SAWMILLS. 315 



In vertical frame-saws the number of strokes in old mills varied 

 from about 70 to 120 per minute, while in new mills it may 

 average about 180 to 200 strokes per minute. Circular saws 

 of medium size can be made to travel with a comparatively 

 low motive-power at from 50 to 70 ft. per second for hardwoods 

 and from 50 to 100 ft. per second for softwoods, according to 

 the driving-power. Band-saws require less driving-power, and 

 waste less in sawdust. With circular saws, the greater the 

 speed the more is the waste in sawdust. The number of 

 revolutions per minute can be easily calculated from the speed 

 of the saw at its circumference, and usually is 400 to 500 for 

 a 48 in. saw. Saws of a smaller diameter and proportionately 

 thinner require less driving - power than larger and thicker 

 saws; and the greater the diameter, the less should be the 

 driving-speed. A good cross-cut saw should be 3J ft. in 

 diameter and number 1 2 gauge, while the ripping - saws can 

 be 36 in. diameter and 14 to 16 gauge. Saws should be of the 

 very best cast-steel, and varying in thickness according to the 

 work to be done ; but the thicker the saw, the greater the 

 waste in sawdust, which may amount to about 25 to 35 per cent 

 of the total volume of wood (and which was intended to be 

 covered by the square-of-quarter-girth measurement of logs). 



There are no absolute standard sizes for converted timber, 

 for the dimensions most in demand vary locally. 



Logs may be rough-hewn with axes, or sawn on all four sides 

 to form squares or balks. The strongest beam obtainable from 

 a log is when the sides are cut in the proportion of 1 : v'2, or 

 5:7, as this ratio gives the maximum product of Ireadth 

 x height. A log or balk halved lengthways forms two 

 half-balks, and if divided lengthways into four equal pieces is 

 quartered, and forms four quarter-balks; and when sawn to 

 smaller dimensions, but in the full length of the log or log- 

 section, timber is known by the trade terms of scantlings, 

 planks, deals, and battens, which are always obtainable of more 



