The theorems are of use, not only for finding the volumes or areas of solids or surfaces of revolution, but also, conversely, for finding centroids or centres of gravity.
The first is by the method of instantaneous centres, already exemplified in 63, and rolling centroids, developed by Reuleaux in connection with his method of analysis.
Again, imagin d fixed, then the instantaneous centre Obd of b with regard n d is found by producing the links c and a to intersect in Obd and the shapes of the centroids belonging respectively to thi links Ii and d can be found as before.
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