970 ON FORM AND MECHANICAL EFFICIENCY [ch. 



When the engineer constructs an iron or steel girder, to take 

 the place of the primitive wooden beam, we know that he takes 

 advantage of the elementary principle we have spoken of, and saves 

 weight and economises material by leaving out as far as possible 

 all the middle portion, all the parts in the neighbourhood of the 

 "neutral zone"; and in so doing he reduces his girder to an upper 

 and lower "flange," connected together by a "web," the whole 

 resembling, in cross-section, an I or an I . 



But it is obvious that, if the strains in the two flanges are to 

 be equal as well as opposite, and if the material be such as cast-iron 

 or wrought-iron, one or other flange must be made much thicker 

 than the other in order that they may be equally strong* ; and if at 

 times the two flanges have, as it were, to change places, or play 

 each other's parts, then there must be introduced a margin of 

 safety by making both flanges thick enough to meet that kind of 

 stress in regard to which the material happens to be weakest. 

 There is great economy, then, in any material which is, as nearly 

 as possible, equally strong in both ways; and so we see that, from 

 the engineer's or contractor's point of view, bone is a good and 

 suitable material for purposes of construction. 



The I or the H-girder or rail is designed to resist bending in one 

 particular direction, but if, as in a tall pillar, it be necessary to 

 resist bending in all directions alike, it is obvious that the tubular 

 or cylindrical construction best meets the case; for it is plain that 

 this hollow tubular pillar is but the I-girder turned round every 

 way, in a "solid of revolution," so that on any two opposite sides 

 compression and tension are equally met and resisted, and there is 

 now no need for any substance at all in the way of web or "filHng" 

 within the hollow core of the tube. And it is not only in the 

 supporting pillar that such a construction is useful ; it is appropriate 

 in every case where stiffness is required, where bending has to be 

 resisted. A sheet of paper becomes a stiff rod when you roll it up, 

 and hollow tubes of thin bent wood withstand powerful thrusts in 

 aeroplane construction. The long bone of a bird's wing has Httle 

 or no weight to carry, but it has to withstand powerful bending 



* This principle was recognized as soon as iron came into common use as a 

 structural material. The great suspension bridges only became possible, in Telford's 

 hands, when wrought iron became available. 



