266 On the relation which subsists between a 



the supports of large ones. Architects, to be sure, are accustomed 

 to enlarge and reduce in proportion ; but nature, whose structures 

 possess infinitely more symmetry, beauty and variety, than those of 

 which art can boast, is content to change her proportions at each 

 change of size. Let us conceive an animal having the proportions 

 of an elephant and only the size of a mouse ; not only would the 

 limbs of such an animal be too strong for it, they would also be so 

 unwieldy that it would have no chance among the more nimble and 

 better proportioned creatures of that size. Reverse the process, 

 and enlarge the mouse to the size of an elephant, and its limbs, totally 

 unable to sustain the weight of its immense body, would scarely 



have strength to disturb its position even when recumbent. 



The very same remarks apply to that case in which the weight, 

 instead of compressing, distends the support. The chains of Trin- 

 ity pier are computed to be able to bear nine times the load put on 

 them. But if a similar structure were formed, of ten times the lin- 

 ear dimensions, the strength of the new chain would be one hundred 

 times the strength of that at Trinity, while the load put upon it would 

 be one thousand times greater ; so that the new structure would pos- 

 sess only nine tenths of the strength necessary to support itself. Of 

 how little importance, then, in bridge building, whether a model con- 

 structed on a scale of perhaps one to a hundred support its own weight! 

 yet, on such grounds, a proposition for throwing a bridge of two 

 arches across the Forth at Queensferry was founded. Putting out 

 of view the road way. and passengers altogether, the weight of the 

 chain alone, would have torn it to pieces. The larger species of 

 spiders spin threads much thicker, in comparison with the thickness 

 of their own bodies, than those spun by the smaller ones. And, as 

 if sensible that the whole energies of their systems would be expen- 

 ded in the frequent reproduction of such massy webs, they choose 

 the most secluded spots ; while the smaller species, dreading no in- 

 convenience from a frequent renewal of theirs, stretch them from 

 branch to branch, and often from tree to tree. I have often been as- 

 tonished at the prodigious length of these filaments, and have mused . 

 on the immense improvement which must take place in science, and 

 in the strength of material too, ere we could, individually, undertake 

 works of such comparative magnitude. 



When a beam gives support laterally, its strength is proportional to 

 its breadth, and to the square of its depth conjointly. If, then, such 

 a beam were enlarged ten times in each of its linear dimensions, its 



