[ 443 ] 
puting their Velocity on the Hypothefis of their para- 
bolic Motion. The Firft of thele Methods was often 
impracticable, and in all great Velocities extremely 
inaccurate, both on account of the Shortnefs of the 
Time of their Flight, and the Refiftance of the Air. 
The Second is ftill more exceptionable, fince, by 
reafon of the Airs Refiftance, the Velocities thus 
found may be lefs in any Ratio given, than the real 
Velocity fought. Now, to avoid thefe Difficulties, 
the Author has invented a Method of determining 
the Velocities of Bullets, which may be carried to 
any required Degree of Exa&nefs, and is no-ways 
liable to the forementioned Exceptions; for. by this 
Invention, the Velocity of the Bullet is found in any 
Point of its Track, independent of the Velocity it 
had before it arrived at that Point, or of the Velo- 
city it would have after it had pafied it : So that not 
only the original Velocity, with which it ifiiies from 
the Piece, is hence known, but alfo its Velocity, after 
it has pafied to any given Diftance ; and therefore 
the Variations of its Velocity from the Refiftance of 
the Air may be alfo afeertained with great Facility. 
• The Machine for this Purpofe is deferibed in the 8th 
Propofition, and the Principle it is founded on is this 
fimple Axiom of Mechanics ; That if a Body in 
Motion ftrikes on another at Reft , and they are not 
feparated after the Stroke , but move on with one 
common Motion y then that common Motion is equal 
to the Motion with which the Firft Body moved 
before the Stroke : Whence, if that common Motion 
and the Mafles of the Two Bodies are known, the 
Motion of the Firft Body before the Stroke is thence 
determined. On this Principle then it follows, that 
the 
