676 ME. MALLET ON THE TEANSIT-^rELOCITY OE EAETHQUAET] WA^HIS. 
The quartz-rock (the mean of both hard and soft) is crushed by a load, applied in 
the same manner, of 1630 tons per square foot, and its molecular arrangement is per- 
manently injured at less than 1000 tons per square foot. The quartz-rock gives the 
highest measure of ultimate resistance, but it is the less trustworthy material when 
loaded heavily. 
Neither of these sorts of rock, if loaded so as to be pressed in the direction of the 
lamina, would sustain more than about 0*7 of the above loads at the crushing-point and 
at that of permanent injury, respectively. From the extreme inequality found within 
narrow limits in both rocks as quarried, neither should be trusted for safe load in prac- 
tice with more than about -^th of the mean load that impairs their molecular arrange- 
ment, as ascertained from selected specimens, or (say) not to more than 50 tons per 
square foot for passive or 25 tons per square foot for impulsive loads. 
The high relative compressibility of laminated rocks in the direction of the lamina 
might probably be made advantageous use of, where they are employed as a building 
material, for the construction of revetment or other walls of batteries exposed to the 
stroke of cannon-shot, by building the work (under suitable arrangements to obviate 
splitting up) with the planes of the laminae in the direction of the line of fire, ^. e. per- 
pendicular to the faces of the work ; for on inspecting the last column in Table XI., 
which contains the values of T,. under the several conditions of rock and of compression, 
it is at once apparent how much greater is the work done in crushing the slates and the 
quartz in their toughest and most compressible direction, i. e. in the direction of the 
laminae , — twice as much work being, upon the average, consumed in crushing the rock in 
this direction as suffices to destroy its coherence in the one transverse to the laminae, 
and the difference in the two, in the case of the softest quartz (Nos. 6 & 8), being as 
much as about 5 to 1. 
It would be unsuitable, however, to the present memoir here to pursue further such 
practical deductions suggested by the results experimentally obtained. 
