_Saturn_ is found to complete one Revolution round the Sun in twenty-nine Years, one hundred and seventy-four Days, six Hours, and thirty-six Minutes; at the Distance of about seven hundred and seventy-seven Millions of Miles. _Jupiter_ performs a like Revolution in about eleven Years, three hundred and seventeen Days, twelve Hours, and twenty Minutes; distant from the Sun about four hundred and twenty-four Millions of Miles. _Mars_ compleats his Circuit in one Year, three hundred and twenty-one Days, twenty-three Hours, and twenty-seven Minutes; and his mean Distance is about one hundred and twenty-three Millions of Miles. These three are called superior Planets, as being farther from the Sun than the Earth, and circumscribing its Orbit. The Earth circumambulates her Orbit in one solar Year, _viz._ in three hundred and sixty-five Days, five Hours, forty-eight Minutes, and fifty-seven Seconds; at the mean Distance of eighty-one Millions of Miles. The Radius of _Venus's_ Orbit is about fifty-nine Millions of Miles; and that of _Mercury_ nearly thirty-two Millions, _ditto_. The Heliocentric Revolution of _Venus_, is made in two hundred and twenty-four Days, sixteen Hours, forty-nine Minutes, and twenty-seven Seconds; and that of _Mercury_, in eighty-seven Days, twenty-three Hours, fifteen Minutes, and fifty-four Seconds. These two last Planets are called inferior Ones, as being circumscribed by the Earth. The Diameter of the Sun being demonstrated to be nearly seven hundred and sixty-three thousand Miles: The proportional Magnitudes of all the above Planets will be found nearly as follows, _viz._ The Diameter of the Globe, Of _Mercury_ 4,240 } _Venus_ 7,900 } the Earth 7,970 } Miles. _Mars_ 4,440 } _Jupiter_ 81,000 } and _Saturn_ 61,000 } Thus much I have thought proper to premise, and for your immediate Inspection, have added the following Schemes, that nothing may be wanting to give a general Idea of the Order of the celestial Bodies in our own System, before I attempt to lead you through the neighbouring Regions of the Stars to the more remote Tracts of Infinity. PLATE IV. Is a true Delineation of the solar System, with the Trajectories of three of the principal Comets, whose Periods and Orbits have been accurately determined, and are represented in their true Proportion and Position to one another, and the Order of the Planets round the Sun, marked with their respective Characters, _viz._ ♄, for _Saturn_, ♃, _Jupiter_, ♂, _Mars_, ♁, the Earth, ♀, _Venus_, and ☿, _Mercury_. The Scale being nearly five hundred and eighteen Millions of Miles to an Inch. PLATE V.