_Fig. 7._ Is a Fountain running on Wheels, and made by Air condens'd on the Surface of Quicksilver, and so forcing the Quicksilver to ascend through the Pipe G: And is to shew that the Lines of Projectils, or other Bodies, are not alter'd by the common Motion of the whole Instrument or Floor on which they are plac'd; and that all Motions on the Earth, if it move, will be the same as if it stand still. _Fig. 8._ Is a Parabola with the several Lines belonging to it, in order to demonstrate the Doctrine of Projectils; and particularly the Art of Gunnery. _Fig. 9._ Is an Engine moving on Wheels, that lets a Ball fall down from a Groove through a Hole, as it is in Motion; to shew that it will then fall on the same Point of the Frame that it falls upon when it is at rest; as does a Stone let fall from the Top of the Mast of a Ship under Sail: and that all respective Motions on the Earth must be the very same, while it self moves as if it were at rest. _Fig. 10._ Is a Cylindrical Iron A B, swinging on a Pin E F, in the very same time that a pendulous Body D of two thirds of its Length C D does; to shew that two thirds is the Center of Oscillation or Percussion in all such prismatick or cylindrical Bodies. [[Opticks Plate I. ― Sutton Nicholls sculp:]] 7 OPTICKS. An Explication of the First PLATE. Figure 1. Represents the Foundation of Vision, and of all Opticks whatsoever, by exhibiting to the Eye a Specimen how the Rays of Light do as well originally, as after Reflection or Refraction, spread themselves in right Lines from each Point in every visible Object, as P, to each other Point, as R, R, R, R, R, every way, to be receiv'd by the Eye in any direct Position whatsoever. _Fig. 2._ Represents the known Law of Reflection; that the Angle of Incidence C P D, is equal to that of Reflection C P E, or that the Angle of Inclination D P A is equal to the other E P B. _Fig. 3._ Shews the Reason why a plain Looking-Glass, as A E F B, exhibits the Object C D by the Image _c d_, which is equal to C D, and equidistant from the Glass A _c_ = A C: And in an erect Posture; all depending only on the Equality of the Triangles, whose Vertices are C _c_ : D _d_, and have their common Bases below E and above F, which Glass by forming the same Image _c d_, so to the Eye, as if the real Object C D was at _c d_, must needs shew that Picture in the Place assign'd, without any Inequality of Distance or Magnitude, or any Inversion. _Fig. 4._ Shews the Reason why the same or equal Object, as A B, C D, E F, appears larger when it is nearer, and smaller when farther off: _viz._ on account of the Inequality of the Angles A G B, or M G N, and C G D, or K G L, and E G F or H G I, and the consequent Inequality of the Pictures made by the Rays at the Bottom of the Eye.