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710 ON FORM AND MECHANICAL EFFICIENCY [ch 



animal. In an ordinary fish, such as a cod or a haddock, we see 

 precisely the same thing: the backbone is stiffened by the indis- 

 pensable help of its three series of ligament-connected processes, 

 the dorsal and the two transverse series. And here we see (as 

 we see partly also among the whales), that these three series of 

 processes, or struts, tend to be arranged well-nigh at equal angles, 

 of 120°, with one another, giving the greatest and most uniform 

 strength of which such a system is capable. On the other hand, 

 in a flat fish, such as a plaice, where from the natural mode of 

 progression it is necessary that the backbone should be flexible 

 in one direction while stiffened in another, we find the whole 

 outline of the fish comparable to that of a double bowstring 

 girder, the compression-member being (as usual) the backbone, 

 the tension-member on either side being constituted by the inter- 

 spinous ligaments and muscles, while the web or filling is very 

 beautifully represented by the long and evenly graded spines, 

 which spring symmetrically from opposite sides of each individual 

 vertebra. 



The main result at which we have now arrived, in regard to 

 the construction of the vertebral column and its associated parts, 

 is that we may look upon it as a certain type of girder, whose depth, 

 as we cannot help seeing, is everywhere very nearly proportional 

 to the height of the corresponding ordinate in the diagram of 

 moments: just as it is in the girder of a cantilever bridge as 

 designed by a modern engineer. In short, after the nineteenth 

 or twentieth century engineer has done his best in framing the 

 design of a big cantilever, he may find that some of his best ideas 

 had, so to speak, been anticipated ages ago in the fabric of the 

 great saurians and the larger mammals. 



But it is possible that the modern engineer might be disposed 

 to criticise the skeleton girder at two or three points; and in 

 particular he might think the girder, as we see it for instance in 

 Diplodocus or Stegosaurus, not deep enough for carrying the 

 animal's enormous weight of some twenty tons. If we adopt a 

 much greater depth (or ratio of depth to length) as in the modern 

 cantilever, we shall greatly increase the strength of the structure ; 

 but at the same time we should greatly increase its rigidity, and 



