Page 30 - January 2009

DE MOTU SONI
by
Rev. William Derham
Philosophical Transactions, London 1708 – Vol. XXVI
Translated by
Dr. J. C. Welling
1882
Annotated by
Thomas B. Gabrielson
Applied Research Laboratory, The Pennsylvania State University State College, Pennsylvania 16804
Annotator’s note: Unless specifically attributed to either Derham or Welling, all of the annotations and footnotes are mine, are enclosed in brackets, and are printed in blue. Relevant references have been added wherever possible. I have taken small liberties with punctuation and, rarely, changed words (for example, changing interjacent to inter- vening), to avoid excessively awkward constructions. Proper names have occasionally been changed to correspond to modern spelling to facilitate map and web searches for fur- ther information.
An abridged translation appeared as “Experiments and observations on the motion of sound,” Phil.Trans. Royal Soc. Abridged 5, 380-395, 1809, by Hutton, Shaw, and Pearson. This abridgement is a rather dry condensation of the facts of Derham’s paper. Welling attempts to preserve Derham’s style and Welling’s translation is complete. Welling’s translation is handwritten and the original manuscript resides at the NOAA Central Library, Rare Books Division, in Silver Spring, Maryland. A high-quality scanned version is avail- able on the web through the Central Library:
http://docs.lib.noaa.gov/rescue/Rarebook_treasures/QC 222D91708.pdf
J. C. Welling was, at the time of this translation, the President of Columbian College (later named George Washington University) in Washington, DC. Welling also participated in several acoustics experiments performed by Joseph Henry as part of the US Lighthouse Board’s study of the feasibility of fog horns.]
Experiments and Observations on the Motion of Sound, and Other Things Pertaining Thereto,
Made by the
Reverend Mr. W. Derham,
Rector of Upminster Church and
Fellow of the Royal Society of London.
1. Disagreement of the most celebrated authors about the progress of sound and the reason of my undertaking.
The most celebrated philosophers have thought it worth while to inquire into the delightful and mysterious reasoning about sound, and especially about its motion and rate of progress; and since the discrepancy among their observations is great, partly that I might remove my own doubts, partly that I might find amusement in my leisure hours I have
endeavored, as far as I am able; to unfold and decide the whole matter.
And since my instruments are most suitable, and my opportunities for testing the matter were not to be despised, I suppose myself, in doing so, to be merely performing an appropriate duty, as to pay a debt to the philosophical word, especially to our most famous Royal Society which has deigned to admit me into its number.
The dissent among the most celebrated authors about the velocity of sound [See also, J. M. A. Lenihan, “The velocity of sound in air,” Acustica 2(5), 205-212, 1952.] can be seen by a slight glance at the following table, in which is exhibited, (in English feet) the space they ascribe to the progress of sound in a single second of time:
Between the last and the next to the last of these esti- mates, the disagreement is not considerable, and the esti- mate of the French authorities is not much greater than these two. But among the rest the disagreement is great; and the reason of the discord manifestly is that it arises either from a defect in the instruments used or from the inade- quate distance at which the observations were made, or from the effect of wind. [These observations regarding sources of error in measurement represent one of the most significant contributions of Derham’s paper. Derham did not go so far as to assign specific uncertainties to measure- ments—few investigators of this time did—but his identifi- cation of sources of error and his conscious design of exper- iments to minimize these problems sets his work apart from his predecessors.]
A. The apparatus by which some of these distinguished men made their measurement was not automatic, but was a ball suspended by a cord, which vibrates seconds. [The expression, “vibrates seconds,” means that the pendulum period for a full cycle of its motion is two seconds. In other words, the pendulum reached an extreme point in its swing every second—once to the right, then to the left. A pendulum that “vibrates half-seconds” would have a full-cycle period of one second.]
Now to all who are versed in such matters it is obvious that the ball is much less convenient than an automatic instrument, and is not so accurate; since it is necessary that the eye should first be engaged in observing the flash and then should glance towards the ball or pendulum – a process which consumes time and produces confusion. This fact then
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