( J4* ) . 
following Propofirion I Ihew the V or nix or Arch which 
fupports its own Weight to be the fame with the Catena' 
ria. In the two next Proportions I (hew how to find the 
Figures of pliable Surfaces which are charged with the 
Weight of a Fluid- In the zid and Proportions I 
treat of the Motion of a Mufical String, and give the 
Solution of this Problem: To find the Number ol Vibrati- 
ons that a String will make in a certain time, having gi- 
ven its Length, its Weight, and the Weight that ftretch- 
es it. This Problem I take to be entirely new, and in the 
Solution of it (in the lafl part of Prop 23.) there is a re- 
markable Infiance of the Ulefulnefsof the Method of firfl: 
and lafl Ratio s. The 24 th Propofition gives the Inven- 
tion of the Center of Olcillation of all Bodies ; and in 
the 25 -th Propofition I have given the Tnvefiigation ofthe 
Center of Percufiion. It is known that this Problem is 
folved by the fam e Calculus as the foregoing; wherefore 
it is generally thought that thefe two Centers are the lame. 
But that is a Mifiake, becaufe the Center of Ofcillatioh 
can be but one Point; but the Center of Percufiion may 
be any wherein a certain Line, which this Propofirion 
fhews how to find. There is an Error in this Propofiti- 
on, which I was not fenfible of till after the Book was 
pubhlh’d, wherefore I take this Opportunity of correding 
of it. It does not affed the Reafoning by which I find 
the Diftance of the Center of Percufiion from the Axis of 
Rotation 5 but it is thiS; that I fuppofed the Center of 
Percufiion to be in the Plane palling thro’ the Center of 
Gravity, and perpendicular to the Axis of Rotation .* 
which is a Mifiake. It is correded by the following 
Propofition. 
