- Revolving about Fixed Aves. 265 
41 Pd yes ae 
tive of this cmamieatl by s, which 
will be avoided as much as possible on this occasion, I wit show 
by a reference to the familiar examples of the common sling and 
fly-wheel, that in a revolving body centrifugal force, whatever 
be its source, is much greater than the power necessary to give 
rotation to that body, and that it cannot therefore be direetly 
caused by the moving power,—and then explain how it may be 
proved by a simple experiment. 
It has been stated above that writers on dynamics have not 
clearly defined the operation of the laws of curvilinear motion 
on bodies revolving about fixed axes. One only of the many in- 
stances in which erroneous views are given by popular writers in 
noticing the subject of central forces, will be mentioned. In 
the Library of Useful. Knowledge {London edition] a writer, 
after enumerating some of the wonderful effects produced by ac- 
eumulating force in the circumference of a fly-wheel, remarks: 
“the same principle explains the force with which a stone may 
be projected froma sling. The thong is swung several times 
round by the force of the arm until a considerable portion of 
force is accumulated and then it (the stone) is projected with all 
Fig. 2. 
sg 
the collected force.* By observing the facts we may discover 
how all this accumulation of force is produced by the strength 
of thearm. A stone, S, Fig. 2, weighing one pound, secured to the 
end of a string rather less than two feet long, may be whirled in 
acircle of four feet diameter at the rate of two entire revolutions in 
* Vol I, p. 51, Art. Mechanics. 
