iet. vr. Kepler, and Galileo. 95

their motions, Galileo, in Italy, was meditatingupon the doctrine of motion in general, and in-vestigating its principles. This philosopher wasborn at Pisa, in the year x 564, and began hisimprovements in mechanics, by banishing fromthe science those ridiculous distinctions whichhad been made by .Aristotle and his followers,between bodies light and heavy, motions natu-ral and violent, rectilinear and circular. Hethe wed, both by demonstration and experiment*that, in a space void of air, all bodies whatever,fall through equal heights in equal times ; andthat a body impelled by two forces, acting in thedirection of the sides of a parallelogram, willfollow the direction of neither, but proceed inthe diagonal, and describe it in the same time,as by the action of one of the forces alone itwould have described one of the sides.

These principles he also found to be equallyapplicable to the motion of all kinds of pro-jectiles. A ball or shell being thrown fromthe mouth of _ a cannon, is under the in-fluence of two forces, which, by their jointaction, regulate its motion, and determine itsdirection: one of these forces is that of thepowder, by which it endeavours to move con-tinually forward in a straight line; the other isthat of its weight, which inclines it to descend,and fall towards the ground; and from the com-position of these two forces, Galileo demonstrated1 that