The INTRODUCTION:

The Diurnal Rotation of the Planetsround their Axes, was discovered by certainSpots which appear on the Surfaces. TheseSpots appear first in the Margin of the Pla-nets Disks (or the Edges of their Surfaces)and seem by degrees to creep towards theirMiddle; and so on, going still forward, tillthey come to the opposite Side or Edgeof the Disk, where they set or disappear:And after they have been hid for the sameSpace of Time that they were visible, theyagain appear to rise, in or near the samePlace as they did at first; then to creep onprogressively, taking the same Course as theydid before. These Spots have been observedon the Surfaces of the Sun, Venus, Mars,and Jupiter ; by which means it has beenfound that these Bodies turn round theirown Axes, in the Times before - mention’d.It is very probable that Mercury and Saturnhave likewise a Motion round their Axes,that all the Parts of their Surface may al-ternately enjoy the Light and Heat of theSun, and receive such Changes as are properand conveinent for their Nature. But byreason of the Nearness of § to the Sun;and b’s immense Distance from him, no Ob-servations have hitherto been made where-by their Spots (if they have any ) could bediscovered, and therefore their diurnal Mo-tions could not be determined. The diur-nal Motion of the Earth is computed from

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