204 NATURAL THEOLOGY. 



acting in so difterent a manner, there is an inter- 

 mediate neutral or central part C, that may be 

 taken away without materially weakening the 

 beam, which shows that a hollow cylinder is the 

 form of strength. We may observe a further 

 illustration of this when a tree is blown down 

 and broken at the stem : to the windward the 

 broken part gapes ; it has been torn asunder like 

 the snapping of a rope : to the leeward side of 

 the tree the fibres of the stem are crushed into 

 one another and splintered ; whilst the central 

 part is bent. This, we presume, must always be 

 the case, more or less. We may observe, too, 

 why the arch is the form of strength. It this 

 transverse piece of timber were in the form of an 

 arch, and supported at the extremities, then its 

 whole thickness, its centre, as well as the upper 

 and lower parts, would support weight by resisting 

 compression. 



But the demonstration may be carried much 

 further, to show the form of strength in the bone. 

 If the part of the cylinder which bears the pres- 

 sure be made more dense, the power of resistance 

 will be much increased ; whereas, if a ligamentous 

 covering be added on the other side, it will 

 strengthen the part which resists extension ; and 

 we observe a provision of this kind in the tough 

 ligaments whicli run along the vertebrae of the 

 back. 



When we see the bone cut across, we are forced 

 to acknowledge that it is formed on the principle 



