458 THE STRENGTH 



as in the figure, the top part remaining whole ; 

 the weight having been removed as soon as the 

 fracture commenced. 



This is represented by Mr. Tredgold, ** Essay 

 on Strength of Cast Iron,'* to be a very 

 economical form of beam. It would, there is little 

 doubt, have resisted much more tenaciously if the 

 weights had been laid uniformly over it ; but this 

 experiment shows the great weakness of the 

 beam when the weight is applied at a single point, 

 and therefore the danger of using it in practice. 



VI. EXPERIMENT. 



A parabolic arch differing from the last, only, 

 in its having a portion taken from the top rib, 

 and added to the bottom, leaving the height as 

 before, and the ratio of the ribs 1 to 2 nearly. 

 Distance between supports and depth as before. 



Dimensions in section of the ribs. 



Area of top rib = 2.2 x .36 = 0.79 inches. 

 Area of bottom rib = 2.2 x .75 = 1.65 „ 



Weight of casting = 43 lbs. 

 Breaking weight = 5528 lbs. 

 It broke quite off in the same place as the 

 last did. 



Both this casting and the preceding one broke 

 with the first weight laid on them, 55281bs., and 

 probably would have done so, particularly the 

 former, with several cwt. less. The latter casting 



