SEC. VII. OF NATURAL PHILOSOPHY. 32? K L, P O PROPOSITION XXXIV. THEOREM XXV1I1. If iu a rare medium, consisting of equal particles freely disposed at equal distances from each other, a globe and a cylinder described on equal diameters move with equal velocities in the. direction of the axis of the cylinder, the resistance of the globe ivill be but half so great an that of the cylinder. For since the action of the medi um upon the body is the same (by Cor. 5 of the Laws) whether the body move in a quiescent medium, or whether the particles of the medium impinge with the same velocity upon the quiescent body, let us consider the body as if it were quiescent, and see with what force it would be impelled by the moving medium. Let, therefore, ABKI represent a spherical body described from the centre C with the semi-diameter CA, and let the particles of the medium impinge with a given velocity upon that spherical body in the directions of right lines parallel to AC : and let FB be one of those right lines. In FB take LB equal to the semi-diameter CB, and draw BI) touching the sphere in B. Upon KG and BD let fall the per pendiculars BE, LD ; and the force with which a particle of the medium, impinging on the globe obliquely in the direction FB, would strike the globe in B, will be to the force with which the same particle, meeting the cylinder ONGQ, described about the globe with the axis ACI, would strike it perpendicularly in b, as LD to LB, or BE to BC. Again ; the efficacy of this force to move the globe, according to the direction of its incidence FB or AC, is to the efficacy of the same to move the globe, according to the direction of its determination, that is, in the direction of the right line BC in which it impels the globe directly, as BE to BC. And, joining these ratios, the efficacy of a particle, falling upon the globe obliquely in the direction of the right line FBy to move the globe in the direction of its incidence, is to the efficacy of the same particle falling in the same line perpendicularly on the cylinder, to move it in the same direction, as BE2 to BC3. Therefore if in 6E, which is perpendicular to the circular base of the cylinder NAO, and equal to the radius AC, we take £H equal to BEa -— • ; then 6H will be to 6E as the effect of the particle upon the globe t \~i\j the effect of the particle upon the cylinder. Arid therefore the solid which is formed by all the right lines 6H will be to the solid formed by all the right lines />E as the effect of all the particles upon the globe to the effect of all the particles upon the cylinder. But the former of these solids is a