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TWO:Tangent wheels and spiral gearing have only what is termed line contact between the bearing surfaces, and as the action between these surfaces is a sliding one, such wheels are subject to rapid wear, and are incapable of sustaining much pressure, or transmitting a great amount of power, except the surfaces be hard and lubrication constant. In machinery the use of tangent wheels is mainly to secure a rapid change of speed, usually to diminish motion and increase force.In their theory of cognition the Stoics chiefly followed Aristotle; only with them the doctrine of empiricism is enunciated so distinctly as to be placed beyond the reach of misinterpretation. The mind is at first a tabula rasa, and all our ideas are derived exclusively from the senses.37 But while knowledge as a whole rests on sense, the validity of each particular sense-perception must be determined by an appeal to reason, in other words, to the totality of our acquired experience.38 So also the first principles of reasoning are not to be postulated, with Aristotle, as immediately and unconditionally certain; they are to be assumed as hypothetically true and gradually tested by the consequences deducible from them.39 Both principles well illustrate the synthetic method of the Stoics��their habit of bringing into close16 connexion whatever Aristotle had studiously held apart. And we must maintain, in opposition to the German critics, that their method marks a real advance on his. It ought at any rate to find more favour with the experiential school of modern science, with those who hold that the highest mathematical and physical laws are proved, not by the impossibility of conceiving their contradictories, but by their close agreement with all the facts accessible to our observation.