from the new ones by the manner in which they are shaded; -- the old walls being shaded by diagonal lines, and the new ones by vertical lines. The additions, which are formed of plaster, are shaded by dots instead of lines.
Where the too great height of the breast of a Chimney is occasioned, not by the height of the mantle, but by the too great width of the breast, in that case, (which however will seldom be found to occur,) this defect may be remedied by covering the lower part of the breast with a thick coating of plaster, supported, if necessary, by nails or studs, driven into the wall which forms the breast, and properly rounded off at the lower part of the mantle. -- See fig. 14.
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CHAP. III.
Of the Cause of the Ascent of Smoke. -- Illustration of the Subject by familiar Comparisons and Experiments. -- Of Chimnies which affect and cause each other to smoke. -- Of Chimnies which smoke from Want of Air. -- Of the Eddies of Wind which sometimes blow down Chimnies, and cause them to smoke.Though it was my wish to avoid all abstruse philosophical investigations in this Essay, yet I feel that it is necessary to say a few words upon a subject generally considered as difficult to be explained, which is too intimately connected with the matter under consideration to be passed over in silence. -- A knowledge of the cause of the ascent of Smoke being indispensably necessary to those who engage in the improvement of Fire- places, or who are desirous of forming just ideas relative to the operations of fire, and the management of heat, I shall devote a few pages to the investigation of that curious and interesting subject. -- And as many of those who may derive advantage from these inquiries are not much accustomed to philosophical disquisitions, and would not readily comprehend either the language or the diagrams commonly used by scientific writers to explain the phaenomena in question, I shall take pains to express myself in the most familiar manner, and to use such comparisons for illustration as may easily be understood.
If small leaden bullets, or large goose shot, be mixed with peas, and the whole well shaken in a bushel, the shot will separate from the peas, and will take its place at the bottom of the bushel; forcing by its greater weight the peas which are lighter, to move upwards, contrary to their natural tendency, and take their places above.
If water and linseed oil, which is lighter than water, be mixed in a vessel by shaking them together, upon suffering this mixture to remain quite, the water will descend and occupy the bottom of the vessel, and the oil, being forced out of its place by the greater pressure downwards of the heavier liquid, will be obliged to rise and swim on the surface of the water.
If a bottle containing linseed oil be plunged in water with its mouth upwards, and open, the oil will ascent out of the bottle, and passing upwards through the mass of water, in a continued stream, will spread itself over its surface.
In like manner when two fluids of any kind, of different densities, come into contact, or are mixed with each other, that which is the lightest will be forced upwards by that which is the heaviest.
And as heat rarefies all bodies, fluids as well as solids, air as well as water, or mercury, -- it follows that two portions of the same fluid, at different temperatures, being brought into contact with each other, that portion which is the hottest being more rarefied or specifically lighter than that which is colder, must be forced upwards by this last. -- And this is what always happens in fact.
When hot water and cold water are mixed, the hottest part of the mixture will be found to be at the surface above; -- and when cold air is admitted into a warmed room, it will always be found to take its place at the bottom of the room, the warmer air being in part expelled, and in part forced upwards to the top of the room.
Both air and water being transparent and colourless fluids, their internal motions are not easily discovered by the sight, and when these motions are very slow, they make no impression whatever on any of our
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