220 



Journal of Agricultural Research 



Vol. VI, No. 6 



Figures 17, 18, and 19. — After each slab was broken the cracks in 

 the top and bottom were drawn to scale. The heavy full lines forming an 

 approximate circle or ellipse around the load point are the tension cracks 

 on the top of the slab caused by the overhang of the ends, after a large 

 center deflection, at about breaking load. The remarkable symmetry of 



Fig. 14. — Deflection curves for slab 934 on first application of load. 



these cracks is worthy of notice. There seems to be no definite relation 

 between the effective width at working loads and the width over which 

 the cracks extended at failure; in fact, it is hardly reasonable that there 

 should be any definite relation, for one case is dealing with safe working 

 stresses within the limit of elasticity, and the other with breaking loads. 



