( iSp ) 
Quantity of Motion, I am oblig’d to fay foinething 
more for the clearing up of that foint. 
If! underftand them right, they call vis viva a Force, 
whofe Effed is fenfible, as the Force of Gravity, when it 
accelerates Bodies in their Fall j-and vis mortua a Force, 
which being deftroy’d, product no fenfible Effed, as 
the. Force of Gravity ading upon a Weight in one Scale 
of a Balance, when the Weight cannot defcend by rea- 
fon of a Counter poife in the other Scale. But certain- 
ly no Man, that confiders the thing attentively, would 
make that Difiindion. However, lince Polenns allows, 
that the Quantity of Motion in Bodies is as the Mafs 
multiplied into the Velocity (or MV)^ but fays, that 
the Force, with which they ad, which he diftinguiflies 
by the Name of vis viva^ is as the Mafs multiplied 'in- 
to the Square of the Velocity, or M V V : I have made 
the following Experiment to fhew his Notion to be in- 
confiftent , tho all the Fbosnomena of unequal Weights 
applied to a Statera, fo as to make' an ^Equilibrium, 
might ferve for that Purpofe, if it had not been obje- 
ded, that the particular Conftrudion of the Machine 
hinder’d it from agreeing with the fuppos’d Theorem, 
that the Force is as the Matter multiply ’d into the 
Square of the Velocity. 
EXP ER I MEl^T.. 
Let two Bails, A and B, (P/.I. Fig, 2.) be joyn’d by a 
String, which going thro’ the fmooth Hole C of an even 
Table, and under the Pulley P, fufpends a Weight W, 
It is plain, that upon letting go the Balls A and B, from 
the Places A and B, they will move towards C with 
the fame Force, becaufe each of them will be drawn 
towards C by half the Force of the Weight W, whe- 
ther the Balls be equal, or unequal. 
I. The 
