FOREST SAW-MILLS. 683 



9. Bate 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 saw-mills 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. Bate 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 moderately-sized butts 

 70 — 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 — 0*50 m., and give on 

 the average 200 strokes in a minute. It should also be noted 

 that the more rapid the sawing the greater space should be left 

 between the teeth of the saw. 



11. Economical Working. 



The value of a saw-mill depends also on economical construc- 

 tion and labour. It is evident that simple forest saw-mills 

 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 compete with 

 large saw-mills which have more difficulty in securing cheap 

 raw material. As regards the quality of the planking, how- 



