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it was not tested (at least with published results) until Tyndall’s negative results. Tyndall (and others) suggested that inhomogeneity and turbulence in the air caused poor sound transmission. Tyndall attempted to test this hypoth- esis in the laboratory but his apparatus exaggerated the inhomogeneity so much that the test results had little mean- ing. Reynolds took Stokes’ hypothesis, identified a critical prediction, and tested the prediction under realistic condi- tions.
Refraction by temperature gradients
The second half of the 19th century saw rapid progress. Reynolds had verified Stokes’ theory. Furthermore, Reynolds realized that any change in the effective wave speed could cause refraction. Wind has a strong effect on the speed of sound in the air but sound speed is also a function of tem- perature. If the temperature of the atmosphere changed with altitude, then so would the sound speed and waves would, again, change direction—bending toward regions of lower sound speed (lower temperature) and away from regions of higher sound speed (higher temperature).
Even in the 1700’s reliable and portable thermometers were available and explorers often carried them. Mountain climbers recorded a general decrease in temperature with altitude but these measurements were suspect: the tempera- tures were influenced by proximity to the ground. So the observed temperature decline may not have been a funda-
Fig. 5. Wind near the Earth’s surface ordinarily increases in strength with altitude. This motion of the air coupled with the natural wave motion causes the waves to tilt downwind. Waves (the red arcs) that start out into the wind (to the left, above), tilt upward and may eventually pass over the head of an observer on that side of the source. Waves that start out with the wind (to the right, above) tilt downward toward the ground. If this happens, then, an observer upwind (to the left) of the source and in the acoustic “shadow” could regain the sound by climbing higher. George Stokes predicted wind-driven refraction in 1857 and in 1873 Osborne Reynolds made a number of experiments with a battery-driven bell to demonstrate what was happening.
with elevation of the observer; and he performed the test—by climbing a tree.
Derham had suggested that optical and acoustical transparency were linked. This hypothesis was testable but
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