172 



CROSS-BREAKING TESTS. 



sav. the load-measuring shackle a heavy, stiff beam is 

 attached, at or near the ends of which are two supports, 

 upon which rests the beam to be tested, and which 

 at the same time determine the span ; to the other shackle 

 is attached a stirrup carrying a knife-edge, which is made 

 to press upon the centre of the beam to be tested, and thus 

 iorms the point at which the load is applied. In some 

 cases the points of support are simple steel knife-edges, 

 in others the ends of the beam are allowed to rest upon 

 prisms of steel, of semi-circular section, with the flat faces 



FIG. 81. 



against the specimen. The semi-cylindrical surfaces rest in 

 bearings of corresponding form, and as the beam bends 

 under the load, these plates move slightly in their bearings, 

 and in this way accommodate themselves to the 

 varying form of the beam. The span in this case 

 is the distance between the centres of the cylindrical 

 portion of the plates. This arrangement is shown well in 

 the case of one of the smaller Buckton- Wicksteed machines 

 in Fig. 20. Here the supports are shown, but the semi- 

 cylindrical plates are not in position. Reference shouM 

 also be made to Figs. 27, 31, 32, 33, 35, and 36. In the 

 case of the Sydney machine, on Fig. 27, the load is measured 

 through a stirrup hung from the tension shackle, and the 

 supports are formed on the ends of rams working in 



