I HAVE now gone through the first part of my design, and have explained, as far as the nature of my undertaking would permit, what Sir ~ISAAC NEWTON~ has delivered in general concerning the motion of bodies. It follows now to speak of the discoveries, he has made in the system of the world; and to shew from him what cause keeps the heavenly bodies in their courses. But it will be necessary for the use of such, as are not skilled in astronomy, to premise a brief description of the planetary system. 2. THIS system is disposed in the following manner. In the middle is placed the sun. About him six globes continually roll. These are the primary planets; that which is nearest to the sun is called Mercury, the next Venus, next to this is our earth, the next beyond is Mars, after him Jupiter, and the outermost of all Saturn. Besides these there are discovered in this system ten other bodies, which move about some of these primary planets in the same manner, as they move round the sun. These are called secondary planets. The most conspicuous of them is the moon, which moves round our earth; four bodies move in like manner round Jupiter; and five round Saturn. Those which move about Jupiter and Saturn, are usually called satellites; and cannot any of them be seen without a telescope. It is not impossible, but there may be more secondary planets, beside these; though our instruments have not yet discovered any other. This disposition of the planetary or solar system is represented in fig. 89. 3. THE same planet is not always equally distant from the sun. But the middle distance of Mercury is between ⅕ and ⅖ of the distance of the earth from the sun; Venus is distant from the sun almost ¾ of the distance of the earth; the middle distance of Mars is something more than half as much again, as the distance of the earth; Jupiter’s middle distance exceeds five times the distance of the earth, by between ⅕ and 1/6 part of this distance; Saturn’s middle distance is scarce more than 9½ times the distance between the earth and sun; but the middle distance between the earth and sun is about 217⅛ times the sun’s semidiameter. [Illustration] 4. ALL these planets move one way, from west to east; and of the primary planets the most remote is longest in finishing its course round the sun. The period of Saturn falls short only sixteen days of 29 years and a half. The period of Jupiter is twelve years wanting about 50 days. The period of Mars falls short of two years by about 43 days. The revolution of the earth constitutes the year. Venus performs her period in about 224½ days, and Mercury in about 88 days.