 



10. Rate of Sawing. 



The rate of sawing depends on the relation of the amount 

 of motive power available to the mechanism in use; the 

 degree of resistance offered by the wood and of friction by the 

 saw during the sawing ; also the amount of play of the saw, for 

 the greater this is for a given motive power the less rapid is 

 the rate of sawing. In old saws the play of the saw was often 

 0*60 to 0'80 meters, with a moderate water-power and moder- 

 ately-sized butts 70 to 120 strokes were given in a minute. 

 When a return was made to short blades and the play was 

 reduced there was an increase in the number of strokes per 

 minute. Superior saws of new construction have a play of 

 0*30 to 0'50 m., and give on the average 200 strokes in a 

 minute. It should be noted also, that the more rapid the 

 sawing the greater the space left between the teeth of 

 the saw. 



11. Economical Working. 



The value of a sawmill depends also on economical con- 

 struction and labour. It is evident that simple forest sawmills 

 driven by water-power, in which only a small capital is 

 invested and where owing to their situation in the forest 

 transport -charges are minimised, can work cheaply and com- 

 pete with large sawmills which have more difficulty in securing 

 cheap raw material. As regards the quality of the planking, 

 however, which depends on the best mechanism, as a rule, 



