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historical view of the

than nominal: this, however, can in no degree diminishour detestation of the principle of its infliction.

Whilst restricted to his residence at Arcetri, Galileo could hardly venture to direct his attention much toastronomical studies, which, under the circumstances ofhis case, was both unsafe and could not but carry withit the most unpleasant associations. His mind recurred,therefore, with increased satisfaction to the favouritesubject of his earliest enquiries, the doctrine of motion.The fruit of his speculations are presented to us in the“Dialogues on Motion,” which were published in 1636;but as all his works, “edita et edenda,” were placed bythe Inquisition on the prohibited list, there was consider-able difficulty in getting it printed. It appeared, how-ever, at Amsterdam .

These dialogues are a continuation of those on thesystem, and are carried on between the same speakers.They contain the developement of all Galileo ’s researcheson the theory of motion; and form, in fact, the mostcomplete statement of the first principles of dynamics,as far as he had investigated them.

Among the principal topics of discussion, is that ofthe rectilinear descent of bodies. The author’s experi-ments are related in full detail; but, owing to the imper-fection of his methods, he deduced a result differingconsiderably from the truth. He enters largely intothe refutation of the Aristotelian doctrine of the acceler-ation of a falling body by the air; and investigates themotion of projectiles, showing by distinct and satisfac-tory reasoning that their path in a vacuum will alwaysbe a parabola. In this, as well as some other parts ofhis researches, he distinctly introduces the great princi-ple of the composition of forces, which he establishes ongrounds at least sufficiently general for the purposes ofthese investigations.

It is somewhat remarkable, that a philosopher ofGalileo ’s penetration should not have entirely rejectedthe notion of nature’s abhorrence of a vacuum. Yet hedid not consider a vacuum impossible; for he describes

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