1010 ON FORM AND MECHANICAL EFFICIENCY [ch. 



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 this is precisely what, in 

 the circumstances of the case, it would seem that Nature is bound 

 to avoid. We need not suppose that the great saurian was by any 

 means active and limber; but a certain amount of activity and 

 flexibility he was bound to have, and in a thousand ways he would 

 find the need of a backbone that should hQ flexible as well as strong. 

 Now this opens up a new aspect of the matter and is the beginning 

 of a long, long story, for in every direction this double requirement 

 of strength and flexibility imposes new conditions upon the design. 

 To represent all the porrelated quantities we should have to construct 

 not only a diagram of moments but also a diagram of elastic 

 deflection and its so-called ''curvature"; and the engineer would 

 want to know something more about the material of the ligamentous 

 tension-member — its flexibility, its modulus of elasticity in direct 

 tension, its elastic limit, and its safe working stress. 



In various ways our structural problem is beset by "limiting 

 conditions." Not only must rigidity be associated with flexibility, 

 but also stability must be ensured in various positions and attitudes ; 

 and the primary function of support or weight-carrying must be 

 combined with the provision of points d'appui iot the muscles con- 

 cerned in locomotion. We canr^ot hope to arrive at a numerical 

 or quantitative solution of this complicate problem, but we have 

 found it possible to trace it out in part towards a qualitative solution. 

 And speaking broadly we may certainly say that in each case the 

 problem has been solved by Nature herself, very much as she solves 

 the difficult problems of minimal areas in a system of soap-bubbles ; 

 so that each animal is fitted with a backbone adapted to his own 

 individual needs, or (in other words) corresponding to the mean 

 resultant of the many stresses to which as a mechanical system it 

 is exposed. 



The mechanical construction of a bird is a more elaborate aff'air 

 than a quadruped's, inasmuch as it has a double part to play, the 

 bird's whole weight being borne now by its legs and now by its wings. 

 As it stands on the ground our bird is a balanced cantilever, carried 



