'138 LECTURE XIII. 



also that a force capable of extending it to twice its length will only com- 

 press it to two thirds. In this substance, and others of a similar nature, the 

 resistance appears to be much diminished by the faciUty by which a contrary 

 change is produced in a different direction; so that the cohesion and repul- 

 sion thus estimated appears to be very weak, unless when the rigidity is in- 

 creased by a great degree of cold. It would be easy to ascertain the specific 

 gravity of such a substance in different states of tension and compression, 

 and some light might be thrown, by the comparison, on the nature and oper- 

 ation of the forces which are concerned. 



It is difficult to compare the lateral adhesion, or the force which resists the 

 detrusion of the parts of a solid, with any form of direct cohesion. This force 

 constitutes the rigidity or hardness of a solid body, and is wholly absent from 

 liquids, although their immediate cohesion appears to be nearly equal to that 

 of solids. Some experiments have been made on the fracture of bodies by 

 means of detrusion, but it does not appear that the force necessary to pro- 

 duce a temporary derangement of this kind has ever been examined : it may 

 be inferred, however, from the properties of twisted substances, that tl>e 

 force varies in the simple ratio of the distance of the particles from their 

 natural position, and it must also be simply proportional to the magnitude of 

 the surface to which it is applied. 



The most usual, as well as the most important effect, produced by the ap- 

 plication of force, is flexure. When a force acts on a straight column in the 

 direction of its axis, it can only compress or extend it equally through its 

 .whole substance; but if the direction of the force be only parallel to the axis, 

 and applied to some point more or less remote from it, the compression or ex- 

 tension will obviously be partial: it may be shown that in a rectangular 

 column, when the compressing force is applied to a point more distant from 

 the axis than one sixth of the depth, the remoter surface will no longer be 

 compressed but extended, and it may be demonstrated that the distance of 

 the neutral point from the axis is inversely as that of the point to which the 

 force is applied. From the effect of this partial compression, the column 

 must necessarily become curved ; and the curvature of the axis at any point 

 will always be proportional to its distance from the line of direction of the 

 force, not only while the column remains nearly straight, but also when it i* 



