STRENGTH OE WOOD 



117 



W^ , , . 



formula /=|.z-^.. If §=the deflection at the centre in bending m 



inches, the coefficient of elasticity (E) in tons per square inch is 



■TTTTfg 



obtained from the formula E=i^7T^. Sir John Anderson has re- 



duced the results of many experimenters to a simple comparative 

 table of mean breaking weight for beams 1 foot long and 1 inch 

 square in timbers employed in England, which, with some sKght 

 modifications, is as follows : — 



Ash (Frdxinus exclhior), 



Beech (Fdgtis sylvdtica), - - - - 



Elm ( tflmus cavvpestris), - - - 



Larch {Ldrix e%ro^da), - - - - 



Memel Fir {Firms sijhestris), 



Riga Fir ,, ,, - - - 



Scots Fir ,, j> - - " 



Christiana Fir ,, ,, 



American Eed Pine ,, 



,, White Spruce - - - * 

 Oak, English {Quercus Rdlur), 



,, Dantzic, - . - . . 



,, Adriatic, 



,, Canadian _ - _ - - 



,, or Teak, African [Oldfieldia africdTia), 

 Mahogany {Swietinia Mahdgoni\ - 



TQd^{Tect6na grandis), - - - - 



690 lbs. 



625 



405 



440 



561 



457 



381 



574 



501 



570 



591 



513 



460 



580 



855 



631 



814 



The ultimate strength of a material is that stress which is required 

 to produce rupture, and this may be either tensile stress or that 

 exerted longitudinally or parallel to the axis of a beam, crushing 

 stress, or resistance to compression in the direction of the fibres, 

 or shearing stress, i.e. tangential. '• r " 



Professor XJnwin figures details of various instruments employed 

 for testing timbers, more especially for tensile strength, including 

 Bauschinger's roller and mirror extensometer, and several shackles 

 for holding the test-specimens. The principle of most modern in- 

 struments for these purposes is the same, the weight being applied 

 gradually, either by small weights or by hydrauHc action, to a 

 system of levers, the force exerted being shown by a deUcately 

 adjusted steelyard. Thus the comparatively simple instrument of 

 American design, introduced at Woolwich in 1854 by Sir John 

 Anderson, and figured in his work,^ consists of a combination of two 

 levers which together give a purchase of 200 to 1, that is to say, 1 lb. 

 applied to the end of the long arm of the upper lever will exert a 

 stress of 200 lbs. on the specimen attached by shackles to the 

 lower one. 



^ The Strength of Materials and Structures, London, 1872, p. 16. 



