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A PRIMER OF BIOLOGY 



obviously doing nothing towards supporting the 

 weight, and we may therefore cut it away very 

 .considerably, so long as we leave sufficient to keep 

 the two outer regions at the same distance apart. 

 Far less material than what we have available will 

 be required for that purpose, so that we may hollow 

 out the sides of the bar and leave a central region or 

 " web," as it is technically termed, to keep the two 

 " flanges " apart. The " girder " thus formed will 

 be almost as strong as the solid bar, whilst its own 



weight will have 

 been greatly de- 

 creased and 

 material econo- 

 mised. 



In the illus- 

 trative case we 

 have just con- 

 sidered we have 

 assumed that the 

 weight is press- 

 ing only in one of two directions (for manifestly, 

 the beam might have to resist an upward as well as 

 a downward pressure). Let us now, however, 

 suppose the weight or pressure to affect the beam 

 Principle laterally as well as vertically. A girder structure 

 honow must in that case be provided laterally also, and the 

 column, two webs would cross each other at right angles. 

 We thus get in cress section such an appearance as 

 that seen at Fig. 38. Lastly, let us suppose the 

 pressure to be exerted in any direction ; we must 

 then provide an infinite number of girders whose 

 flanges must face every point of the compass. Under 

 these circumstances, the flanges will obviously keep 

 each other apart, and we may then get rid of the 



FIG. 38. Principle of the hollow column 

 A, crossed girders ; B, hollow column. 



