TESTING BUILDING MATERIALS. 305 



imitate the effect of frost. Though this process has been much relied 

 on, and generally employed, recent investigations made -by Dr. Owen 

 lead us to doubt its perfect analogy with that of the operations of na- 

 ture. He found that the results produced by the actual exposure to 

 freezing and thawing in the air, during a portion of winter, in the 

 case of the more porous stones, produced very different results from 

 those obtained by the use of the salt. It appears from his experiments 

 that the action of the latter is chemical as well as mechanical. 



The commission, in consideration of this, have attempted to produce 

 results on the stone by freezing and thawing by means of artificial 

 cold and heat. This process is, however, laborious ; each specimen 

 must be inclosed in a separate box fitted with a cover, and the amount 

 of exfoliation produced is so slight that in good marble the operation 

 requires to be repeated many times before reliable comparative results 

 can be obtained. In prosecuting this part of the inquiries unforeseen 

 difiiculties have occurred in ascertaining precisely the amount of the 

 disintegration, and it has been found that the results are liable to be 

 vitiated by circumstances which were not foreseen at the commence- 

 ment. 



It would seem at first sight, and the commission when they under- 

 took the investigation were of the same opinion, that but little diffi- 

 culty would be found in ascertaining the strength of the various spe- 

 cim^ens of marbles. In this, however, they were in error. The first 

 difliculty which occurred was to procure the proper instrument for the 

 purpose. On examining the account of that used by Eennie, and 

 described in the Transactions of the Koyal Society of London, the com- 

 mission found that its construction involved too much friction to allow 

 of definite comparative results. Friction itself has to be overcome as 

 well as the resistance to compression, and, since it increases in pro- 

 portion to the pressure, the stronger stones would appear relatively 

 to withstand too great a compressing force. 



The commission first examined an hydraulic press, which had pre- 

 viously been employed in experiments of this kind, for the use of the 

 government, but found that it was liable to the same objection as that 

 of the machine of Eennie. They were, however, extremely fortunate 

 subsequently in obtaining, through the politeness of Commodore Bal- 

 lard, commandant of the navy yard, the use of an admirable instru- 

 ment 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 consists of a compound lever, the 

 several fulcra of which are knife edges, opposed to hardened steel sur- 

 faces. _ The commission 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 hvo hundred. In the use of 

 this instrument the commission were much indebted to the experience 

 and scientific knowledge of Lieutenant 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 

 20 s 



