:N'o. 104.] 213 



to form two pyramidal figures with their apices opposed to each other 

 at the center 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 interposed lead, the stone 

 first gives way along the outer lines, or those of least resistance, and 

 the remaining pressure must be sustained by the central portions 

 around the vertical axis of the cube." This fact, so clearly demon- 

 strated, shows very conclusively that all experiments made upon blocks 

 of stone with the intervening yielding material are fallacious, and 

 really give us but one-half the actual power of resistance possessed by 

 the stone tested. When we add to this fact also the practice of engi- 

 neers as usually stated, that owing to imperfections of the material 

 and other causes, it is not considered safe to load a stone with more 

 than one-eighth of its crushing weight,* it will be seen that we are 

 very far within the safe limits to which any stone may be loaded and 

 retain its integrity. 



By this process, Prof Henry has shown that the marble of Lee, 

 Massachusetts, will sustain a pressure of 23,917 pounds to the square 

 inch. This marble was adopted for, and has been used in the capitol 

 extension or new Capitol at Washington. In strength it is not supe- 

 rior to many other marbles, nor equal to some of the ordinary com- 

 pact limestones, and is much inferior to the granites. In composi- 

 tion it consists of the carbonates of lime and magnesia, and is a true 

 dolomite, as shown by the analysis of Dr. Genth and Dr. Torrey, con- 

 taining minute proportions of iron and manganese. The experiment 

 of using a dolomite on so large a scale will ultimately demonstrate 

 whether a stone of this composition can be relied upon as a durable 

 building material. As before stated, however, the simple presence of 

 magnesia is not of itself evidence of the rapid decay of the stone ; a 

 small proportion of iron in some form, or combined with some other 

 mineral, may effect the destruction of a magnesian limestone that 

 otherwise appears sound and durable. 



Less attention seems to have been given to the lateral strength of 

 stone, than its importance would warrant.. When we see, even in 

 buildings of recent erection, the window sills and caps cracked through, 

 and these parts giving way and becoming dilapidated and unsightly, 

 it is evidently a matter of no small importance to be able to decide 

 what amount of weight can be borne by stones of certain dimensions. 

 This knowledge also becomes of the highest importance in view of the 

 manner in which the foundations of heavy buildings are laid; the 

 gradual retraction of the width above relieving the lower and outer 

 layers of stone from the direct crushing force of the superincumbent 

 walls, but testing its lateral strength, f 



In estimating the strength of a stone to resist pressure, it is not 

 safe to predicate an opinion upon examples of cracking or breaking in 

 the walls of a building, whether before or after its completion ; for a 

 little inequality in the bedding may produce such a result, when, if 

 evenly bedded, the stone would have borne many times the load it has 



■ « • ~^~^~' 



* According to some engineers with but one-twentieth of its crushing weight. 

 + The results of experiments, showing the power of resistance to pressure of several of 

 our limestones, marbles, granites, etc., will be found in an appendix to this report. 



