COMPUTATION OF RELATIVE STRENGTH OF TEST BARS.
The rule for computing the relative strength of test bars (see page 476) is to divide the breaking load by the area of the bar, at its point of fracture. It is to be understood that this rule can be applied only to bars of the same length and cross section, or made from the same pattern, in sizes or areas to equal i)^, inch to 2l/> inches diameter or such bars as shown .on pages 536 and 573, for the purpose of making comparisons of any difference that may exist in the area of test bars made from off the same pattern, due to a straining, etc., of the mould in which the bars were cast. While the compilation derived by the rules in Table 99, page 476, are placed under the head of " Strength per square inch " in most of the Tables of tests in this work, such is given as a matter of form, or for relative comparisons, and not as absolute strength per square inch. The author has presented the rule given in Table 99 for the reason that it is the simplest for ordinary shop testing, and takes better cognizance of the practical elements for everyday use in a standard bar than any other formula of which he has knowledge. Whatever systems are advanced for making relative comparisons in the transverse or tensile strength of iron, no matter what size of a bar we use, be it of one inch, two inches, or three inches urea, square or round, the author claims that none should be recognized asulphur oi" brimstone was plaeed in the ladle after No. 30 had been poured. The* whit<* rin# at II, No. 29, shows the hardening effect of sulphur.aragraph.