Provisions of the Building Act affecting the bricklayer, do not stand back to back, may be built as follows: from the external face of the party-wall to the inward face of the back of the chimney in the cellar story, one brick and a half thick, and in the upper stories, one brick thick from the hearth to twelve inches above the mantle. If such chimney is built against any other wall, the back may be half a brick thinner than above stated. Those backs of chimneys which are not in party-walls of the second, third, and fourth rate, must be in every story one brick thick at least, from the hearth to twelve inches above the mantle. These backs may also be half a brick thinner, if such chimney be built against any other wall. The breasts of chimneys, whether in party-walls or not, are not to be less than one brick thick in the cellar story, and half a brick thick in every other story. All partitions between flues must not be less than half a brick thick. Flues may be built opposite to each other in party-walls, but they must not approach nearer to the centre of such wall than two inches. All chimney breasts next to the rooms, and chimney backs, and all flues, are to be rendered or pargeted. Backs of chimneys, and flues in party-walls against vacant ground, must be lime whited, or marked in some durable manner, but must be rendered or pargeted as soon as any other building is erected to adjoin them. No timber must be over the opening of any chimney for supporting the breast; but all chimneys must have a brick or stone arch, or iron bar or bars. All chimneys must have slabs or foot paces of stone, marble, tile, or iron, at least eighteen inches broad, and at least one foot longer than the opening of the chimney when finished; and such slabs or foot paces must be laid on brick or stone trimmers at least eighteen inches broad from the face of the chimney breast, except there be no room or vacuity beneath, in which case they may be bedded on the ground. Brick funnels must not be made on the outside of the first, second, third, or fourth rate, next to any street, square, court, road, or way, so as to extend beyond the general line of the buildings in such situations. No metallic funnel or other pipe, for conveying smoke or steam, is allowed to be fixed near any public street, square, court, or way, to the first, second, third, or fourth rate, and no such pipe is to be fixed on the inside of any building nearer than fourteen inches to any timber, or other combustible material. Definitions. MECHANICS: THE Science of Mechanics has been very concisely defined, the geometry of motion. It is divided by Sir Isaac Newton into the two branches of practical and rational mechanics. Practical mechanics treats of the six mechanical powers, of one or more of which every machine is composed; and rational mechanics comprehends the whole theory of motion, shows how to determine the motions produced by given powers or forces; and conversely, when the phenomena of the motions are given, how to trace the powers or forces from which they arise. OF MATTER. Every branch of natural philosophy acquaints us with some new properties of matter, of the general nature of which, and of those properties of it which respect mechanical science, we shall here introduce an account. The terms matter, substance, and body, though so nearly allied that they are occasionally employed in the same sense, without creating much confusion of ideas, have different significations, which ought to be understood. Matter is the most general term of the three, and comprehends whatever is capable of making resistance, and is possessed of weight, without regard to figure or quantity. The word substance is compounded of the Latin preposition sub (under,) and the verb stare (to stand,) and approaches very nearly to the signification of the word matter, as it implies that which supports or stands under the different forms and appearances which are presented to our senses. Its meaning is, however, more restricted, and it is generally accompanied by the article, to denote a particular portion of matter. The term body comes from the Saxon, and originally signified the person or form of a man, or other creature; hence it ought to be applied only to a substance possessing a definite form. An examination of the general properties of matter will afford us a better idea of its nature, than can possibly be given by a definition. Some kinds of matter, as metals, wood, stone, &c. are visible, a property dependent upon their opacity, or power of reflecting to our eyes, some or all of the rays of light which fall upon them. Other kinds of matter are invisible, on Properties of matter.-Extension.-Divisibility. account of their perfect transparency, and their existence is ascertained only by their effects. Of this class are the various kinds of gases, for example, the atmosphere or air that we breathe, which, though totally invisible when dry and pure, yet is matter, as much as iron or the hardest body in nature. Among the properties attributed to all matter, the following are too important to be passed over without particular notice, viz. SOLIDITY, EXTENSION, DIVISIBILITY, MOBILITY, INERTIA, ATTRACTION and REPULSION. The solidity of matter here meant is not opposed to fluidity, but expresses that property which every body possesses of not permitting any other body to occupy the same place with it at the same time. This fact is an axiom in philosophy of the most incontestable kind. If a piece of wood or stone occupy a certain space, it must be removed before another body can be put into that space, and though the tyro may suggest that fluids do not oppose such resistance, it is only the facility with which they escape that induces the supposition of their being an exception to this universal property of matter. Under proper circumstances, their solidity is as obvious as that of the most solid substance: the piston of a syringe drawn full of water, cannot be thrust down if the aperture for the jet be stopped; and a pair of bellows filled with air, cannot be compressed if the pipe be closed. The solidity of matter thus understood, is the same with what some writers call its impenetrability. These words, in common language, denote the property of not being easily separated into parts, a meaning very different from that attached to them in the sense just explained, with which it must therefore not be confounded. Extension is another property of matter inseparable from its existence. The idea which we obtain of solidity by the resistance of bodies, and the impossibility of two bodies co-existing in the same identical place, immediately suggest and prove to us that matter is extended, or occupies a certain portion of space. Divisibility is that property by which matter is capable of being separated into parts removeable from each other. We cannot conceive a particle of matter to be so small, as not to consist of two halves; this being the case, we are directly led to the conclusion that matter is capable of being divided to infinity. But, however natural this mode of reasoning appears, it has had many opponents, and those who suppose it just, have been considered as involving themselves in a cloud of palpable contradictions. To assume, it has been said, as a first principle in philosophy, that matter is infinitely divisible, is to Properties of matter.-Divisibility. assert that it has no beginning of substance; that there are no limits between matter and nothing; and that a finite thing has infinite properties. Such being the difficulties attending the assumption of the infinite divisibility of matter, Sir Isaac Newton closes an admirable disquisition on the nature, laws, and constitution of matter, by stating the great probability that God in the beginning formed matter into solid, massive, impenetrable, moveable particles or atoms, of such sizes and figures, and with such other properties, and in such proportion to space, as most conduced to the end for which he formed them; and that these primitive particles being absolute solids, are incomparably harder than any of the bodies compounded of them, even so hard as to be incapable of wearing or breaking in pieces, nothing but infinite power being able to destroy what Infinite Power made one in the first creation. That nature may be lasting, the changes of corporeal things are to be attributed only to the various separations and new associations of these permanent particles, and when compound bodies break, it is not in the midst of solid particles, but where these are laid together and touch only in a few points. By adopting this theory of ultimate atoms, we avoid the toils of metaphysics, although, at the same time, we take for granted what we cannot directly prove. But those propositions which are proved by the absurdity of supposing the contrary, are often as important, and nearly as well entitled to be received, as those which admit of the most palpable demonstration; and although Sir Isaac Newton's conjecture respecting solid, indestructable atoms, has been buffeted among men of science for about one hundred years, it remains to be the prevalent opinion at this day; not because much new light has been thrown upon the subject, since the time of that justly admired philosopher, but because it comports so well with the phenomena of nature, that an assent to it can hardly be denied. The extreme tenuity of certain substances to our general perception, is no proof against their being composed of particles perfectly solid; if a wet bladder be tied over the mouth of a pneumatic jar (that is, a jar open at the bottom) and then gently dried, so as to remain well stretched, and the jar be then placed upon the air-pump, as soon as it is exhausted of air, the atmosphere pressing upon the exterior, will burst the bladder, and falling upon the pump plate, will produce a loud report, like a gun. This effect could not be produced without the intervention of solid particles in air. Further, it seems impossible to account for the power of the most subtil agents of nature, if their ultimate atoms however few they may be in a given compass, were not equal in Properties of matter.-Divisibility. solidity to those which appear to us the hardest. Though the velocity of the electric fluid is immeasurably great, that velocity alone would be insufficient to produce its well-known effects on bodies of the closest texture, if it contained no principle of hardness within itself. Having premised these considerations, we shall now take a view of parts actually separate, and if we shall find that these are so small and so numerous as to surpass imagination, we shall have approximated as nearly as the human understanding can do, to the attainment of an idea of the inconceivable minuteness of those solid, ultimate, indivisible atoms which constitute matter. A pound of so gross a substance as cotton, may be spun into a thread exceeding one hundred yards in length; and the celebrated Boyle speaks of a thread of silk three hundred yards in length, which weighed no more than three grains and a half. But the ductility of gold is still more astonishing; a grain of gold can be hammered by the goldbeaters, until it will cover fifty square inches, and may be divided into two millions of visible parts. The gold which covers the silver wire used in making silver lace, is spread over a surface twelve times greater than in the last mentioned instance. In making this wire, a cylindrical bar of silver is strongly gilt, and afterwards converted into wire by drawing it successively through holes diminishing in magnitude, formed in plates of steel. By this means the surface is prodigiously augmented; but the wire still remains gilt, and preserves a uniform appearance, even when examined by the microscope. Sixteen ounces of gold, which would not occupy more space than one cubical inch and a quarter, will completely gild a wire sufficient to encompass the whole globe of the earth. The metallic particles in acid solutions are still more minutely divided. A single grain of copper dissolved in an ounce of diluted nitrous acid, will impart a green colour to a gallon of water, or cover one thousand square inches of bright iron with a coat of copper. The odour of all bodies that excite the sensation of smell cannot be given out without a waste of their substance; yet this waste is so very small, that is, the fragrant parts of bodies are endued with such prodigious divisibility, as in general, for long periods, to occasion no perceptible diminution of the substance by which it is sustained. The odour of a grain of musk will continue for twenty years in an apartment where fresh air is admitted every day. Instances of the wonderful divisibility of matter are very abundant. Gunpowder, when exploded, expands to two hundred and forty-four times the bulk it occupied |