6. Sharpness in the points and features of the fern-like structure of the crystals, is a sign of fine weather; but when they begin to break up, and are badly defined, unsettled weather may be expected. Admiral FitzRoy, in _The Weather Book_, writes of this instrument as follows:--"Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, _preferably_, in the outer air, the chemical mixture in a so-called storm-glass varies in character with the _direction_ of the wind--not its force, _specially_ (though it _may_ so vary in _appearance_, only from another cause, _electrical tension_). "As the atmospheric current veers toward, comes from, or is only _approaching_ from the polar direction, this chemical mixture--if closely, even microscopically watched--is found to grow like _fir_, _yew_, fern leaves, or hoar-frost--or like crystallizations. "As the wind, or great body of air, tends more from the _opposite_ quarter, the lines or spikes--all regular, hard, or crisp features--gradually diminish, till they vanish. "Before, and in a continued southerly wind, the mixture sinks slowly downward in the vial, till it becomes shapeless, like melting white sugar. "Before, or during the continuance of a northerly wind (polar current), the crystallizations are beautiful (if the mixture is correct, the glass a _fixture_, and duly _placed_); but the least motion of the liquid disturbs them. "When the main currents meet, and turn _toward the west_, making _easterly_ winds, stars are more or less numerous, and the liquid dull, or less clear. When, and while they _combine by the west_, making westerly winds, the liquid is clear, and the crystallization well-defined, without loose stars. "While _any hard_ or _crisp_ features are visible below, above, or at the top of the liquid (where they form for polar winds), there is _plus_ electricity in the air; a _mixture_ of polar current co-existing _in that locality_ with the opposite, or southerly. "When nothing but soft, melting, sugary substance is seen, the atmospheric current (feeble or strong as it may be) is southerly with _minus_ electricity, unmixed with, and _uninfluenced_ by, the contrary wind. "Repeated trials with a delicate galvanometer, applied to measure electric tension in the air, have proved these facts, which are now found useful for aiding, with the barometer and thermometer, in forecasting weather. "Temperature affects the mixture much, but not solely; as many comparisons of winter with summer changes of temperature have fully proved. "A confused appearance of the mixture, with flaky spots, or stars, in motion, and less clearness of the liquid, indicates south-easterly wind, probably strong to a gale.