hk. Mallet-—Temperature attainable by Rock-crushing. 261 
e e 
buildings the material of which is overloaded, where crushing 
or spalling off of the ashlar stones only occurs at and near the 
joint.* In either case, whether the prism be homogeneous or 
not, the crushing must be localized either to the end or ends of 
the prism, or to the plane of weakness where it first yields, and 
which then becomes the crushing surfaces of two opposed 
prisms. It is these physical conditions which ‘determine the 
localization” of crushing in the prism, and which conditions 
ave been disregarded in the Rev. O. Fisher's objection. Let 
us now consider the subsequent effects of the successive*crush- 
ing of a column of prismatic mass of rock, one extremity of 
which is continually urged against the face of a fixed mass of 
rock which does not yield, a case which approximates to that 
which most frequently occurs in nature, and which, to fix our 
ideas, we may suppose presents a face for crushing one square 
foot; and being continually urged forward, and the pressure 
being greatest where the pressing column comes into contact 
with the fixed mass of rock, the extremity of the column sup- 
posed homogeneous, or the parts adjacent thereto, are continu- 
ally crushed by a succession of per saltum movements. The 
first cubic foot of the column that is crushed has its tempera- 
ture raised, let us suppose, by the minimum of 217°. The 
crushed fragments at this temperature are pushed aside by the 
y the unerushed column from the hotter portions of 
u 
material surrounding it that have already been heated by 
* See also E. Hodgkin son’s experiments on the directions of fracture of crushed 
rial, Brit. Assoc. Report, vol. vi; and Tredgold on Cast Iron, by Hodgkinson, 
Part 2. p. 319, and plate 1. 
