56 Mayow 



it is implied in our idea of a perfectly rigid body that 

 none of its sides can be lengthened or shortened. 



We may gather from what has been said that the 

 matter of a rigid body undergoes when bent a notable 

 compression — and that, especially, at the middle, as is 

 obvious from Plate I., Fig. 5. For when the convex 

 surface b^ e^ d^ of the bent rigid body has been brought 

 inwards towards the concave surface a^ c, the matter of 

 the rigid body at e must suffer a notable compression. 

 Hence the reason is obvious why rigid bodies when 

 bent too much usually break near the middle. Just as 

 in making bows this alone is attended to that their 

 middle part be thick and strong enough, while it is of 

 no consequence if the ends are thinner. 



Indeed if a rigid body be so compact and solid that 

 there are no interstices between its parts, and it 

 cannot, in consequence, be compressed into less space, 

 then clearly such a rigid body cannot be bent but will 

 rather break. For it should be observed that rigid 

 bodies which can be bent, although their outer surfaces 

 are extremely solid and firmly compacted, have very 

 many little spaces in their interior, as is clearly the 

 case in iron and glass, which are specially rigid. For 

 when glowing iron or glass is plunged into water, that 

 they may cool quickly and become rigid, their outer 

 parts cool sooner than their inner, and on that account 

 become more rigid and solid. For the nitro-aerial 

 particles which are in the still heated interior open 

 little spaces here and there, in order to continue their 

 motions, and when they gradually extricate themselves 

 outwards they are detained in the now cooled surface 

 and render it exceedingly solid, although very many 

 empty little spaces are left in the interior. 



Thus far then of bodies of the greatest rigidity — that 

 is bodies whose sides can neither be lengthened nor 



