Abstract
BEFORE making a few comments on Prof. Milne's second letter under the above title (NATURE, June 12, p. 127), I should like to express my warm appreciation of his devotion to seismological research, and the great impetus it has given to observational work. In pure seismology—apart from applications of elastic solids to earth problems—Prof. Milne's reading is doubtless more extensive than mine, but if he is correct in regarding my first letter as containing nothing new to seismologists, they must, as a class, be singularly prone to a policy of meliora scio deteriora sequor. Novelty in results is, of course, much a matter of opinion. When Prof. Milne says, however, that there is no occasion for my warning as to Young's modulus, I must in reply give a quotation from his first letter, relating to the material of his hypothetical core, “it follows that the density is 5.96, or approximately 6. The elastic modulus for a core of this density which conveys vibrations with a speed of at least 9.5 km. per second is 451 × 1010 C.G.S., or roughly speaking, a little more than twice the Young's modulus for Bessemer steel.” The italics are mine. If “the modulus” is not Young's modulus, E, a comparison between it and the E for steel is misleading, because a comparison of numerical results naturally implies that they refer to the same physical quantity. On this view the statement is doubly misleading, because there are two wave moduli, viz. m+n and n. If, as one would infer from Prof. Milne's second letter, “the” modulus was intended for the wave modulus m+n, the futility of the comparison becomes obvious when we remember that on the ordinary theory (m+n)/E may have any value between 1 and, according to the value of Poisson's ratio. As a matter of fact, “the” modulus must, I think, have been intended at the time for Young's, though this must have escaped Prof. Milne's memory. If it were meant for m+n, we should have (451÷5.96)½ “at least” 9.5, whereas it is really only 8.7. If, however, we multiply 451 × 1010 by 6/5—which would be correct if 451 × 1010 were a Young's modulus in a material where Poisson's ratio had the uniconstant value 0.25—and substitute this, we deduce a wave velocity of 9.53 km. per second.
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CHREE, C. Seismometry and Gêite. Nature 68, 176 (1903). https://doi.org/10.1038/068176b0
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DOI: https://doi.org/10.1038/068176b0
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