316 THE STONE ABCH. [CHAP. XV 



we assume that any section along a bed-joint resists only a per- 

 pendicular pressure due to the parts above, and a force paral- 

 lel to the joint which must not exceed the resistance to sliding 

 due to friction. If this parallel force is greater than the resist- 

 ance of friction, the upper part will slide upon the joint. 



If we represent the greatest angle of repose by <, then the 

 resultant of the vertical forces, acting upon the joint in ques- 

 tion, must make an angle with the normal to the joint less than 

 the angle </>. Thus at the joint A (PI. 24:, Fig. 95), this angle is 

 greater than $, and the upper part will slide along this joint. 

 At B this angle is less than <, and no sliding can take place. 



The ratio of the force of friction due to the component of P 

 normal to the joint, to the component of P parallel to the joint, 

 we call the coefficient of safety against sliding. It is evidently 



equal to 2L-. or to the distance G-N divided by PN. 



tan P N ' J 



Since we can dispose the bed-joints at pleasure, we may 

 always make them perpendicular to the direction of the pres- 

 sure, for instance in Fig. 95 horizontal ; or at least so place 

 them that their normals vary from the direction of the resultant 

 of the outer forces, at most by an allowable angle P N. 



The sliding of the joints can then always be prevented by the 

 position of the bed-joints. 



17O. Forces acting upon a Cros-section Neutral Axis. 

 Let us consider what happens when the resultant of the outer 

 forces acting upon a joint, instead of acting at the centre of 

 gravity, approaches the edge of a joint, under the assumption 

 that sliding cannot take place, or that the direction of this re- 

 sultant is perpendicular to the joint. There is no reason for 

 assuming the distribution of pressure upon the joint surface 

 any different from the case of a beam. The stone, as well as 

 the mortar, is elastic, though in a less degree than wood or iron, 

 and accordingly the pressure at any portion of the joint is pro- 

 portional to the approach of the limiting surfaces of the upper 

 and lower portions of the wall. If, then, we assume that these 

 surfaces are plane before and after loading, if the resultant 

 pressure does not act at the centre of gravity, but near to one 

 edge, the pressure at different points will vary, and there will 

 be a neutral axis, or line of no pressure, either within or wholly 

 without the joint surface. 



Every Gross-section is therefore acted upon by a system of 



