Revolving about Fixed Axes. 



263 



proportions of the moving power, and the forces that it produces 

 directly and indirectly — the manner in which the central forces 

 are excited — and the combined operation of all the forces upon 

 a body whilst revolving and when projected, have not been satis- 

 factorily explained. 



It is not intended, however, at present to enter into an investi- 

 gation of the subject upon principles purely dynamical, but the 

 object of these remarks is to show by mathematical reasoning, 

 founded upon experiment and familiar examples, that the power 

 employed to revolve a body about a fixed axis is wholly expend- 

 ed in giving velocity to that body in the direction of the circle, 

 and that, consequently, the central forces must be excited in obe- 

 dience to a law of nature ; and, in the second place, that the mo- 

 ving and excited forces act in conformity with the principles of 

 " the composition of forces." 



Fig. 1. 



If the bar of soft iron m. Fig. 1, be prepared as a horse-shoe 

 magnet and secured in a proper manner to the rod r, working 

 horizontally on an axle at c, it maybe connected at pleasure with 

 a galvanic battery, by means of its wires and the usual arrange- 

 ments of cups containing quicksilver at the centre. The iron 

 bar A, of a suitable size and description, moving with a given 

 uniform velocity along the straight line A^, would be attracted at 

 B by the magnet, if it were connected at that moment with the 

 galvanic battery, and would be made to move in the curve Bv 

 of the circle BD, but in virtue of its inertia it would, in the ab- 

 sence of friction and atmospheric resistance, continue to move in 

 that circle with the same uniform velocity. For the deflecting 

 force being independent of the projectile force, and acting at all 

 times in the direction of the radii of the circle, it cannot in any 

 respect increase nor diminish the original velocity of the bar. 

 And for the same reasons the force with which the bar is moving 



