Sound: A Course of Eight Lectures

I have already mentioned the remarkable observation made by General Duane: that during a northeast snowstorm blowing against the sound, the signal at Cape Elizabeth is always heard at Portland, a distance of nine miles. Our observations at the South Foreland—where sound was proven to reach more than twelve miles against the wind—supported by decisive experiments, turn General Duane’s guesses into certainties.

It has, for example, been proven that a couple of gas flames placed in a chamber can, in a minute or two, make its air so uneven in density original: "non-homogeneous" that it practically cuts off the sound. Meanwhile, that same sound passes without any noticeable hindrance through showers of paper scraps, seeds, bran, raindrops, and even the densest fumes and fogs. Sound also passes through thick layers of calico, silk, wool original: "serge", flannel, coarse cloth original: "baize", thick felt, and through pads of cotton netting that even the strongest light cannot penetrate.

As long as the air in which snow, hail, rain, or fog is suspended remains uniform original: "homogeneous", the sound will pass through it, appearing to ignore the suspended particles.¹ This point is illustrated on a large scale by my own observations on the Mer de Glace A large glacier in the French Alps where Tyndall conducted many of his scientific studies., and by those of General Duane at Portland; both prove that snow-filled air from the northeast is a highly uniform medium for sound.

Professor Henry explains why the northeast snow-wind allows the Cape Elizabeth signal to be heard at Portland in this way: he imagines a "theoretical wind" in the higher levels of the atmosphere. This wind blows in the opposite direction of the wind on the ground, always accompanying it and more than canceling out its effect. In making this speculation, he relies on the—

¹ This does not seem more surprising than the passage of light, or radiant heat, through rock salt. Tyndall is comparing how sound travels through "dirty" air to how light travels through a solid crystal; if the medium is uniform, the "clutter" doesn't stop the wave.