Page 25 - January 2009

tried to avoid the pitfalls. He understood that it was difficult to depend on vision both to watch the timing pendulum and to watch for the flash of a distant gun; he replaced the simple pendulum with an automatic pendulum that he could hear while watching, often through a telescope, for the muzzle flash. He attributed Kircher’s faulty conclusions to confusion between clarity of sound and the apparent speed.
He also understood uncertainty in measurement and he realized that the longer the measurement baseline, the less the impact of an uncertainty in timing.
He did not ascribe to the then popular (and incorrect) belief that optical transparency implied acoustical trans- parency and the optical opacity implied acoustical opacity.
Summary
When considered in the context of the times, Derham’s work was exceptional. He first analyzed the weaknesses with other investigators’ techniques and then he designed his own measure- ments to avoid these problems. He was rarely dogmatic; he would often point out inconsistencies in his own observations. [And the few times that he made unsubstantiated statements, he was more often wrong than right. There’s a lesson here.] Unfortunately, several of his less-well-considered statements were quoted then re-quoted well into the 1800’s – even then, the “sound bite” was more attractive than dogged pursuit of truth.
Derham’s work had a substantial impact on research and literature for about 150 years after which it faded into obscu- rity. The impact was not always positive—quotes often prop- agated from paper to paper bereft of their context. [Certainly this would “never” happen today.] Derham wrote that fresh- ly fallen snow seemed to deaden sounds dramatically until the snow became compacted. This statement is true but it was twisted by subsequent authors into a statement that falling snow hindered the transmission of sound.
More often than not, Derham bases his conclusions (or lack of conclusion) on multiple observations and, if those observations are inconsistent, he says so. Before his brief statement about heavy fog, Derham shares contradictory observations regarding the effects of light rain and fog. One has to wonder, given Derham’s openness about contrary observations and his deference to “experts,” whether the dog- matic statement regarding heavy fog is anything but an echo of the expert opinion of the day.
In the cultural climate of England at Derham’s time, there was no great divide between the pursuit of religion and the pursuit of science. Derham was twice invited (1711 and 1712) to give the prestigious Boyle Lectures—lectures for the pur- pose of communicating to the public the connection between science and religious thought. Derham’s most widely distrib- uted book—Physico-Theology—expounded this connection. His earliest book was technical rather than philosophic: The Automatic Clockmaker is a summary of the state of the art of
timekeeping instruments. This background positioned him well for timing his sound speed measurements.
He published 44 papers for the Proceedings of the Royal Society of London on topics ranging from meteorology to astronomy and entomology. He edited and published the sci- entific memoirs of Robert Hooke and the naturalist, John Ray. Derham was Rector of Upminster and his access to the church was particularly useful. A doorway and platform were installed in the tower of St. Laurence’s Church in Upminster. Derham used this platform as an observation point for many of his sound speed measurements and his astronomical and meteorological investigations. (See Fig. 4.)
William Derham and James Welling, who translated De Motu Soni, each stood at crossroads: Derham’s paper at the start of the 18th century on the motion of sound marked the beginning of high-quality measurements of the speed of sound; the connection he drew between wind and sound opened new avenues. Welling—another amateur scientist— contributed to one of the many experiments near the end of the 19th century that established refraction as critical to the understanding of sound propagation in the atmosphere.
Prior to Derham’s paper, confusion reigned in the realm of meteorological effects on sound. While other investigators believed that wind had some effect, Derham demonstrated an effect and got the magnitude and direction about right with respect to sound speed. But it wasn’t until the latter half of the 19th century that refraction or bending of sound by gradients in wind and temperature was demonstrated. Whereas John Tyndall, with his curious idea of flocculence is better known in the community of acousticians, it was left to George Stokes, Osborne Reynolds, and Joseph Henry to elu- cidate refraction of sound.
A. D. Atkinson provides a fitting conclusion [A. D. Atkinson, “William Derham, F.R.S. (1657-1735),” Annals of Science 8(4), 368-392, 1952.]:
“If not an outstanding character, Derham yet emerges from his half-forgotten books and letters with surprising clarity...His busy, rather self-important, investigations, now being rowed round the marshes, now climbing the Chapel at Windsor or the Observatory at Greenwich, discussing the wheel-work of the Hampton Court clock with the Fenchurch Street watch-maker, listening from his church-tower for the echoes of gun-fire from half a dozen neighbouring parishes...all these ...go to make the portrait of a far from unworthy man. If the ...zeal which persuaded him to taste ear-wax and beetles, raise a smile, such trifles add humanity to the picture. If his genius was limited, his curiosity and devotion seem unquestion- able, his career an admirable example of a way of life which, now impossible, has often in the past contributed to the progress of thought and literature in England.”
24 Acoustics Today, January 2009