MINUTES OE PEOOEEDINGS OF 
12 
Grolerfs Machine of Rotation .* Pig. 2. 
3. Col. Grobert, a Prench officer, modified this proceeding of Mattei 
about the year 1804. His apparatus consisted of two very thin discs of 
card-board, each divided into 360° by equal radii, that is, equally graduated. 
These discs were attached to the extremities of a shaft, to which a rapid 
movement of rotation was given. If the machine was at rest, the ball 
when fired would pierce both screens on the same radii, but when a rotation 
was given, there was a certain interval between the time of passing the 
first screen and the second, and the difference between the degrees of radii 
indicated the angle, and from it was calculated the time of flight of the 
shot in passing from the first screen to the second, and consequently the 
velocity. Thus, let x equal the angle denoted by the two radii struck, 
y the interval between the two discs, and T the duration of one revolution 
of the discs, the time that the projectile has taken in traversing the distance 
y will be _ a? 
1 ~ 360 T ‘ 
and the velocity of the ball will be 
F _y360. 
Tx 
Grobert’s Machine of Eotation. 
This instrument was unable to measure the time with greater accuracy 
than the -fa °f a second, and consequently gave no practical results. 
Gregory's Machine of Rotation. Pig. 3. 
4. Next in order of time, about the year 1818, comes Gregory's 
machine of rotation, which is very similar to that of Grobert. But none of 
Gregory’s Kevolving Discs. 
* Traits de Ballistique, par M. Didion. 
