THE FORCE OF
7 8
is a block of wood placed two or three feet from thewheel to receive the balls; it is of elm, that the pene-trations may be uniform.
In using this machine it is requisite to ascertain when themotion of the wheels is equable, and how time long it takesin each revolution ; for this purpose various expedients may bedevised. At Turin we fitted a little excentric wheel I L tothe axis C D, which at each revolution gave a vibratoryhorizontal motion to a tongue of wood at the extremity ofwhich appended a common pendulum that, was shortened orlengthened till its vibrations were isochronous to those of thetongue : the length of the pendulum will shew how long thewheel is in making each revolution. So soon as the isochro-nisin between the pendulum and the tongue is established,fire the gun and stop the wheel; the two holes made in therim of paper by the ball, will he easily distinguished fromeach other by the edges being turned to the point that theball went out at. Stretch a thread in the direction M R,passing through the centre of the hole H where the ball en-tered the paper. The distance of the other hole K from thepoint B will be then known : this distance shews how muchone point of the wheel turned, while the shot was traversingthe diameter A B.
165. If the mechanism and use of this machine be wellunderstood, it will be easy to find the uniform velocity withwhich the shot passes through the diameter A B of the wheel,and consequently its uniform velocity during one second oftime. Let D be the diameter of the wheel, c its circumfe-rence, t the time that the wheel takes to make one revolu-tion, m the distance that a point of the circumference of thewheel turns, while the shot is traversing the diameter; thenC D
m : D :: C : ———, the last term will express the space passed
through by the shot with an uniform velocity during one re-volution of the wheel: then expressing by u the space that theshot passed through with an uniform velocity in one second,
CD CD
or its initial velocity, t: 1 :: -;-; the last term = u
J m tm
will express the velocity sought. With this machine, whenthe wheel turns with an equable motion, t= -j- of a second,