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- Let V i and V2 be the potentials at points just outside and inside the surface dS, and let n l and n 2 be the
**normals**to the surface dS drawn outwards and inwards; then - dV i /dn i and - dV 2 dn 2 are the normal components of the force over the ends of the imaginary small cylinder. - But, as originally pointed out by Euler, the difficulty can be turned if we notice that in the ordinary trajectory of practice the quantities i, cos i, and sec i vary so slowly that they may be replaced by their mean values,, t, cos 7 7, and sec r t, especially if the trajectory, when considerable, is divided up in the calculation into arcs of small curvature, the curvature of an arc being defined as the angle between the tangents or
**normals**at the ends of the arc. - Properties of the limagon may be deduced from its mechanical construction; thus the length of a focal chord is constant and the
**normals**at the extremities of a focal chord intersect on a fixed circle. - Apollonius' genius takes its highest flight in Book v., where he treats of
**normals**as minimum and maximum straight lines drawn from given points to the curve (independently of tangent properties), discusses how many**normals**can be drawn from particular points, finds their feet by construction, and gives propositions determining the centre of curvature at any point and leading at once to the Cartesian equation of the evolute of any conic. - Now, if one of these latter children of the second brother marries a cousin - a child of the first brother, - their offspring, if large enough, will consist of some pure
**normals**N, impure**normals**N(A), and of albinoes A.

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