STRENGTH OF WOOD. 135 



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 slight modifica- 

 tions, is as follows : 



Ash (Frdxinus exclesior), - - 690 Ihs. 



Beech (Fdgus sylvdtica), - " 625 ,, 



Elm ( Ulmus camp&stris), - 405 ,, 



Larch (Ldrix europda), - 440 ,, 



Memel Fir (Pinus sylvdstris), 561 ,, 



Riga Fir 457 



Scots Fir ,, 381 ,, 



Christiania Fir ,, ,, . . 574 jf 



American Red Pine, 501 ,, 



White Spruce, - - 570 



Oak, English (Quercus Rdbur), ~ . '_ - 591 ,, 



,, Dantzic, - - 513 ,, 



,, Adriatic, - - 460 



,, Canadian, ... . . 580 ,, 



or Teak, African (Oldfieldia africdna), 855 



Mahogany (Swiet6nia Mahdgoni), - - - - 531 ,, 



Teak (Tectdna grdndis), - 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. 



Professor Unwin 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 

 instruments for these purposes is the same, the weight being 

 applied gradually, either by small weights or by hydraulic action, 

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

 delicately adjusted steelyard. Thus the comparatively simple 

 instrument of American design, introduced at Woolwich in 1854 

 by Sir John Anderson, and figured in his work, 1 consists of a 

 combination of two levers which together give a purchase of 

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



