On Sal Nitrum and Nitro- Aerial Spirit 57 



shortened — but since, as is probable, there is nothing 

 so perfectly rigid, we must suppose that, in the bend- 

 ing of such rigid bodies as we know, not only does the 

 convex side come nearer to the concave, but that also 

 the planes at the ends incline to each other in the 

 manner already described, also that the convex side is 

 a little elongated, and lastly that the concave side is 

 just such a little shortened. For the force by which a 

 rigid body is bent tends to produce all these results. 

 For I think it is the case always that if a force seeks 

 to effect something and there are various ways in 

 which it may attain the result, while yet there is in all 

 of them great resistance and difficulty, the force I say 

 endeavours to perform the thing in these several ways. 

 Hence since a rigid body can be bent, as was pre- 

 viously shown, either by the approximation of the 

 convex to the concave side, or by the elongation of the 

 one or the shortening of the other, or lastly by the in- 

 clination to each other of the end planes ; and since 

 there is at the same time great resistance to the 

 bending of the rigid body by any of the methods (for 

 the matter of a rigid body is so dense and compact, 

 that its sides cannot without difficulty be drawn 

 towards each other ; its sides also are so firm and solid 

 that they cannot be lengthened or shortened without 

 great pressure), hence I say the force by which one 

 tries to bend a rigid body has enough to do in effecting 

 its purpose in those several ways. Still, however, 

 while the rigid body is bending at the same time in all 

 of these ways, its matter suffers a notable compression, 

 especially at the middle, as is evident from what has 

 been said. It should also be remarked here that when 

 the force by which a rigid body is bent tends to draw 

 the convex surface towards the concave, it tends also 

 by compressing the matter of the rigid body to pro- 



