The Encyclopedia Britannica, in its CD97ô is silent on the subject of brontides, but explains that one of the four waves of energy accompanying an earth tremor — specifically, a "body wave" called a "P [for Primary] wave" –resembles a sound wave. It is a compressional or longitudinal seismic wave near the earth's surface which gives back-and-forth motion to rock particles along the path of the tremor.
The implication is that when the energy of the subterranean stretching and compression reaches the surface, it sets the air above in motion, producing a short, low sound. This theory was first propounded in 1936, in a study of a recent earthquake at Helena, Montana.3 That would be sufficient to satisfy a Lewisian curiosity, were it not for the fact that the vicinity of Great Falls, Montana, is an area of low potential for earthquakes. The men of the Corps of Discovery would more likely have heard the effects of P waves in the vicinity of the Three Forks of the Missouri River, 200 river miles upstream.
1. Webster's Third International Dictionary (Springfield, Massachusetts: G. & C. Merriam Company, 1971).
2. Robert W. Durrenberger, comp., Dictionary of the Environmental Sciences (Palo Alto, CA: National Press Books, 1973).
3. F. P. Ulrich, "Helena Earthquakes," Bulletin of the Geological Society of America, Vol. 27 (1936), pp. 323-339.