Page 18 - January 2009

BACKGROUND AND PERSPECTIVE WILLIAM DERHAM’S DE MOTU SONI (ON THE MOTION OF SOUND)
Thomas B. Gabrielson
Applied Research Laboratory, The Pennsylvania State University State College, Pennsylvania 16804
Author’s note: To make this article consistent with the following article and easier to read, I have enclosed footnotes and comments in square brackets, printed them in blue, and placed them in the article where their reference numbers would have appeared.
“Derham's extensive collection of sound speed measurements forced Newton's hand; Newton's prediction was well below the range of Derham's measurements.”
Introduction
In 1706, the Rector of St. Laurence’s Church in Upminster, William Derham, traveled to the eastern coast of England on a mis- sion. [W. Derham, “De Motu Soni,” Philosophical Transactions, Vol. 26, (1708 - 1709), pp. 2-35.]
“But that nothing might be
wanting in confirmation of
these facts, I made a journey to
Essex Coast. These Sands, washed and covered by the daily time of the sea, make a great and smooth plain for many miles. Upon this plain I measured off only six miles, because neither the time nor my leisure permitted that I should measure a greater distance. At the end of almost each mile I made experiments by the firing of muskets, not without great peril to my life from the influ- ence of the sea and the darkness of night. From these experiments I found that all my former observations were most exact and true, to wit, that sound traverses one mile in 9 1⁄4 half seconds...”
Measuring the speed of sound was a hobby for Derham. Though infrequently referenced in modern literature, Derham’s measurements over a period of several years over- shadowed previous efforts in quantity, quality, and support- ing weather observations. He not only confirmed the inaccu- racy of Newton’s prediction for the speed of sound in air; his measurements also exposed the role of wind in the apparent speed of sound.
Nearly 170 years later, the American scientist Joseph Henry organized a series measurements along another coast, the southern coast of Block Island in Long Island Sound: [J. Henry, Scientific Writings of Joseph Henry, Report of the US Light-house Board for 1875]
“...on the 19th of August [1875] ...General Woodruff and Dr. Welling, starting from the bottom of the cliff below the light-house, went along the beach, one [to the east]...and the other [to the west]. General Woodruff found that the sound of the siren was dis- tinctly heard all the way to the breakwater...Dr. Welling...entirely lost the sound within a quarter of a mile...the wind was in the direction traversed by
General Woodruff and contrary to that of Dr. Welling...”
Although it was not obvious at the time, Henry’s experiments, inspired by the work of Stokes and Reynolds, were part of a revolution in understanding the role of wind in the propagation of sound. The strange behavior of sound had evoked recurring comment for at
least two hundred years. At times, the smoke and flame of nearby gunfire could be seen clearly but the gun’s report could not heard; at other times, cannon discharging far from sight could be heard distinctly. Some of these stories inspired Rev. Derham—a Fellow of the Royal Society of London— to conduct several years’ worth of experiments; some of the same stories led Henry and Dr. James Welling to the beaches of Block Island.
Joseph Henry is well known, though mostly for his con- tributions to the study of magnetic fields (the unit of electri- cal inductance is the henry). In contrast, neither Welling nor Derham appear with any frequency in the literature of acoustics. Welling and Derham were both amateur physicists. Welling was the President of Columbian University—later renamed George Washington University— and an anthro- pologist by vocation; Derham was Rector of the Upminster parish of the Church of England. Both published on acoustics, Derham in the Philosophical Transactions of the Royal Society, Welling in the Bulletin of the Washington Philosophical Society.
Welling’s interest in Henry’s experiments on sound led him to Derham’s paper; however, Derham’s paper was pub- lished in Latin. Rather than relying on fragments of translat- ed passages in other papers, Welling translated the entire paper into English. Welling presented his handwritten trans- lation to the U.S. Weather Service in 1883 and the manuscript survives in the Rare Books Collection at the NOAA Central Library in Silver Spring, Maryland. [A high-quality scanned version of the handwritten manuscript is available on line. See http://docs.lib.noaa.gov/rescue/Rarebook_treasures/QC222D 91708.pdf ]
In the decades preceding Henry’s experiments, there were two competing theories regarding the surprising behav- ior of sound in the atmosphere. Alexander von Humboldt, during his 1799-1800 expedition to the Orinoco River basin,
Foulness Sands on our
Derham’s De Motu Soni 17