564 Mr., vince’s Inveftigation of the 
PROP. IX. 
If a force alls upon a body in any given direction not faf- 
fing through the center of gravity ; to determine the 
plane of rotation , the diredlion in which the center of 
gravity begins to move , and its motion after . 
Conceive a plane a>'bz (fig. 6.) to be fupported upon 
a line ab paffing through its center of gravity g, and 
fuppofe a force to act at any point d in that line, and in 
a direition perpendicular to the plane ; then it is raani- 
feft, that fuch a force can give the plane no rotatory 
motion about ab. Imagine now the fupport to be taken 
away whilft the force is ailing at d, then it is evident,, 
that as the plane had no tendency to move about ab as 
an axis, and the taking away of the fupport can give it 
no fuch motion, it will, by Cor. 2. Prop. vm. begin its 
progreflive motion in the direction in which the force 
aits ; and as the force is fuppofed not to ail at the cen- 
ter of gravity, it muft at the fame time have a rotatory 
motion about fome axis, which, as it has no motion 
about ab, muft lie fomewhere in the plane, and per- 
pendicular to ab ; and confequently in ipfo motus initio 
the plane of rotation muft be perpendicular to the plane 
AyBz. 
