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THREE:Isometrical perspective is often useful in drawing, especially in wood structures, when the material is of rectangular section, and disposed at right angles, as in machine frames. One isometrical view, which can be made nearly as quickly as a true elevation, will show all the parts, and may be figured for dimensions the same as plane views. True perspective, although rarely necessary in mechanical drawing, may be studied with advantage in connection with geometry; it will often lead to the explanation of problems in isometric drawing, and will also assist in free-hand lines that have sometimes to be made to show parts of machinery oblique to the regular planes. Thus far the remarks on draughting have been confined to manipulation mainly. As a branch of engineering work, draughting must depend mainly on special knowledge, and is not capable of being learned or practised upon general principles or rules. It is therefore impossible to give a learner much aid by searching after principles to guide him; the few propositions that follow comprehend nearly all that may be explained in words.It has been the object thus far to show that machines, processes, and mechanical manipulation generally may be systematised and generalised to a greater or less extent, and that a failure to reduce mechanical manipulation and machine construction to certain rules and principles can mainly be ascribed to our want of knowledge, and not to any inherent difficulty or condition which prevents such solution. The same proposition is applicable to invention, with the difference that invention, in its true sense, may admit of generalisation more readily than machine processes.