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PHYSICAL AND MATHEMATICAL SCIENCES. 227

theory of motion the inertia of matter, regarded as areal active power, and not merely a passive indifferenceto motion or rest. With respect to curvilinear motion,he pointed out distinctly the necessity of supposing adeflecting force, which being removed, the motion wouldbe rectilinear and in the direction of the tangent. Helaid it down as a general principle, that there is alwaysthe same quantity of motion in the universe, which, itwould seem, is the clue to his notions of inertia, &c.just mentioned. He appears to have regarded motionas a sort of quality superinduced upon matter, butwhich might, in some cases, be, as it were, latent. Allthis arose out of the theoretical principle of the perma-nence of such qualities ; and this he deduced from theimmutability of the divine attributes. The inherentfallacy of such reasoning must be sufficiently apparent;and we shall not be surprised to find that though, inseveral cases, he has brought out true results, yet, inothers, as in the whole theory of the collision of bodies,he has run into palpable errors.

This last subject, indeed, is one which long remainedwithout a completely satisfactory investigation. Suchinvestigations were first given by Dr. Wallis and Sir C.Wren, nearly at the same time, in l6t>8 ; and, also inde-pendently of these, by Huyghens in 1 66 f). They eachfounded their reasonings on the principle, then firstfully developed, that action and reaction are equal andin opposite directions.

In 1633, Des Cartes had completed his “ System ofthe World,” having previously broached the metaphy-sical doctrines on which he founded his entire methodof philosophical reasoning.

With regard to this system, he must be allowed thecredit of having been the first who attempted to suggestany one physical principle to explain and connect allthe planetary motions. For we can hardly class underthe designation of philosophical theories the crystallinespheres of Ptolemy , or the vitality assigned to theearth by Kepler. Des Cartes proceeded upon principlesQ 2

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