1855] WRITINGS OF JOSEPH HENRY. 349 



experiments embracing samples of nearly all the marbles 

 under trial, and in no case did an exception occur to vary 

 the result. 



The explanation of this remarkable phenomenon — now 

 that it is known, is not difficult. The stone tends to give way 

 by bulging out in the centre of each of its four perpendicular 

 faces, and to form two pyramidal figures with their apices 

 opposed to each other at the centre of the cube, and their 

 bases against the steel plates. In the case where rigid equa- 

 ble pressure is employed, as in that of the thick steel plate, 

 all parts must give way together. But in that of a yielding 

 equable pressure, as in the case of interposed lead, the stone 

 first gives way along the outer lines or those of least resis- 

 tance, and the remaining pressure must be sustained by the 

 central portion around the vertical axis of the cube. 



After this important fact was clearly determined, lead and 

 all other interposed substances were discarded, and a method 

 devised by which the upper and lower surfaces of the cube 

 could be ground into perjFect parallelism. This consists in 

 the use of a rectangular iron frame into which a row of six 

 of the specimens could be fastened by a screw at the end. 

 The upper and lower surfaces of this iron frame were wrought 

 into perfect parallelism by the operation of a planing machine. 

 The stones being fastened into this, with a small portion of 

 the upper and lower parts projecting, the whole were ground 

 down to a flat surface until the iron and the face of the cubes 

 were thus brought into a continuous plane. The frame was 

 then turned over and the opposite surfaces ground in like 

 manner. Care was of course taken that the surfaces thus 

 reduced to perfect parallelism, in order to receive the action 

 of the machine, were parallel to the natural beds of the stone. 



All the specimens tested were subjected to this process, 

 and in their exposure to pressure were found to give concord- 

 ant results. The crushing force exhibited was therefore much 

 greater than that heretofore given for the same material. 



The commission also determined the specific gravities of 

 the different samples submitted to its examination, and also 

 the quantity of water which each absorbs. 



