OF IRON BEAMS. 443 



draw it asunder, to crush it. The bottom rib 

 must then be several times as large as the top 

 one to resist best an ultimate strain. 



32. This last matter must be considered with 

 some modifications; it would not perhaps be 

 proper to make the size of the ribs just in the 

 ratio of the disruptive to the crushing forces, as 

 the top rib would be so slender that it would be 

 in danger of being broke by accidents. 



33. The thickness too of the middle part of 

 the beam, or that between the ribs, is not a 

 matter of choice ; independent of the difficulty 

 of casting, it cannot be rendered thin at pleasure, 

 but must have a certain thickness, though in 

 long beams the breaking weight is small, and a 

 very small strength of middle is necessary. 



The neutral line being the boundary between 

 two opposing forces, those of tension and com- 

 pression, it seems probable that bending the 

 beam would produce a tendency to separation 

 at that place. Moreover, the tensile and com- 

 pressive forces are, strictly speakmg, not 

 parallel; they are deflected from their parallelism 

 by the action of the weight, which not only 

 bends the beam, but tends to cut it across in 

 the direction of the section of fracture; and 

 3 K 2 



