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walls, steeples, and towers; the static momentum of levers, with their combinations into weighing-scales, windlasses, pulleys, funicular machines, inclined planes, screws, catenaria, and all kinds of gearing.

Strength.-Strength is the resistance which a body opposes to a disintegration or separation of its parts. Tensile strength is the absolute resistance which a body makes to being torn apart by two forces acting in opposite directions. Crushing strength is the resistance which a body opposes to being battered or flattened down by any weight placed upon it. Transverse strength is the resistance to bending, or flexure, as it is called. Torsional strength is the resistance which a body offers to any external force which attempts to twist it round. Detrusive strength is the resistance which a body offers to being clipped or shorn into two parts by such instruments as shears or scissors. Working strength. The term "working strength" implies a certain reduction made in the estimate of the strength of materials, so that, when the instrument or machine is put to use, it may be capable of resisting a greater strain than it is expected on the average to sustain.

Tools. By the term tools, according to the definition given by Rennie, we understand instruments employed in the manual arts for facilitating mechanical operations, by means of percussion, penetration, separation, and abrasion, of the substances operated upon, and for all which operations various motions are required to be imparted either to the tool or to the work.

Torsion. Torsion, in mechanics, is the twisting or wrenching of a body by the exertion of a lateral force. If a slender rod of metal, suspended vertically, and having its upper end fixed, be twisted through a certain angle by a force acting in a plane perpendicular to its axis, it will, on the removal of the force, untwist itself, or return in the opposite direction with a greater or less velocity, and, after a series of oscillations, will come to rest in its original position. The limits of torsion, within which the body will return to its original state, depend on its elasticity. A fine wire of a few feet in length may be twisted through several revo

lutions, without impairing its elasticity; and, within those limits, the force evolved is found to be perfectly regular, and directly proportional to the angular displacement from the position of rest. If the angular displacement exceeds a certain limit (as in a wire of lead, for example, before disruption takes place), the particles will assume a new arrangement, or take a set, and will not return to their original position on the withdrawal of the disturbing force. Velocity. Velocity is the rate of motion. Velocity is independent of space and time, but, in order to obtain its value or expression as a quantity, we compare space with time. Thus, when the value of the velocity of a moving body is required, we measure the space which the body passes through, and divide that space by the time of passage, and the quotient is the velocity. Weight. The weight of a body is the force of attraction between the earth and that body. The weight of a body is greatest at the surface of the earth, and decreases above or below that surface. Above the surface, the weight decreases as the square of its distance from the centre of the earth, and below the surface the weight decreases simply as its distance from the centre.

Weights and Measures. -The weights and measures of this country are identical with those of England. In both countries they repose, in fact, upon actually existing masses of metal (brass), which have been individually declared by law to be the units of the system. In scientific theory, they are supposed to rest upon a permanent and universal law of nature the gravitation of distilled water at a certain temperature and under a certain atmospheric pressure. In this aspect, the origination is with the grains, which must be such that 252,458 of these units of brass will be in just equilibrium with a cubic inch of distilled water, when the mercury stands at 30 inches in a barometer, and at 62 degrees in a thermometer of Fah. Unfortunately, the expounders of this theory in England used only the generic term brass, and failed to define the specific gravity of the metal to be employed; the consequence of this omission is to leave room for an error of 0000 in every attempt to reproduce or compare the results. This is the

minimum possible error; the maximum would be a fraction of the difference in specific gravity between the heaviest and lightest brass that can be cast.

The Wheel and Axle.-The wheel and axle may be considered as a perpetual lever, from the constant renewal of the points of suspension and resistance. The fulcrum is the centre of the axis, the longer arm is the radius of the wheel, and the shorter arm the radius of the axis. As the diameters of different circles bear the same proportion to each other that their respective circumferences do, the power is also to the weight as the diameter of the wheel is to the diameter of the axle. If one wheel move another of equal circumference, no power will be gained, as they will both move equally fast. But if one wheel move another of different diameter, whether larger or smaller, the velocities with which they move will be inversely as their diameters, circumferences, or number of teeth. The Wedge. The wedge is a double inclined plane, consequently its principles are the same. Hence, when two bodies are forced asunder, by means of the wedge, in a direction parallel to its head, multiply the resisting power by half the thickness of the head or back of the wedge, and divide the product by the length of one of its inclined sides; the quotient is the force equal to the resistance. The breadth of the back or head of a wedge being 3 inches, its inclined sides each 10 inches, required the power necessary to act upon the wedge so as to separate two substances whose 150 × 1.5 resisting force is equal to 150 lbs.

10

22.5 lbs.

Work is a term in mechanics of recent origin, but of great utility; it means a compound of force, pressure, and motion. Work is said to be performed when a pressure is exerted upon a body, and the body is thereby moved through space. The unit of pressure is a pound, the unit of space a foot; and work is measured by a foot-pound as a unit. Thus, if a pressure of so many pounds be exerted through a space of so many feet, the number of pounds is multiplied into the number of feet, and the product is the number of foot-pounds of work.

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Iron is the most important of all the metals known to man, as

well as the most useful. It has been one of the principal agents

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in the civilization of the human race, and is at the present day more extensively employed in the mechanical arts than any other metal. It is found in different conditions, but always in the state of oxides, or as iron ore, that is, a sort of rusty metallic state. The most common kind the hematite or blood-stone may be described as iron-rust solidified, or rendered concrete by water. After being taken from the ground in the condition of ore, it is placed in a blast-furnace and smelted, after which it is rendered fibrous and ductile by puddling. Spiegel iron or specular cast-iron is, as its name implies, largely crystalline, presenting bright, mirror-like, cleavage planes.

Wrought-iron varies in specific gravity from 7-8 to 7.6; taking the mean at 7-7, a cubic foot will weigh 479-8721664 lbs., or nearly 480 lbs. Cast-iron varies in specific gravity from 7.5 to 6.9, the average being 7.2.

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Cast-iron is composed of about 91 per cent. of iron, 5 of carbon, 2 of silicon, and 2 parts of sulphur phosphorus, and other impurities. It also contains manganese, nickel, cobalt, chromium, vanadium, titanium, and tungsten, in minute quantities. The parts of steam-engines generally made of wrought-iron are the link, eccentric-rods and straps, valve- and piston-rods, connectingrods, air-pump levers, cross-heads for pumps, arms, etc.

Rust. The red powder that falls from iron which has long been subjected to the action of moisture, is the oxide of the metal, and is termed rust.

Steel is one of the chemical modifications of iron, a combination of iron and carbon. It is composed of 98.6 of iron and 1·4

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