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FORCES WHICH ACT IN ONE PLANE BUT NOT UPON THE SAME
POINT OF A RIGID BODY.
39–57. The Theory of Couples.....
58–63. Parallel forces in a plane
64-68. Non-parallel forces in a plane ......
FORCES NOT IN ONE PLANE
WHICH AOT UPON DIFFERENT
POINTS OF A RIGID BODY.
71–77. Parallel forces not in a plane
78–80. Resultant of three couples
81–95. Any forces acting on a rigid body
96. Equilibrium of three forces acting on a rigid body
98—101. Conditions of equilibrium of any forces made general
186—230. MECHANICAL INSTRUMENTS
189–194. The Lever
195–207. The Pulley .......
208–211.· The Wheel and Axle...........
212–214. The Inclined Plane
215. The Screw
216. The Wedge
217. General Property of Machines
218. White's Pulley
219. Hunter's Screw
220. Compound Wheel and Axle
221. The Genou.
223—225. Toothed Wheels.....
226. The Endless Screw
227. The Common Balance
228. The Steelyard
229. The Danish Balance
230. Roberval's Balance
249. Roofs and Bridges........
250–267. The Catenary.......
S TA TICS.
DEFINITIONS AND PRELIMINARY NOTIONS.
1. In the Science of MECHANICS of which STATICS forms a part, matter is considered as essentially possessing extension, figure and impenetrability. The least conceivable portion of matter is called a particle.
2. We conceive of matter that it can exist either in a state of rest, or motion. If then matter, once at rest, pass into a state of motion, the change, not being essential to the existence or nature of matter, is of necessity ascribed to some agent, which, as to its nature, is essentially independent of the matter influenced.
Whether this agent reside in the matter influenced, or in external objects, or in both, are questions which can only be answered after experimental investigation. This agent is called force; and
. it will be perceived from this statement, that a force is judged of entirely by the effects which it produces : and hence, if in the same circumstances two forces produce equal effects, we infer that the forces are equal.
3. It is assumed, that the effect of two equal forces acting in concert, is double the effect of one of them; three, treble; and so on.
The reason of its being necessary to make this an assumption is, that in our ignorance of the nature of force,
are compelled to judge of it by the change which it produces in the state of rest or motion of matter; and it is obvious, that we can no more judge that one such change