QUILLS AND STRAY/. 27 



j' in mind, then, that in a rod or pole the centre is the 

 part which most requires to be strengthened, we can see, in 

 cases too numerous to mention, how art has followed, though 

 perhaps unconsciously, in the footsteps of nature. Take, for 

 example, the beam of a steam-engine, such as is given in the 

 sketch, and for which the great engine at Chatham acted as 

 model. The reader will observe that in this case the beam is 

 gradually thickened towards the centre, the ends, where the 

 strain is slightest, being comparatively small. 



Another point also must be noticed. Equal strength could 

 have been obtained had the beam been solid, but at the expense 

 of weight, and consequent waste of power. Lightness is there- 

 fore combined with strength by making the beam consist of a 

 comparatively slight centre, but having four bold ridges, as 

 shown in the section given in the accompanying illustration. 

 This plan, as the reader will see, is exactly the same as that 

 which is adopted in the iron mast and porcupine quill, except 

 that the ridges are external instead of internal. The same 

 mode of construction is employed in ordinary cranes, the prin- 

 cipal beam of which is almost identical in form with that 

 of the engine, both being thickest in the centre, and both 

 strengthened with external ridges. 



There are also other analogies between the hollow mast and 

 natural objects. Keeping still to the animal world, we find 

 the quill feathers of the flying birds to supply examples of the 

 combination of great strength with great lightness and very 

 little expenditure of material. Their wing bones, too, aro 

 hollow, communicating with the lungs, and are consequently 

 light as well as strong. 



Passing to the vegetable world, we find a familiar example 

 of this structure in the common Wheat Straw. The ripe ear is 

 so heavy, when compared with the amount of material which 

 can be spared to carry it, that if the stalk were solid it would 

 give way under the mere weight of the ear. Moreover, the 

 full-grown corn has to endure much additional weight when 

 wetted with rain, and to resist much additional force when 

 bowed by the wind, so that a slight and solid stalk would be 

 quite inadequate to the task of supporting the ear. 



The material of the stalk is therefore utilised in a different 

 manner, being formed into a hollow cylinder, the exterior of 



