238 CELESTIAL MECHANICS. I. vii. 25. 



notions on this subject ; they had been perhaps in some 

 degree misled by an accidental error committed by New- 

 ton in computing the precession of the equinoxes : the 

 experiments of Smeaton served to set the question in a 

 clearer point of view, and Dr. Wollaston has more lately 

 removed every remaining obscurity from the subject, in 

 one of his Bakerian Lectures, published in the Philoso- 

 phical Transactions. Mr. Smeaton's apparatus consisted 

 of a vertical axis, turned by a thread, passing over a pulley, 

 and supporting a scale with weights ; the thread was ap- 

 plied to different parts of the axis, having different dia- 

 meters, and the axis supported two arms, on which two 

 leaden weights were fixed, their distances being variable 

 at pleasure. The experiment being thus arranged, the 

 same force produces, in the same time, but half the velocity, 

 in the same situation of the weights, when the thread is ap- 

 pHed to a part of the axis of half the diameter : and if the 

 weights are removed to a double distance from the axis, 

 a quadruple force will be required in order to produce an 

 equal angular velocity in a given time. ' 



340. Definition. " 350." The centre 

 of gyration is a point, into which if all the 

 particles of a revolving body were condensed, 

 with its actual velocity, the body would retain 

 the same quantity of rotatory power ; and 

 the radius of gyration is the distance of this 

 point from the axis of motion. 



341. Definition. The rotatory inertia 

 of a body with respect to any given axis, is 

 the sum of all the products of the elementary 



