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we can see that they will both turn about the fixed points a and d, and so roll upon each other like the pitch lines of non- circular spur-wheels. 9 It is very difficult, however, to realize
Fig. 25.
the matter from this point of view on account of the peculiar form of the centroids, and some method which can be more easily comprehended, for showing at least the velocity-ratio of the arms, is required. For this purpose let us suppose a cylindric spur-
