272 Remarks on the Central Forces of Bodies 
acid upon two contiguous plates of zinc and copper. Centrifugal 
force may therefore with propriety be considered a physical agent, 
which is called into action, by an inscrutable law of nature, when- 
ever matter is made to move in a curve ;—which ought to be no 
more a subject of surprise than that magnetic force should be ex- 
cited in a bar of iron by certain chemical operations, the precise 
nature of which is as little understood as that of inertia. , 
The centrifugal principle has been employed as a projectile 
force from the earliest ages. It would be interesting to notice 
the extent to which it was used in ancient wars; and particular- 
ly to point out, as might be done even with the feeble lights af- 
forded us, how much Archimedes was indebted to the central 
forces for the destructive effects of his engines, which I believe 
to have been no fabled nor imaginary productions of genius. 
As I shall here come in conflict with some generally received 
opinions, I will give a short extract from Professor Renwick’s 
Elements of Mechanics. Not that he differs from other writers 
on this subject, but I find that the extract will be useful in ex- 
plaining what is to follow. “The simplest case of central force 
is where a body connected with a fixed point by an inflexible 
straight line is impelled by a projectile force, at right angles to 
that line. The latter force would have impressed upon the body 
a motion with a uniform velocity. The body, then, in conse- 
quence of its connection with a fixed point, describes a circle of 
which that. point is the centre. If the connection were to cease 
at any point in the curve, the deflecting force would cease to act, 
and the body would go in a straight line whose direction would 
be a tangent to the curve. The force acting at any point in the 
curve must therefore be decomposed into two, one of which is in 
the direction of the curve, the other in that of the radius.”* 
_ If a ball at A, Fig. 4, weighing one pound, and attached to 
ea: inflexible -_ AC, two feet long, be impelled by a projec 
tile force or moving power at the rate of two entire revolutions 
ina second, or ore feet per second, it will have a re 
constitute the aggregate a ening on the body at 
any point of the curve or circle; the former acting in the direc- 
tion of | the curve, and the latter i in that of the radius—one caused 
‘ Lae ee eee oe eae 
™ Page 62. eee se +t Cavallo, p. 66. 
