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THREE:With a thorough knowledge of practical machine operation, and an acquaintance with existing practice, an engineer proceeding upon such a plan, will, if he does not overlook some of the conditions, be able to generate designs which may remain without much modification or change, so long as the purpose to which the machinery is directed remains the same.Modern admirers of Aristotle labour to prove that his errors were inevitable, and belonged more to his age than to himself; that without the mechanical appliances of modern times science could not be cultivated with any hope of success. But what are we to say when we find that on one point after another the true explanation had already been surmised by Aristotle��s predecessors or contemporaries, only to be scornfully rejected by Aristotle himself? Their hypotheses may often have been very imperfect, and supported by insufficient evidence; but it must have been something more than chance which always led him wrong when they were so often right. To begin with, the infinity of space is not even now, nor will it ever be, established by improved instruments of observation and measurement; it is deduced by a very simple process of reasoning, of which Democritus and others were capable, while Aristotle apparently was not. He rejects the idea because it is inconsistent with certain very arbitrary assumptions and definitions of his own, whereas he should have313 rejected them because they were inconsistent with it. He further rejects the idea of a vacuum, and with it the atomic theory, entirely on �� priori grounds, although, even in the then existing state of knowledge, atomism explained various phenomena in a perfectly rational manner which he could only explain by unmeaning or nonsensical phrases.195 It had been already maintained, in his time, that the apparent movements of the heavenly bodies were due to the rotation of the earth on its own axis.196 Had Aristotle accepted this theory one can imagine how highly his sagacity would have been extolled. We may, therefore, fairly take his rejection of it as a proof of blind adherence to old-fashioned opinions. When he argues that none of the heavenly bodies rotate, because we can see that the moon does not, as is evident from her always turning the same side to us,197 nothing is needed but the simplest mathematics to demonstrate the fallacy of his reasoning. Others had surmised that the Milky Way was a collection of stars, and that comets were bodies of the same nature as planets. Aristotle is satisfied that both are appearances like meteors, and the aurora borealis��caused by the friction of our atmosphere against the solid aether above it. A similar origin is ascribed to the heat and light derived from the sun and stars; for it would be derogatory to the dignity of those luminaries to suppose, with Anaxagoras, that they are formed of anything so familiar and perishable as fire. On the contrary, they consist of pure aether like the spheres on which they are fixed as protuberances; though314 how such an arrangement can co-exist with absolute contact between each sphere and that next below it, or how the effects of friction could be transmitted through such enormous thicknesses of solid crystal, is left unexplained.198 By a happy anticipation of Roemer, Empedocles conjectured that the transmission of light occupied a certain time: Aristotle declares it to be instantaneous.199

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