In 1977 physicist Jearl Walker offered a more plausible explanation than the P wave theory. Thunder! Loud sounds such as explosions or thunder normally die away to inaudibility at a distance of about 15 miles, so even if one could see distant thunder clouds above mountainous terrain, one would not necessarily hear the accompanying thunder. However, the normally inverse ratio of temperature to altitude can be turned upside down by a frontal inver-sion, in which a cold air mass is driven under warm air, forcing the latter up-ward.1 The thunderclap that accompanies lightning begins close to the ground, radiat-ing sound waves outward and upward. Walker explained:
The speed of sound in air increases as the air temperature increases. Thus, as the sound wave reaches the increasingly warmer air in the stratosphere, the higher portion of the wave will travel faster than the lower portion, causing the wave path to bend over and eventually turn downward. Sound will be heard where it reaches the ground.2
The effect is similar to that of a whisper gallery in a science museum, where two people separated by a considerable distance beneath a parabolic ceiling that is low at both ends and high in the middle, can converse at a whisper, but cannot be heard by persons standing between them.
The phenomenon continued to attract the attention of acousticians and meteorologists throughout the second half of the 20th century. In 2001, what may be the last word on the subject appeared in the book Civil War Acoustic Shadows, by Charles D. Ross.3 Surprisingly, the facts on which it was based had first been recorded immediately after the battle at Gaines's Mill near Richmond, Virginia, on June 27, 1862. Two observers—the Confederate Secretary of War, George Wythe Randolph, and a member of his staff—had stood within a mile and a half of the action and could see the smoke from muskets and the flash of cannon fire, but had not heard a sound, yet the noise of that same battle was clearly heard as far away as 140 miles west of the conflict.
Comparable acoustic anomalies were witnessed in several ensuing engagements, and actually affected battlefield tactics and outcomes. Furthermore, the same coincidence of silent zones with long-distance audibility of gunfire was recorded several times during World War I.
Owens Dictionary (1754)
"Hence it is, that sound is conveyed from one side of a whispering-gallery to the opposite one, without being perceived by those who stand in the middle. . . . Let ABC represent the segment of a sphere; and suppose a low voice uttered at D, the vibrations expanding themselves every way, some will impinge upon the points E, C, &c. and from thence be reflected to the points F, from thence to G, and so on till they all meet in C."
Thus a clearly reasonable explanation for what Lewis described as the "unaccountable artillery of the Rocky Mountains," is based on a combination of acoustic shadows and sound reflectance, or lensing, has been known for nearly 150 years. In fact, Lewis had the key to the solution at his very fingertips. In Owen's Dictionary, the encyclopedia he had brought along on the expedition, was an illustrated article titled "Whispering Places," including descriptions of well-known examples located in two English cathedrals.4 Had Lewis found the "leasure" to ponder the principle, he might have imagined the correlation between a whispering-gallery and the terrestrial atmosphere.
On the other hand, Washington Irving's decidedly post-Enlightenment perspective toward the question is still worth bearing in mind:
In whatever way this singular phenomenon may be accounted for, the existence of it appears to be well established. It remains one of the lingering mysteries of nature which throw something of a supernatural charm over her wild mountain solitudes, and we doubt whether the imaginative reader will not rather join with the poor Indian in attributing it to the thunder spirits, or the guardian genii of unseen treasures, than to any commonplace physical cause.
Well, perhaps the "commonplace physical cause" of this mysterious phenomenon has indeed been settled once and for all. On the other hand, why not throw in with Washington Irving's "poor Indian" and his "supersticious immagination"?
Do we really need to know all the answers?
1. Encyclopædia Britannica Online (accessed 18 November 2001), s.v. "Temperature inversion."
2. Jearl Walker, The Flying Circus of Physics, with Answers (New York: John Wiley & Sons, 1977), pp. 227-28. See also E. G. Richardson, "Sound and the Weather," Weather (London: Royal Meteorological Society), Vol. 2 (June 1947), 169–73, 205–10; and Martin A. Uman, Lightning (New York: Dover, 1969), 196-98.
3. Shippensburg, Pennsylvania: White Mane Books, 2001.
4. A New and Complete Dictionary of Arts and Sciences (London: Printed for W. Owen, 1754), s.v. "Whispering places."