THREE:To make this plainer, the laws of forces, the proportion of parts, strength of material, and so on, are subjects of general knowledge that may be acquired from books, and understood without the aid of an acquaintance with the technical conditions of either the mode of constructing or the manner of operating machines; but how to construct proper patterns for castings, or how the parts of machinery should be moulded, forged, or fitted, is special knowledge, and must have reference to particular cases. The proportions of pulleys, bearings, screws, or other regular details of machinery, may be learned from general rules and principles, but the hand skill that enters into the manufacture of these articles cannot be learned except by observation and experience. The general design, or the disposition of metal in machine-framing, can be to a great extent founded upon rules and constants that have general application; but, as in the case of wheels, the plans of moulding such machine frames are not governed by constant rules or performed in a uniform manner. Patterns of different kinds may be employed; moulds may be made in various ways, and at a greater and less expense; the metal can be mixed to produce a hard or a soft casting, a strong or a weak one; the conditions under which the metal is poured may govern the soundness or shrinkage,things that are determined by special instead of general conditions.
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THREE:A learner should, in fact, consider the application and operative conditions of gearing as one of the main parts of the subject, and the geometry or even the construction of wheels as subsidiary; in this way attention will be directed to that which is most difficult to learn, and a part for which facilities are frequently wanting. Gearing may be classed into five modificationsspur wheels, bevel wheels, tangent wheels, spiral wheels, and chain wheels; the last I include among gearing because the nature of their operation is analogous to tooth wheels, although at first thought chains seem to correspond more to belts than gearing. The motion imparted by chains meshing over the teeth of wheels is positive, and not frictional as with belts; the speed at which such chains may run, with other conditions, correspond to gearing.The means of supporting cores must be devised, or at least understood, by pattern-makers; these supports consist of 'prints' and 'anchors.' Prints are extensions of the cores, which project through the casting and extend into the sides of the mould, to be held by the sand or by the flask. The prints of cores have duplicates on the patterns, called core prints, which are, or should be, of a different colour from the patterns, so as to distinguish one from the other. The amount of surface required to support cores is dependent upon their weight, or rather upon their cubic contents, because the weight of a core is but a trifling matter [96] compared to its floating force when surrounded by melted metal. An apprentice in studying devices for supporting cores must remember that the main force required is to hold them down, and not to bear their weight. The floating force of a core is as the difference between its weight and that of a solid of metal of the same sizea matter moulders often forget to consider. It is often impossible, from the nature of castings, to have prints large enough to support the cores, and it is then effected by anchors, pieces of iron that stand like braces between the cores and the flasks or pieces of iron imbedded in the sand to receive the strain of the anchors.
THREE:Lalage entered gently. He stood in the pitchy darkness for some time. He could not hear a sound. Presently his patience was rewarded. There was the click of a key in the door and something swished by him.On the other hand, if Stoicism did not make men pitiful, it made them infinitely forgiving. Various causes conspired to bring about this result. If all are sinners, and if all sins are equal, no one has a right, under pretence of superior virtue, to cast a stone at his fellows. Such is the point of view insisted on with especial emphasis by Seneca, who, more perhaps than other philosophers, had reason to be conscious how far his practice fell short of his professions.94 But, speaking generally, pride was the very last fault with which the Stoics could be charged. Both in ancient and modern times, satirists have been prone to assume that every disciple of the Porch, in describing his ideal of a wise man, was actually describing himself. No misconception could be more complete. It is like supposing that, because Christ commanded his followers to be perfect even as their heavenly Father is perfect, every Christian for that reason thinks himself equal43 to God. The wise man of the Stoics had, by their own acknowledgment, never been realised at all; he had only been approached by three characters, Socrates, Antisthenes, and Diogenes.95 May the sage fall in love? asked a young man of Panaetius. What the sage may do, replied the master, is a question to be considered at some future time. Meanwhile, you and I, who are very far from being sages, had better take care not to let ourselves become the slaves of a degrading passion.96
