Oblique Sphere, is that pofition of the globe when either pole is above the horizon lefs than yo degrees.

Oppofition, when two ftars or planets are 180 degrees distant from each other.

Orbit, the path a planet describes in its course round the Sun.

Orbis magnus, the orbit of the Earth.

Parallax, the difference between the places of any celestial body, as feen from the centre, and from the furface of the Earth.

Parallax of the Earth's annual orbit, is the angle at any planet which is fubtended by the diftance between the Sun and Earth.

Parallels of latitude, are small circles of the fphere, parallel to the equator.

Perigeon, that point of a planet's orbit in which it is nearest the Earth.

Perihelion, that point of a planet's orbit nearest the Sun. Pole ftar, a ftar of the second magnitude in the tail of the Greater Bear, fo called from being fituated near the North Pole of the world.

Poles of the world, the two points at the extremities of the Earth's axis.

Preceffion of the equinoxes, a flow motion of these two points, whereby they are found to go backwards about 50 feconds in a year.

Quadrant, the fourth part of a circle; also an inftrument for measuring angles.

Retrograde, is that motion by which fome of the planets feem to go backwards, or contrary to the order of the figns.

Right afcenfion, is that degree of the equator which comes to the meridian with any celestial body, reckoning from the first point of Aries.

Satellites, the fecondary planets.

VOL. II.

MM

Second

Second, the fixtieth part of a minute, either of time, or fpace.

Solftitial points, are the two points in the ecliptic through which the folftitial colure paffes.

Stationary, a planet is faid to be ftationary when it has no apparent motion.

Syftem, a number of bodies revolving round a common centre; as the folar system.

Syzigies, thofe points of the Moon's orbit where he is at the new and full.

Telescopic ftars, are ftars only difcoverable by means of a telescope.

Tranfit, is the paffing of celeftial bodies before oue another. Twilight, that faint light we perceive before the rifing and after the fetting of the Sun, occafioned by the Earth's atmosphere.

Vector radius, a line fuppofed to be drawn from any planet to the Sun, which moving with the planet, defcribes equal areas, in equal times.

Zenith, that point of the heavens directly over our heads. Zodiac, that zone furrounding the heavens on each side of the ecliptic, in which all the planets perform their motions.

CHAP.

СНАР.
C HÁ P. XV.

OF MECHANICS.

1.

Definitions.

THE mechanical powers are certain fimple machines used for raising greater weights, or overcoming greater resistances, than the natural ftrength of man can perform without them.

2. Thefe fimple machines are reckoned fix in number: viz. 1. The lever; 2. the wheel; 3. the pulley; 4. the screw; 5. the wedge; 6. the inclined plane.

3. Force is a power exerted on a body to move it; if it act instantaneously, it is called percuffion, or impulse; if conftantly, it is an accelerative force.

4. Gravity is that force wherewith the body endeavours to fall downwards: it is called abfolute gravity when in an empty space, and relative gravity when immersed in a fluid.

5. Specific gravity is the proportion which the weight of one body bears to that of another.

6. The centre of gravity is a certain point in a body, upon which the body, when fufpended, will rest in any pofition.

7. The centre of motion is a fixed point round about which a body moves. And the axis of motion is that fixed line about which it moves.

8. Power and weight, when opposed to each other, fignify the body that moves another, and the body that is moved;

the

the body which communicates the motion is the power, and that which receives the motion is the weight.

9. Friction is the resistance which any machine suffers by the parts rubbing against each other.

In the practice of mechanics, though all bodies are rough in fome degree, and all engines imperfect; yet it is necessary to confider all planes as perfectly even, all bodies perfectly fmooth, and all bodies and machines to move without friction or refiftance, all lines ftraight and inflexible, all cords very pliable, &c.

SECT. I.

ON THE SIX MECHANICAL POWERS.

THE whole principles of relative motion in mechanics depend upon this one fingle rule;-That the whole force of a moving body is the refult of its quantity of matter multiplied by the velocity of its motion. Thus, when the product arifing from the multiplication of the particular quantities of matter in any two bodies by their respective velocities are equal, the entire forces are fo too. For example:fuppofe a body, A, which weighs 40 pounds, to move at the rate of two miles in a minute; and another body B, which weighs only four pounds, to move 20 miles in a minute: the entire forces with which thefe two bodies will ftrike' against any other would be equal to each other, and therefore it would require equal powers to stop them; for 40 multiplied by 2, gives So, the force of the body A: and 80

is

is also the product of 4, multiplied by 20, the force of the body B. Thus, the heavier any body is, the greater is the power required, either to move or ftop it. And again, the swifter it moves, the greater is its force; therefore, when two bodies are fufpended on any machine, so as to act contrary to each other, if the machine be put in motion, and the perpendicular afcent of one bedy, multiplied into its weight, be equal to the perpendicular defcent of the other body, multiplied into its weight; those bodies, how unequal foever in their weights, will balance one another in all fituations; for as the whole afcent of one is performed in the fame time with the whole defcent of the other, their refpective velocities must be directly as the fpaces through which they move; and the excess of weight in one body is compensated by the excefs of velocity in the other. Upon this principle the power of any machine may be easily computed; for it is only finding how much swifter the power moves than the weight does (that is, how much farther in the fame time), and juft fo much power is gained by the engine.

A lever is a bar, either of iron or of wood, one part of which is supported by a prop, as its centre of motion. And the velocity of every part or point in the lever is directly as its distance from the prop.

There are four kinds of levers:-1. The common lever, where the prop is placed between the weight and power, but much nearer the weight than the power. 2. Where the prop is at one end of the lever, the power at the other end, and the weight between them. 3. Where the prop is at one end, the weight at the other end, and the power applied between them. 4. The bended lever, which differs froin a lever of the first fort only in being bent. Levers of the first and second kind are often used in mechanical engines; but the third kind are feldom ufed, as no power can be gained by them.

When the power is at the same diftance from the prop as

the