Second, the fixtieth part of a minute, either of time, or Space. 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 the is at the new and full. Telefcopic ftars, are stars only difcoverable by means of a telescope. Tranfit, is the paffing of celeftial bodies before another. Twilight, that faint light we perceive before the rifing and after the setting 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 fide of the ecliptic, in which all the planets perform their mo tions. CHAP. Oblique Sphere, is that pofition of the globe when either pole is above the horizon less than 90 degrees. Oppofition, when two ftars or planets are 180 degrees diftant from each other. Orbit, the path a planet defcribes in its course round the Sun. Orbis magnus, the orbit of the Earth. Parallax, the difference between the places of any celestial body, as seen from the centre, and from the surface of the Earth. Parallax of the Earth's annual orbit, is the angle at any planet which is fubtended by the distance between the Sun and Earth. ' Parallels of latitude, are small circles of the sphere, parallel to the equator. Perigeon, that point of a planet's orbit in which it is nearest the Earth. Peribelion, that point of a planet's orbit nearest the Sun. Pole fiar, a ftar of the fecond magnitude in the tail of the Greater Bear, so 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; alfo 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 celeftial body, reckoning from the first point of Aries. Satellites, the fecondary planets. VOL. II. Mm Second, 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 fuffers 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 neceffary 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 refpective 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 zo miles in a minute: the entire forces with which these two bodies will strike against any other would be equal to each other, and there. fore it would require equal powers to ftop them; for 40 multiplied by 2, gives So, the force of the body A: and 80 |