Nor is the interest theoretical only, for the question is oneof direct importance to the practical mechanician. In a largenumber of machines and scientific apparatus it is requisitethat some point or points should move accurately in a straightline with as little friction as possible. If the ruler principleis adopted, and the point is kept in its path by guides, wehave, besides the initial difficulty of making the guides trulystraight, the wear and tear produced by the friction of thesliding surfaces, and the deformation produced by changesof temperature and varying strains. It becomes therefore ofreal consequence to obtain, if possible, some method whichshall not involve these objectionable features, but possess theaccuracy and ease of movement which characterise our circleproducingapparatus.
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