Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/348

equilibrated, and in their greatest curvity, one while with their convexity towards the upper part, and another while towards the lower part of the Solar Discus. And because those positions are in continuall alteration, making the inclinations and incurvations now greater, now lesser, and sometimes reduce themselves, the first sort to perfect libration, and the second to perfect perpendicularity, it is necessary to assert that the self same Axis of the monethly revolution of the spots hath a particular revolution of its own, whereby its Poles describe two circles about the Poles of another Axis, which for that reason ought (as I have said) to be assigned to the Sun, the semidiameter of which circles answereth to the quantity of the inclination of the said Axis. And it is necessary, that the time of its Period be a year; for that such is the time in which all the appearances and differences in the courses of the spots do return. And that the revolution of this Axis, is made about the Poles of the other Axis parallel to that of the Ecliptick, & not about other points, the greatest inclinations and greatest incurvations, which are always of the same bigness, do clearly prove. So that finally, to maintain the Earth fixed in the centre, it will be necessary to assign to the Sun, two motions about its own centre, upon two several Axes, one of which finisheth its conversion in a year, and the other in lesse than a moneth; which assumption seemeth, to my understanding, very hard, and almost impossible; and this dependeth on the necessity of ascribing to the said Solar body two other motions about the Earth upon different Axes, describing with one the Ecliptick in a year, and with the other forming spirals, or circles parallel to the Equinoctial one every day: whereupon that third motion which ought to be assigned to the Solar Clobe about its own centre (I mean not that almost monethly, which carrieth the spots about, but I speak of that other which ought to passe thorow the Axis and Poles of this monethly one) ought not, for any reason that I see, to finish its Period rather in a year, as depending on the annual motion by the Ecliptick, than in twenty four hours, as depending on the diurnal motion upon the Poles of the Equinoctial. I know, that what I now speak is very obscure, but I shall make it plain unto you, when we come to speak of the third motion annual, assigned by Copernicus, to the Earth. Now if these four motions, so incongruous with each other, (all which it would be necessary to assign to the self same body of the Sun) may be reduced to one sole and simple motion, assigned the Sun upon an Axis that never changeth position, and that without innovating any thing in the motions for so many other causes assigned to the Terrestrial Globe, may so easily salve so many extravagant appearances inthe