Tyndall, John, 1820-1893 · London : Longmans, Green · leaf 163
s + and — employed in these figures denotethat the two squares on which they occur are always mov-ing in opposite directions. When the squares marked +are above the average level of the plate, those marked ■—are below it; and when those marked — are above theaverage level, those marked + are below… … Scattering sand once more over its sin-face, I damp oneof the corners of the plate, and excite it by drawing thebow across the middle of one of its sides. The sanddances over the surface, and finally ranges itself in twosharp]y-defined ridges along its diagonals, fig. 65. Thenote here produced is a…
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