689. Why does smoke ascend in a straight line in mild and fine weather?
Because the air is still, and being dry and warm it does not chill the smoke, nor drive it out of its course.
690. Why do the wings of wind-mills turn round?
Because the wind, striking at an angle upon the wings, forces them aside; and as there are four wings all upon the same angle, and fixed upon the same centre, the oblique pressure of the wind causes the centre to rotate.
There is a world of miniature phenomena which has never been fully recognised, in which we may see the mightier works of nature pleasingly and truthfully illustrated.
When the wind blows into the corner of a street, and whirling around, catches straw, dust, and feathers in its arms, and then wheels away, flinging the troubled atoms in all directions,—it is a miniature of the mightier whirlwind, which wrecks ships, uproots trees, and levels houses with the earth.
When a cloud of dust, on a hot summer's day, rises and flies along the thirsty road, making the passenger close his eyelids, and dusting the leaves of wayside vegetation,—it is a miniature of the terrible simoom, which blows from the desert sands, scattering death and devastation in its track.
When steam issues from the tea-urn, and becomes condensed in minute drops upon the window-pane,—the miniature is of the earth's heat, evaporating the waters, and the cold air of night condensing the vapours into dew.
When grass and corn bend before the wind, and are beaten down by its force; when the pond forgets its calm, and rises in troubled waves, casting the flotilla of natural boats that move upon its surface, in rude disorder upon its windward shore,—the little storm is but a miniature of those great hurricanes which wrecked a fleet in the Black Sea, and levelled the encampments of a mighty army.
When the snow that has gathered upon the house-top, warming beneath the smiles of the sun, slips from its bed, and drops in accumulated heaps from the roof,—it is a miniature of those terrible avalanches which in the Pyrenees bury villages in their icy pall, and doom man and beast to death.