The following Table gives :i summary of the results obtained for 

 T)ouglas Fir : 



BEAM. 



Dimensions in inches- . 



O 



IS 



i<~ 



"~ 

 = - 



'~ a 



u 



NEW TlMHKIi. SPLCIAL1.Y SH. EtTED. 



\KW TIMBER, FIRST QUALITY. 



Ol.ll TlMIIER. 



The following tlafri may be adopted in practice : 



In the case of specially selected timber, free from knot-, with sound 

 clear and straight grain, and cut out of the log at a distance from the 

 heart: 



Average Wright in Ihs. per cubic foot = 40. 



Average co-efficient of elasticity in Ihs. per sij. in. = 2,000,000. 



Average maximum skin stress in Ibs. per square inch = 9000. 



Safe working i-kin stress in Ibs. per square inch = 3000-lbs. 



In the case of first quality timber, such as is ordinarily found in the 

 market : 



Average weight in Ibs. per cubic font = 34. 



Average co-efficient of elasticity in ibs. per square inch = 1,430,000. 



Average maximum skin stress in Ibs. per square inch = GOOO. 



Safe working skin stress in Ibs. per square inch =: 2000. 



In specifying these data it will be observed that 3 is adopted as the 

 factor of safety. Upon this hypothesis the factor of safety for the stick 

 giving the minimum skin stress in more than 2, and this, in the opinion 

 ef the author, is an ample factor for a material which experience und 

 all experiments show, may be strained without danger very nearly up 

 to the point of fracture. 



Further, the results obtained iu the experiments with the old 

 stringers shew that th ; strength of the timber had been retained to a 

 very large extent, and that the rotting had not extended to such a depth 

 below the skin as to sensibly affect the efficiency of the sticks, which 

 still possessed ample strength for the work they were designed to do. 



Thus in It-jam XXII u diminution in the skin stri ss of 1058 Ibs. per 

 square inch, which is equivalent to a diminution in the effective depth 

 of "^ S 7 ^ 05S = 1.076-ins. would still leave 0000 Ibs. per square inch 

 as the skin stress. Thus if the rotting had extended to depth of 

 1.176 ins., the I'act-ir of safety would still remain 3. 



18 



