Page 23 - January 2009

the contrary, his work appears to be the first, clear demon- stration of the effect of wind on sound speed. (See Fig. 3.)
This plot reveals another facet of these measurements. Wind aided the propagation in more instances than it hin- dered the propagation. This may have been a climatological bias—if the prevailing winds were from the west, there may simply have been more opportunities to observe in conditions of wind-aided propagation. However, this could also have been the result of refraction. Once the influence of winds was sus- pected, experimenters realized that they might be able to reduce the effects of the wind on their one-way measurements by repeating the measurement in the opposite direction. On the surface, this seems a good idea but many who attempted the reciprocal measurement were frustrated. Gerrit Moll (1824) writes of his measurements [G. Moll, A. Van Beek, “An account of experiments on the velocity of sound, made in Holland,” Phil. Trans. Roy. Soc. London 114, 424-456, 1824. Zevenboompjes and Kooltjesberg are low hills about 11 miles apart near Amersfoort and Blaricum respectively.]:
“The first night of our experiments, the 23d, 24th, and 25th of January, 1823, we experienced the same annoy- ance of which the French philosophers had to complain the first night of theirs. The report of the shots of Zevenboompjes was not heard at all at the station of Kooltjesberg. But at Zevenboompjes all the shots of Kooltjesberg were distinctly heard.”
Until the effects of wind-induced refraction were under- stood, this was a mystery, but sounds are often far weaker upwind of a source than downwind. Derham’s data set may have been biased by the difficulty in making an observation with propagation against the wind.
It is unfortunate that Derham did not record temperature but the uncertainty in his measurements would have obscured any relation- ship. Criticism in hindsight is wasted effort; it is more useful to examine what he did record. In addition to recording the local wind speed and direction, he also recorded (when possible) the direction of movement of clouds. Cloud motion indicates wind direction at altitude that can be different than the direction at the ground; a difference in wind direc- tion with altitude solved an intrigu- ing mystery a century and a half later. Based on observations, Joseph Henry suspected that, near coastal areas, the surface winds might be directed so as to hinder propagation but that an opposite wind direction at higher altitudes could produce good acoustic transmission. Joseph Henry advocated launching small balloons during acoustical measurements to identify conditions that changed sig-
nificantly with altitude. [Even today, atmospheric acoustic measurements are often made without sufficient supporting meteorological data. Unless the propagation ranges are extremely short, measurements of wind speed vertical gradi- ents and temperature gradients are often necessary (and how many experimenters have neglected to measure humidity in spite of its marked effect on absorption of sound?).]
Where Derham was mistaken
As careful as many parts of Derham’s work were, there are several unfortunate statements in his paper. The first relates to the work of the German Jesuit scholar, Athanasius Kircher:
“Kircher says that he always found the velocity of sound to be different at different times, at morning, at midday, at evening and at night. But I, relying on a better chronometer and using a more suitable distance, never have found that the celerity of sound is different at these times, but in all weather, whether the atmosphere be clear and serene, or cloudy and turbid; whether snow is falling or fog, (which both powerfully blunt the audibili- ty of sound); whether it thunders or it lightnings, whether heat or cold dries the air; whether it be day or night, summer or winter; whether the mercury is rising or falling in the barometer—in a word I may say that in all changes of atmosphere whatsoever (winds only being excepted) the velocity of sound is neither greater nor less. The sound is only more or less clear from this vari- ation of the medium, and perhaps this fact deceives the sagacious Kircher.
In considering this statement, it is important to recog- nize that Derham was disputing more than just Kircher’s
Fig. 4. St. Laurence Church at Upminster. William Derham was Vicar, then Rector of the Upminster parish for much of his adult life. In Derham’s time, there was a door and a platform on the south side of the tower from which he made many of his speed-of-sound, astronomical, and meteorological observations. (Image supplied by Fr. Michael Hore, current Rector of the Upminster Parish.)
22 Acoustics Today, January 2009