Wall or Cloth. And this Experiment, well managed, infallibly confirms the Theory of the Rainbow, as deduced from the Doc trine of Light and Colours in SECT. II. 22. 30. SECTION SECTION VI. Of MOTION and REST. The LAWS of MoTION explained. Of the Compofition and Refolution of Forces. The Naturé and Ufe of PENDULUMS explained. A new PYROMETER defcribed to measure the Expanfion of Metals. Of the CENTERS of Magnitude, Motion, and Gravity. Of the common Center of Gravity in the Syftem of the Heavenly Bodies. The Principles of MECHANICS explained. Of the feveral Mechanical Powers; the LEVER, the WHEEL and AXLE, the PULLEY, the INCLINED PLANE, the WEDGE, and the SCREW. The ftrongest ARCH, to build upon. The Doctrine of WHEEL CARRIAGES, &c. HE Subject of this Section will be "TH to confider the Nature, Laws, and Phænomena of MOTION, and moving Bodies; the Nature and Ufe of Pendulums; the Center of Gravity; the Theory of Mechanical Powers and Machines; and the Doctrine of Wheel Carriages; and to demonstrate each Particular by Experiments. 2. MOTION 2. MOTION is the Change of Place in Bodies; and Rest is their Continuance in the fame Place. The moft general Caufe of Motion with us, is that Power in the Earth and other Bodies we call GRAVITY, by which they attract, gravitate, or move towards one another, unless hindered by some intervening Obstacle; and indeed this is the Caufe of all Motion in the larger Bodies of the System. 3. There is beĥides this an Animal Gaufe or Power of producing Motion in Bodies. For any Animal can, at Will, move any Body in any Direction, provided that his Mufcular Force be not inferior to the Force of Gravity in that Body; and where this happens to be the Cafe, we have Recourse to Art; and by Means of many Kinds of Mechanical Contrivances, we are enabled to raise and move fuch heavy Bodies as far exceed our natural Strength. And hence we derive the most useful Science of MECHANICS, or Conftruction of Mechanical Powers and Machines. 4. The Manner in which we conceive the Idea of Motion, makes it neceffary to distinguish it into two Kinds; viz. Real and Apparent; for we may not only have an Idea of Motion in Bodies when they do really move, but also when they do not move, but are abfolutely at Reft. This Ambiguity in our Ideas of Motion renders us fubject to many and very great Miftakes in our common |