254 PROCEEDINGS OF THE AMERICAN ACADEMY 



consist in his use of a prepared carbon. The natural stone is entirely 

 unfit for ruling purposes. 



But it appeared subsequently that this conclusion was quite too 

 hastily formed, as far as the capability of receiving graphite is con- 

 cerned. During all these observations, the position of the diamond in 

 its holder remained unchanged ; but it was afterwards found that, by 

 giving it a certain inclination with respect to the surface of the ruled 

 plate, it was possible to rule lines, both coarse and fine, which would 

 receive the graphite in the most perfect manner. In general, however, 

 lines ruled with a carbon will take the plumbago perfectly but once. 

 If they ai'e filled and the surface of the glass is afterwards cleaned by 

 rubbing, it is not possible to fill them equally well again. As the fill- 

 ing is not disturbed by mounting in balsam, the better way is to clean 

 the glass thoroughly before ruling, and then mount permanently after 

 the first filling. 



Though the carbon is reduced so perfectly to a true knife-edge that 

 the intersection of the two faces appear as a line when examined with 

 an eye-piece of high magnifying power, it is apparent, nevertheless, 

 that the cutting-edge is composed of distinct and separate crystals ; 

 for in many cases two lines have been ruled at the same time. Gen- 

 erally one is much coarser than the other. Indeed, by regulating the 

 pressure, companion lines can be ruled so fine that it is impossible 

 to see them until they are filled. The setting of the diamond to rule 

 lines of a given kind and quality is simply a question of time and 

 patience. In one hundred trials, jjerhaps two or three may give lines 

 which will receive plumbago, four or five may give double lines, and 

 one or two may give lines of great delicacy. Great care is neces- 

 sary in the preservation of the minute cutting crystal when once found. 

 Notwithstanding the most careful manipulation, it often gives way 

 without visible cause. In several instances, I have been able to locate 

 the exact point where it was destroyed. 



In general, the best results have been obtained with the prepared 

 carbon. It is, however, somewhat capricious in its action. The labor 

 of preparation is also much greater than with the African or the 

 Brazilian diamond. The process of grinding occupies from five to 

 ten days. That it is much harder than any other kind of diamond is 

 conclusively shown by the fact that one specimen in my possession 

 has been used in shaping a jewel weighing 180 carats, with only a 

 trifling abrasion of its surface. 



In conclusion I ought to say, in explanation of the somewhat incom- 

 plete and fragmentary character of this investigation, that it has been 



