348 WRITINGS OP JOSEPH HENRY. [1855 



strument devised by Major Wade, late of the United States 

 Army, and constructed, under his direction, for the purpose 

 of testing the strength of gun metals. This instrument con- 

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

 knife-edges opposed to hardened steel surfaces. The com- 

 mission verified the delicacy and accuracy of the indications 

 of this instrument by actual weighing, and found, in accord- 

 ance 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 was much in- 

 debted to the experience and scientific knowledge of Lieu- 

 tenant J. A. Dahlgren, of the Navy Yard, and to the liberality 

 with w^hich all the appliances of that important public estab- 

 lishment were put at its disposal. 



Specimens of the different samples of marble were pre- 

 pared in the form of cubes of one inch and a half in dimen- 

 sion, 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 being ground 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 parallelism and flatness on the two oppo- 

 site 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 Rennie, and the one which 

 appears to have been used by most — if not all — of the subse- 

 quent experimenters, in researches of this kind. Some doubt 

 however was expressed as to the action 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 in- 

 terposed. This interesting fact was verified in a series of 



