. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 408 MOTION. ar« swimming, flying, crawling, climbing, leaping, running, walking, &c. The con- sideration of these diversified methods of progression involves the theory of the mo- tion of bodies in general, of the lever, the pulley, the centre of gravity, specific gravity, and the resistance of fluids, &c.; and, as we shall have occasion for constant refer- ence to the mechanical principles connected with these subjects, they will be first dis- cussed; but for the convenience of those who are unacquainted with th
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 408 MOTION. ar« swimming, flying, crawling, climbing, leaping, running, walking, &c. The con- sideration of these diversified methods of progression involves the theory of the mo- tion of bodies in general, of the lever, the pulley, the centre of gravity, specific gravity, and the resistance of fluids, &c.; and, as we shall have occasion for constant refer- ence to the mechanical principles connected with these subjects, they will be first dis- cussed; but for the convenience of those who are unacquainted with the algebraic method of computation and analysis, the latter will generally be separated from the text. Fundamental Axioms. — First, every body continues in a state of rest or of uniform mo- tion in a right line until a change is effected by the agency of some mechanical force. Secondly, any change effected in the quiescence or mo- tion of a body is in the direction of the force impressed, and is proportional to it in quantity. Thirdly, reaction is always equal and con- trary to action, or the mutual actions of two bodies upon each other are always equal and in opposite directions. Thus if M (Jig. 203) be a particle of matter free to move in any direction, and if the lines MA, MB, represent the intensity of two forces Fig. 203. A -B acting on it in the direction MC, the particle M will move towards C by the combined action of the two forces, and it will require a force in the direction of CM, equal to MA+ MB to keep it in a state of rest: but if MA and MB (Jig. 204) represent the intensities and directions of two forces acting on the par- ticle M in opposite directions, if MA be Fig. 204. M. greater than MB, the particle M will be moved towards A by the difference of these two forces, and it will require a force equal to that difference to keep it at rest. The composition and resolution of forces.— In the composition of forces it is proposed to find the resultant, arising f
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