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TWO:Machines and tools operating by percussive action, although they comprise a numerous class, and are applied in nearly all mechanical operations, have never received that amount of attention in text-books which the importance of the machines and their extensive use calls for. Such machines have not even been set off as a class and treated of separately, although the distinction is quite clear between machines with percussive action, and those with what may be termed direct action, both in the manner of operating and in the general plans of construction. There is, of course, no lack of formul? for determining the measure of force, and computing the dynamic effect of percussive machines acting against a measured or assumed resistance, and so on; but this is not what is meant. There are certain conditions in the operation of machines, such as the strains which fall upon supporting frames, the effect produced upon malleable material when struck or pressed, and more especially of conditions which may render percussive or positive acting machines applicable to certain purposes; but little explanation has been given which is of value to practical men.Meanwhile the old principle of universal doubt could no longer be maintained in presence of the certainties already won by modern science. Man, in the time of Newton, had, as Pope tersely puts it, ��too much knowledge for the sceptic side.�� The problem was not how to establish the reality, but how to ascertain the origin and possible extent of that knowledge. The first to perceive this, the first to evolve criticism out of scepticism, and therefore the real founder of modern philosophy, was Locke. Nevertheless, even with him, the advantage of studying the more recent in close connexion with the earlier developments of thought does not cease; it only enters on a new phase. If he cannot, like his predecessors, be directly affiliated to one or more of the Greek schools, his position can be illustrated by a parallel derived from the history of those schools. What Arcesilaus and Carneades had been to Socrates and his successors, that Locke was, in a large measure, to Bacon and the Cartesians. He went back to the initial doubt which with them had been overborne by the dogmatic reaction, and insisted on making it a reality. The spirit of the Apologia is absent from Plato��s later dialogues, only to reappear with even more than its original power in the teaching of the New Academy. And, in like manner, Descartes�� introspective method, with its demand for clear ideas, becomes, in the Essay concerning Human Understanding, an irresistible solvent for 420the psychologyy and physics of its first propounder. The doctrine of innate ideas, the doctrine that extension is the essence of matter, the doctrine that thought is the essence of mind, the more general doctrine, held also by Bacon, that things have a discoverable essence whence all their properties may be deduced by a process analogous to mathematical reasoning,��all collapsed when brought to the test of definite and concrete experience.