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.