Salv.I will use my utmost endeavours to render my self intelligible,The causes of the inequality of the additions and substractions of the diurnal conversion from the annual motion. but the difficulty of the accident it self, and the great attention of mind requisite for the comprehending of it, constrains me to be obscure. The unequalities of the additions and substractions, that the diurnal motion maketh to or from the annual dependeth upon the inclination of the Axis of the diurnal motion upon the plane of the Grand Orb, or, if you please, of the Ecliptick; by means of which inclination the Equinoctial intersecteth the said Ecliptick, remaining inclined and oblique upon the same according to the said inclination of Axis. And the quantity of the additions importeth as much as the whole diameter of the said Equinoctial, the Earths centre being at the same time in the Solstitial points; but being out of them it importeth lesse and lesse, according as the said centre successively approacheth to the points of the Equinoxes, where those additions are lesser than in any other places. This is the whole businesse, but wrapt up in the obscurity that you see.
Sagr.Rather in that which I do no not see; for hitherto I comprehend nothing at all.
Salv.I have already foretold it. Neverthelesse we will try whether by drawing a Diagram thereof, we can give some small light to the same; though indeed it might better be set forth by solid bodies than by bare Schemes; yet we will help our selves with Perspective and fore-shortning. Let us draw therefore, as before, the circumference of the Grand Orb, [as in Fig. 4.] in which the point A is understood to be one of the Solstitials, and the diameter AP the common Section of the Solstitial Colure, and of the plane of the Grand Orb or Ecliptick; and in that same point A let us suppose the centre of the Terrestrial Globe to be placed, the Axis of which CAB, inclined upon the Plane of the Grand Orb, falleth on the plane of the said Colure that passeth thorow both the Axis of the Equinoctial, and of the Ecliptick. And for to prevent confusion, let us only draw the Equinoctial circle, marking it with these characters DGEF, the common section of which, with the plane of the grand Orb, let be the line DE, so that half of the said Equinoctial DFE will remain inclined below the plane of the Grand Orb, and the other half DGE elevated above. Let now the Revolution of the said Equinoctial be made, according to the order of the points DGEF, and the motion of the centre from A towards E. And because the centre of the Earth being in A, the Axis CB (which is erect upon the diameter of the Equinoctial DE) falleth, as hath been said, in the Solstitial Colure, the common Section of which and of the Grand Orb, is the diameter PA, the said line PA shallbe
