OTHER PROPERTIES OF CAST IRON. 179 
neutral line.” He then goes on to illustrate 
the theory by a diagram to show that the sum 
of the forces exerted by the extended fibres is 
equal to the sum of the forces exerted by the 
compressed ones, and thus concludes :—‘‘ Now 
it is evident that the extensions or compres- 
sions of any particles within these surfaces will 
be as their distances from the line A B (mean- 
ing the neutral line;) and the forces exerted 
by those particles must be in the same propor- 
tion, so long as the elasticity remains perfect ; 
for then the forces are found to be as the ex- 
tensions or compressions. Afterwards the forces 
of the particles would be as some different fune- 
tions of their distances from the neutral line.” 
In further illustration of this subject, suppose 
we place a bar of cast iron upon the supports 
E E in the figure, and subject it to pressure, 
by weights suspended from the middle; it is ob- 
vious, in this case, that the resisting forces of 
extension and compression immediately come 
into operation ; the particles forming the con- 
vex side of the bar, become more widely sepa- 
rated, whilst those on the concave are more 
closely condensed. It is evident, therefore, 
that a change of position must take place in 
