Page 24 - January 2009

ideas regarding the speed of sound. In 1700, there were many strange ideas. Some workers thought that wind had no effect on the speed of sound; others thought that any wind, no mat- ter what the direction, retarded sound; still others believed that sound was affected strongly by barometric pressure. Most of these ideas were ill-considered and supported only by observations made in ambiguous circumstances. For example, Kircher did not demonstrate the proper depend- ence of sound speed on temperature. Derham countered sloppiness with careful, repeated observations but his tech- niques could not resolve the small temperature dependence of sound speed. Derham said that “...whether heat or cold dries the air,...whether summer or winter,...the velocity of sound is neither greater nor less...” In hindsight, he was clearly wrong. He would have been safe to say that such changes must be smaller than his ability to resolve. He would also have been safe to say that others in writing about appar- ent variations in sound speed were not making careful enough measurements to support their claims. But he didn’t.
Later, Derham writes, “But as regards thick fogs, it is cer- tain that they are dampers of sound in the highest degree.” The fact that this statement follows a lengthy discussion of the variability of observations in clear and foggy air is uni- formly ignored by those who lifted this quote to support their opposition to acoustic fog signals in the mid 1800’s. Derham is wrong but the lack of discussion makes one wonder whether he is simply repeating the opinion of others. In any event, this remark is a mystery: contrary evidence is easy to find. Derham did not describe any personal observations in heavy fog; his sound-speed measurement could not be made if the source could not be seen and this lack of personal observation may excuse his naïve acceptance of current belief. Regarding other questions, Derham constructs meas- urements to support or refute the question; here, he does not.
Finally, Derham speculates on the connection between wind and sound and the discussion reveals a peculiar (to us today) view:
“...only will I observe as to sounds, to wit, that while their motion is accelerated by wind it is plain that those parts of the atmosphere by which sounds are impressed or propagated are not the same as those from which winds are blown, but certain other more ethereal and volatile parts, as one may suppose. For the fleetest winds do not pass through more than 60 miles in an hour, but sounds travel more than ...778 miles in the same time.”
Derham assumes that, because the speeds of wind and sound are so much different, that they must be carried by dis- tinctly different parts of the air, one more ethereal than the other. It is quite possible that Derham didn’t understand the basis of Newton’s expression for the speed of sound since Newton based his calculation on the ordinary properties of air.
Where Derham was correct
Notwithstanding Derham’s detractors, he was right more often than he was wrong. His measurements were sound and showed remarkably little variance considering the equipment that was available to him. The care with which he made his measurements served as a model for those to follow:
“I have selected these observations from very many oth- ers, all of them being cautiously made and each one repeated two or three times or oftener...”
Derham was the first to establish clearly the dependence of sound speed on wind speed and direction.
He understood the psychology of measurement and
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Derham’s De Motu Soni 23