Let, therefore, A B C (in the first figure) be a sphere, or the portion of a sphere, whose centre is D; and let it be transparent and homogeneous, as crystal, glass, or water, and objected to the sun. Wherefore, the foremost part A B C will, by the simple motion of the sun, by which it thrusts forwards the medium, be wrought upon by the sunbeams in the strait lines E A, F B, and G C; which strait lines may, in respect of the great distance of the sun, be taken for parallels. And seeing the medium within the sphere is thicker than the medium without it, those beams will be refracted towards the perpendiculars. Let the strait lines E A and G C be produced till they cut the sphere in H and I; and drawing the perpendiculars A D and C D, the refracted beams E A and G C will of necessity fall, the one between A H and A D, the other between C I and C D. Let those refracted beams be A K and C L. And again, let the lines D K M and D L N be drawn perpendicular to the sphere; and let A K and C L be produced till they meet with the strait line B D produced in O. Seeing, therefore, the medium within the sphere is thicker than that without it, the refracted line A K will recede further from the perpendicular K M than K O will recede from the same. Wherefore K O will fall between the refracted line and the perpendicular. Let, therefore, the refracted line be K P, cutting F O in P; and for the same reason the strait line L P will be the refracted line of the strait line C L. Wherefore, seeing the beams are nothing else but the ways in which the motion is propagated, the motion about P will be so much more vehement than the motion about A B C, by how much the base of the portion A B C is greater than the base of a like portion in the sphere, whose centre is P, and whose magnitude is equal to that of the little circle about P, which comprehendeth all the beams that are propagated from A B C; and this sphere being much less than the sphere A B C, the parts of the medium, that is, of the air about P, will change places with one another with much greater celerity than those about A B C. If, therefore, any matter combustible, that is to say, such as may be easily dissipated, be placed in P, the parts of that matter, if the proportion be great enough between A C and a like portion of the little circle about P, will be freed from their mutual cohesion, and being separated will acquire simple motion. But vehement simple motion generates in the beholder a phantasm of lucid and hot, as I have before demonstrated of the simple motion of the sun; and therefore the combustible matter which is placed in P will be made lucid and hot, that is to say, will be fire. Wherefore I have rendered a possible cause of fire; which was to be done. [Sidenote: The generation of fire from collision.]