202 
Sm&leSig 1 ,m *‘ ™ •• — 
“h.TSi« 0rm ’ f iS n- Ne ' rt0n speaks of “l®>st parts “into 
that theW, v! ? dlV1SlM f’, l,IKlnlll S P arts of magnitudes so small 
at their qualities cannot be immediately discerned by our senses 
According to he above-stated rule, these parts can have no othtr 
essenhal qnaht.es than those which we are co^za"of by our 
senses, when we see and handle masses ; namely, the qualities of 
magnitude, form, mobility, and inertia. This inference is expressly 
insisted upon by Newton in his Third Rule of Philosophizing 
By taking .magnitude to be an essential quality, we, of eoursl ex- 
Newton’s “least parts “‘as md Pr ° perly desi S nate 
■%SS 
loll P n 5 perfectl l’ sraootil horizontal plane a perfectly 
smooth sphere and suppose the sphere to be pushed with the hand 
oml) with ° tT" in V tr 1 sUt “™ » the plane (without 
3 interval of hm ‘ "“SiwtTI ^ ^ , ***• * 
sufficient accuracy for the purposes of the experiment, by rewvda'tinw 
the motion communicated to the sphere by the haid* so°that if 
.all be parallel and equal to the motion of another sphere f which 
might be called a pilot sphere), the latter having be?n cans d to 
move by mechanical arrangements in the above -specified manner 
Let the same thing be done with spheres of the same material of 
morion' ,*/ he 8120 ° f firet - and in cacl ‘ <we let the 
not on dc regulated by the same motion of the pilot sphere Tlmn it 
would certainly he felt that the motion of the sphere iris In each“n 
stance produced by a personal effort, and it would bo perceived tint 
the effort was greater the greater the size of the sphere effect 
• , , ct1011 assumed to be inconsiderable. The experiment 
oht even suggest the law that the effort was in exact proportion 
to the size of the sphere ; but it is not adapted toTo« ZtT 
different' 'grounds I ’ “ wm , bo stated subsequently, rests on 
m ? tion rf . ^ 
Now the inertia of the body may be d JE \ T ,y , 0XCTC ‘ s < !d - 
si^,sz 
