J. Henry on testing Building Materials. 33 



consists of a compound lever, the several fulcra of which are 

 knife-edges, opposed to hardened steel surfaces. The commis- 

 sion verified the delicacy and accuracy of the indications of this 

 instrument by actual weighing, and found, in accordance with 

 the description of Major Wade, the equilibrium was produced 

 by one pound in opposition to two hundred. In the use of this 

 instrument the commission were much indebted to the experience 

 and scientific knowledge of Lietenant Dahlgreen, of the Navy 

 Yard, and to the liberality with which all the appliances of that 

 important public establishment were put at their disposal. 



Specimens of the different samples of marble were prepared 

 in the form of cubes of one inch and a half in dimension, and 

 consequently exhibiting a base of two and a quarter square 

 inches. These were dressed by ordinary workmen with the use 

 of a square, and the opposite sides made as nearly parallel as 

 possible by grinding by hand on a flat surface. They were then 

 placed between two thick steel plates, and in order to insure an 

 equality of pressure, independent of any want of perfect paral- 

 lelism and flatness on the two opposite surfaces, a thin plate of 

 lead was interposed above and below between the stone and 

 the plates of steel. This was in accordance with a plan adopted 

 by Eennie, and that which appears to have been used by most, 

 if not all, of the subsequent experimenters in researches of 

 this kind. Some doubt, however, was expressed as to the ac- 

 tion of interposed lead, which induced a series of experiments to 

 settle this question, when the remarkable fact was discovered, 

 that the yielding and approximately equable pressure of the lead 

 caused the stone to give way at about half the pressure it would 

 sustain without such an interposition. For example, one of the 

 cubes, precisely similar to another which withstood a pressure of 

 upwards of 60,000 pounds when placed in immediate contact 

 with the steel plates, gave way at about 30,000 with lead inter- 

 posed. This remarkable fact was verified in a series of experi- 

 ments, embracing samples of nearly all the marbles under trial, 

 and in no case did a single 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 bulg- 

 ing 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 equable 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 inter- 

 posed lead, the stone first gives way along the lines of least re- 

 sistance, and the remaining pressure must be sustained by the 

 central portions around the vertical axis of the cube. 



SECOND SERIES, VOL. XXII, NO. 64. — JULY, 1856. 5 



