624 THE STRENGTH 



be best to deduce it from that beam, which most 

 nearly resembles in section the beam we intend 

 to obtain the strength of. 



Cor. If, in different beams of these forms, 

 the ratio of the length to the depth be the 

 same, the strength will be as the area of a 

 middle section of its bottom rib, c being a 

 constant quantity. 



66, In the preceding theorem, the quantity 

 c would perhaps be nearly constant for sections 

 of any particular forms such as we have used; 

 but the rule, more strictly speaking, only applies 

 where the whole tensile force exerted by the beam 

 lies in its bottom rib ; all the superior part 

 being in some degree of compression, more or 

 less : or exerting so little force by tension that 

 it may be neglected in comparison with what 

 was borne by the bottom rib or flange. This, 

 in the earlier part of our experiments on beams 

 was by no means the case; for in them the 

 bottom flange was so small, that the lower 

 extremity of the vertical part, between the 

 flanges, must have exerted considerable tensile 

 influence, in some cases ; and in the earliest 

 experiments, perhaps more than the bottom 

 flange itself did. In the latter ones however, 

 the bottom rib became so large, and the neutral 



