18 THE CORN-FIELD. 



suppose that a stem of this description would be 

 more liable to be broken off than a solid one 

 which contained an equal quantity of substance; 

 but it has been found, by experiment, that the 

 reverse of this is the case. If a bar of iron, or 

 any other metal, were to be beaten out and 

 formed into a hollow cylinder, it would sustain 

 a much greater weight, or require a far greater 

 power to be exerted in order to break it than if 

 it were suffered to remain in its solid state. In 

 like manner, a stem of one year's growth, when its 

 fibres are arranged in a tubular form, will bear a 

 much greater weight of seed, and withstand the 

 violence of the wind very much better, than the 

 same quantity of substance arranged in any other 

 way. The bones of birds afford a similar example 

 of fitness for the particular part they are called on 

 to perform. Many birds easily remain on wing for 

 a very long time without resting, though exercising 

 a power which enables them to fly at the rate of 

 from thirty to sixty miles an hour. In order that 

 they may do this, the bones of their wings must 

 be very light, or the weight would soon bring 

 them to the ground ; and it is necessary also that 

 they should at the same time be very strong, 

 otherwise the force of the air pressing against 

 their expanded feathers would snap them off. 

 Accordingly, if we examine the bones in the wing 

 of a bird, we shall find that they are composed of 

 a strong substance as tough as horn, but very 

 thin, and not filled with marrow like the bones of 

 quadrupeds, but hollow, and connected with in- 

 ternal organs, by means of which the bird is 

 enabled to fill the bones with air or to empty them 

 at his will, and accordingly as he may be disposed 



