XVI] ON STRENGTH AND FLEXIBILITY 1015 



a level head " by their help and guidance. Then, between the level 

 of the skull and the level of the great pelvic cantilever a continuous 

 field of force governs and defines the S-shaped curvature. Man's 

 vertebral column shews, mutatis mutandis, the same phenomenon 

 of continuous but alternating curvature. The dorsal region is, of 

 necessity, concave towards the cavity of the chest, and as a simple 

 consequence the cervical and lumbar regions curve the other way. 

 The typically aquatic birds, such as swim under water as 

 penguins and divers do, have characteristic features and adaptations 

 of their own. Just as the cantilever girder becomes obsolete in the 

 aquatic mammal so does it tend to weaken and disappear in the 

 aquatic bird. There is a marked contrast between the high-arched 

 strongly built pelvis in the ostrich or the hen, and the long, thin, 

 comparatively straight and apparently weakly bone which represents 

 it in a diver, a grebe or a penguin. Wings large enough foi; air 

 would be an obstruction under water, and small wings are enough ; 

 for they have to produce thrust only, not Uft, and the former is 

 but a small fraction of the latter load. The feet also are now mainly 

 concerned with the same forward thrust, and we begin to see how 

 the long narrow pelvis gives just the point d^appui which that 

 thrust requires. 



The woodcock, as ornithologists are aware, shews us an osteological paradox, 

 which is commonly described by saying that this bird's ear is in front of its 

 eye ! If we hold a woodcock's skull level, beak and all, this indeed seems to 

 be the case, but no woodcock does so. Standing or flying, the woodcock 

 holds its beak pointing downwards, and its skull is then level, like that of 

 other birds ; in other words, its beak is not in a line with the basi-cranial axis, 

 as a guillemot's is, but bends sharply downwards. When the axis of the skull 

 is horizontal, the beak points downwards at an angle of nearly 60°, and the 

 auditory aperture is then as much behind the eye as in other birds. 



There is a certain other principle much to the fore in the con- 

 struction of the skeleton, well known to the designer of a hydroplane 

 or "flying boat," and not wholly neglected by the bridge-building 

 engineer; it is the principle of non-rigid, flexible or elastic stability*. 

 A homely comparison between a basket and a tin-can tells us in 

 a moment what it means, and shews us some at least of its peculiar 

 advantages. This method of construction helps to distribute the load, 



* India-rubber has great elastic stability. It is not compressible, but is almost as 

 incbmpressible as water itself, as J. D. Forbes discovered a century ago. 



