lyS 



WILLIAM H. HOBBS 



is about yV- Although the actual load lifted increases but little 

 after this ratio has reached \, the percentage which is, lifted of the 

 total load upon the arch continues to increase up to stage 6, where 

 underturning begins and from which point the competence falls off 

 at a rapid rate. These deductions are graphically reproduced in 

 Fig. 17, where the abscissae are the ratios of rise to span, and the 

 ordinates have a different significance for each curve. The upper 

 curve gives the percentage of total load which is hfted from inferior 



.50 ftise -to Span. 



Fig. 17. — -The curves of relative competency and of relative volume of an anticline 

 in different stages up to the stage of underturning; and (in dotted lines) the purely 

 hypothetical competence and volume on the assumption that the length of the arch is 

 not increased after extended underturning without failure. 



beds within the arch ; the middle curve gives in arbitrary units the 

 relative loads which are lifted; while the lowest curve gives in 

 arbitrary units (not those of the middle curve) the relative volumes 

 inclosed by the anticline. 



Initiation of new anticlines in series. — The stage marked 6 in our 

 series is where underturning begins, for the steeper limb includes a 

 point at which the tangent to the curve is vertical. The active 

 compressive force, which up to this stage has been deflected upward 



