Measure being a certain Quantity of Sensation interwove with our Ideas of Distance and Duration, proceeding from a Reflection of what is impressed upon the Mind by some external Object, I must again return to our Mother of Ideas the Earth, and from thence, as I did, of Distance, frame the original Images best suited to the Understanding, proper for our Judgment of Duration. Time takes its first Denomination from the diurnal Rotation of the Earth upon its Axis, which we call a natural Day, and this for obvious Reasons we subdivide in twenty-four Parts or Hours. This diurnal Motion having been successively repeated, and the Day renewed three hundred and sixty-five Times, we find that all the vegetable World has gone through all its Variegations, and Nature has again put on the same Face, adapted to the Season; during which Time, and indeed which occasions this general Change and Repetition, the Earth is found to make one intire Revolution round the Sun. This Space, or Period of Time, we call a solar, or rather a natural Year; and from our Sensibility of this, and its constituent Parts, both horary and diurnal, we form our general Judgment of Duration. _Saturn_, the most remote, and most regular Planet in our System, as has been said before, performs one Revolution round the Sun in about twenty-nine of the above solar Years: The great Comet of 1680 makes but one periodical Return in five hundred and seventy-five of those Years, and the general Motion of the Stars, arising from the Procession of the Equinoxes, altogether continually changing their Aspect, or Position, at the Rate of 50″ _per_ Year round the ecliptic Poles, compleats but one Revolution in 25920 Years; in which Time the whole sidereal Frame of Heaven has changed, and every Star returned to the same Point of the solar Sphere it set out from. This is by many called the great _Saturnian_ Year: Concerning which, Mr. _Addison_ has thus translated an eminent Author. When round the great _Saturnian_ Year has turn'd, In their old Ranks the wandering Stars shall stand, As when first marshall'd by the Almighty's Hand. Addison. Now, if this sidereal Revolution, arising from a secondary Cause, require this Number of Years to perfect one Rotation, what must their primitive Orbits take to circumscribe the _Vortex Magnus_.