audible quite across the glacier, the distance being half a mile, while the experiment was rendered reciprocal by one of my assistants making his voice audible to me.
The flakes here were so thick that it was only at intervals that I was able to pick up the retreating forms of the men. Still the air through which the flakes fell was continuous. Did the flakes merely yield passively to the sonorous waves, swinging, like the particles of air themselves, to and fro as the sound-waves passed them? Or did the waves bend by diffraction round the flakes, and emerge from them without sensible loss? Experiment will aid us here by showing the astonishing facility with which sound makes its way among obstacles, and passes through tissues, so long as the continuity of the air in their interstices is preserved.
A piece of mill-board or of glass, a plank of wood, or the hand, placed across the open end t' of the tunnel a b c d (see page 689), intercepts the sound of the bell, placed in the padded box P, and stills the sensitive flame k (described in the last article).
An ordinary cambric pocket-handkerchief, on the other hand, stretched across the tunnel-end produced hardly an appreciable effect upon the sound. Through two layers of the handkerchief the flame was strongly agitated; through four layers it was still agitated; while through six layers, though nearly stilled, it was not entirely so.
Dipping the same handkerchief into water, and stretching a single wetted layer across the tunnel-end, it stilled the flame as effectually as the mill-board or the wood. Hence the conclusion that the sound-waves in the first instance passed through the interstices of the cambric.
Through a single layer of thin silk the sound passed without sensible interruption; through six layers the flame was strongly agitated; while through twelve layers the agitation was quite perceptible.
A single layer of this silk, when wetted, stilled the flame.
A layer of soft lint produced but little effect upon the sound; a layer of thick flannel was almost equally ineffectual. Through four layers of flannel the flame was perceptibly agitated. Through a single layer of green baize the sound passed almost as freely as through air; through four layers of the baize the action was still sensible. Through a layer of close hard felt, half an inch thick, the sound-waves passed with sufficient energy to sensibly agitate the flame. I did not witness these effects without astonishment.
A single layer of thin oiled-silk stopped the sound and stilled the flame. A single layer of gold-beater's-skin did the same. A leaf of common note-paper, or even of foreign post, stopped the sound.
The sensitive flame is not absolutely necessary to these experiments. Let a ticking watch be hung six inches from the ear, a cambric handkerchief dropped between it and the ear scarcely sensibly affects the ticking, a sheet of oil-skin or an intensely heated gas column cuts it almost wholly off.
