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Maxwell and Acoustics
As seen below, also relevant to subsequent developments in acoustics, Maxwell noted that his theory indicated a char- acteristic stress relaxation time of τ = μ/P. He estimated its value for air at normal conditions of 2 × 10−10 seconds, not- ing that “This time is exceedingly small, even when com- pared with the period of vibration of the most acute audible sounds; so that even in the theory of sound, we may consider the motion as steady during this very short time and use the equations we have already found, as has been done by Pro- fessor Stokes” (Maxwell, 1867, p. 83). In a broader context, his paper included the viscoelastic nature of gases.
Thermodynamics, Cosmogony, Molecules, Atoms, and Controversy
By 1868, Maxwell’s attention was drawn to various issues of cosmogonic significance. For example, from thermodynam- ic considerations, he considered what he termed “physical indications of a beginning and an end” (Harman, 1995, p. 367). He examined related issues in his BAAS address of 1870 along with the stability of molecular processes as indicated by the similarity of terrestrial and stellar spectra, drawing attention to the wisdom of the perspective associated with the Christian faith in Hebrews 11:3 of the Biblical New Tes- tament (Marston, 2016). (Here, Maxwell indicated “ ... we seem to have advanced along the path of natural knowledge to one of those points at which we must accept the guid- ance of that faith by which we understand that ‘that which is seen was not made of things which do appear’” [Maxwell, 1870, p. 421]). Better known, however, was his discussion of molecules having “the essential character of a manufactured article” in his BAAS address in September 1873 (Maxwell, 1873b) that drew criticism from John Tyndall in his widely publicized Belfast Address at the BAAS meeting in August 1874 (Tyndall, 1874). In January 1874, Maxwell had devel- oped aspects of his own reasoning in an anonymous book review in Nature (Anonymous, 1874; Marston, 2007). In 1875, he expanded his observations and theistic perspec- tive in an article on Atoms for the Encyclopedia Britannica by examining favorable “collocations” and “instances of be- nevolent design” (Niven, 1890). Many scientists even doubt- ed the reality of atoms and molecules at that time. During this period, Maxwell also considered the consequences of a hypothetical superhuman “agent” or “doorkeeper” (eventu- ally known as Maxwell’s demon) that had the ability to sense molecular motion and utilize that information (Flood et al., 2014).
Maxwell, Stokes, Rayleigh,
and Peer Review
In Britain during the Victorian era, the Royal Society of Lon- don published the most prestigious scientific journals: the Proceedings and the Philosophical Transactions. Typically, an abstract of scientific results would be read at a meeting of the Royal Society, followed by peer review of a full-length manu- script administered by the Society’s secretary, a position held by George Gabriel Stokes (1819-1903) from 1854 to 1885. Often, the anonymity of reviewers was not maintained dur- ing the peer-review process. It was expected that opinions of the more senior reviewers would be respectfully valued during the review process, although there is some evidence that highly original manuscripts would be published even if the reviews were not in full agreement. (See, for example, the discussion of William Thomson’s review of Maxwell’s 1865 electromagnetic theory paper given in Marston [2016].) We know about Maxwell’s growing involvement in the peer-re- view process in the 1860s and more specifically with papers associated with acoustics because of compilations of his re- views by Harman (1995, 2002). The first of this category of manuscript reviewed by Maxwell was from Stokes himself (Stokes, 1868), a paper still recognized as the first thorough analysis of sound production by bounded vibrating objects. Maxwell considered the paper “an important contribution to Mathematics and to Acoustics” and was favorable to its pub- lication (Harman, 1995, p. 415). The next important exam- ple in this category is Maxwell’s review of the manuscript by Rankine (1870), the first of the papers leading to the famed Rankine-Hugoniot relationships of shock wave physics. Rankine’s manuscript emphasized the thermodynamics of waves and Maxwell was clearly supportive (Harman, 1995), incorporating Rankine’s approach in the relevant section on wave propagation in his text Theory of Heat (Maxwell, 1871). In his associated discussion of the “condensation” of a sound wave becoming “more sudden” as the wave propagates, he illustrated the process with the “waves of the sea on com- ing into shallow water becoming steeper in front and more gently sloping behind, till at last they curl over on the shore” (Maxwell, 1871, Chapter 15).
The next examples concern Lord Rayleigh (birth name John William Strutt until mid-1873), who became a major con- tributor to acoustics. It is appropriate to first review early aspects of Strutt's life and education. Strutt entered Trinity College Cambridge in January 1861, and there are several parallels between his studies and those of Maxwell a de-
24 | Acoustics Today | Winter 2016