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480                     METALLURGY OF CAST IRON.
practice might almost as well omit their testing, for they are as liable to be misled as be correct in their conclusions. In obtaining the area of a round or square bar two measurements, at least, should be taken, added together, and then divided by two to obtain the average of their sizes to assure a tester that he has knowledge of what is closely the true total area of bars. Those desirous of closely following mixtures, etc., by physical tests to obtain true knowledge of the strength of their product, can not ignore the value of micrometer measurements. For scientific research, at least, such methods must be strictly followed. To find decimal equivalents for use in micrometer measurements, see Table 139, page 594. strengths, etc., deceive a tester 200 to 400 pounds in acceptiny; common rule measurement and tlie actual load in thinking he has a true record of the iron's strength, the reader is referred to Table 89, tests Xos. 6 and <S, on pu^'c 460, showing transverse tests of ^un metal. There we find two bars which, if the actual breaking loads were accepted, would deceive the tester .\(n) pounds, or in other words, instead of his believing lie had one bar only 72 pounds stronger than the other, he actually had ;i difference of .j6() pounds, us stated above. Tin's should aid to clearly illustrate the importance of micrometer measurements, wherever the tester desires to truly ascertain whether any difference actually exists in the* strength of his mixtu/es or the character of the iron produced.