HOW FAILURE UNDER STRESS OCCURS IN TIMBER. 439 
Fig. 11. Oak wood. A general view of a tangential section of fractured wood, x 20 diams., showing 
that the fracture takes place more readily at the medullary rays (m) than at the apparently weaker 
trachee (f). 
Fig. 12. Oak wood. A tangential section from a partially crushed block, x 176 diams., showing that 
the fracture is caused by the wood-fibres separating from the medullary rays (m). 
Puate III, 
Fig. 13. Ash. A normal transverse section, x 240 diams. It shows portions of trachee (¢), medullary 
rays (mm), wood-fibres (f), etc. Tracheides shown only about one-third the size of those of Oak. 
Fig. 14. Box. A normal transverse section, x 240 diams. It shows trachez (¢), medullary rays (m), 
wood-fibres (f), ete. The relative size of the trachee in these three different woods may now be 
compared. 
Fig. 15. Ash. A normal tangential section, x 240 diams., showing medullary rays (m) and wood- 
fibres (f). The sinuous path of the wood-fibres making their way past the medullary rays is well 
shown, : 
Fig. 16. Ash. A tangential section, x 176 diams., of a fractured specimen. It shows early stage in the 
erumpling of the fibres and their splitting from the medullary rays. 
Fig. 17. Box. A normal tangential section, x 240 diams. It shows trachez (¢), medullary rays (m), 
wood-fibres (f), ete. Most of the medullary ray cells are filled with starch. 
Fig. 18. Box. A tangential section, x 176 diams., of specimen that has been compressed. It also 
shows crumpling of the fibres and their splitting from the medullary rays. 
Puate LY. 
Fig. 19. Oak. A set of strips, x % ns., that have been fractured by tension. (a) exhibiting its 
narrow side, and (0) exhibiting its broad side, were both cut with their broad sides tangential ; (c) and (d), 
similarly shown, were cut with their broad sides radial. (5) and (c) therefore show a tangential face, and the 
fracture is a ragged one. In (a) and (d), where a radial face is exposed, there is an abrupt break extending 
more or less straight across the face. This is due to little or no slip taking place between the annual rings 
or wood-fibres radially adjacent, but a considerable slip takes place between the medullary rays and the 
adjacent fibres. 
Fig. 20. Box. Three strips, x 32 n.s., that have been fractured by tension. (a) and (0) exhibit 
tangential, and (c) radial, faces. The fracture runs through the wood to a considerable extent tangentially, 
but radially it is very abrupt. 
Puate V. 
Fig. 21. Ash. One strip, x 4 n.s., fractured by tension. It has been cut with broad sides tangential, 
and exhibits one of these sides showing usual ragged break. 
Fig. 22. Oak. A tangential section, x76 diams., from a beam fractured by cross-breaking, on the 
tension side. It shows the separation of tissues takes place most readily between medullary rays and 
adjacent tissues. 
Fig. 23. Box. A tangential section, x 176 diams., from a strip broken by tension. 
Fig. 24. Box. A tangential face of beam, x { n.s. 
Puate VI. 
Fig. 25. Oak. A tangential face, x Z n.s., of a beam fractured by cross-breaking. The ragged break on 
tensional part similar to that obtained in direct tension. 
Vig. 26. Oak. Radial face of beam fractured by cross-breaking, and showing abrupt break, x { n.s. 
Fig. 27. Ash. A beam cut with the broad sides radial, and arranged to show one of these sides, 
X zo 0.8., after cross-breaking. Note the bulging on the upper side due to compression. 
Fig. 28. Oak. Transverse sections of two blocks ext with the broad longitudinal sides tangential, 
x § n.s., and subjected to a tangential stress which is less in (a) than in (0). 
