Because the draught of air is not sufficient to supply the wants of the fire, and enable it to create an _upward current_. 684. _What is the best method of conveying air to fires?_ Tubes built in the walls, communicating with the outer air, and terminating _underneath the grates_. 685. _Why is this the best method of ventilation?_ Because doors and windows may then be made air-tight, and _draughts across rooms be prevented_. 686. _Why do chimneys that stand under elevated objects, such as hills, trees, and high buildings, smoke?_ Because the wind, striking against the elevated object, _flies back_, and a part of it _rushes downward_. 687. _Why do sooty chimneys smoke?_ Because the accumulation of the soot _diminishes the size of the flue_, and lessens the ascensive power of the draught, by reducing the quantity of _warm air_. It also obstructs the motion of the air, by the _roughness of its surface_. 688. _Why do chimneys smoke in damp and gusty weather?_ Because the ascending air is _suddenly chilled_ by gusts of damp and cold air, and driven down the chimney. [Verse: "Remember that thou magnify his work, which men behold. Every man may see it; man may behold it afar off."--JOB XXXVI.] 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_.