144 Mr. Hatchett’s Experiments on the various Alloys, 
It seemed to me, that the most probable way to prevent the 
little furrows or gullies from being thus formed, would be, to 
construct the instrument in such a manner, that the direction in 
which the pieces rubbed upon each other should continually 
vary. The following contrivance was therefore adopted, by which 
the pieces were prevented from rubbing together twice in the, 
same direction. 
In this method, the lower frame, as well as the upper, is sup- 
ported on two moveable vertical boards ; but, whereas the boards 
supporting the upper frame are placed parallel to B b, in conse- 
quence of which the frame can move only in the direction BA, 
these are placed parallel to BA, so that the frame can move 
only in the direction B b. 
EE is the axis by which the upper frame is moved : this turns 
in fixed sockets at SS, and is turned at each end into the form 
of an eccentric circle, which acts as a crank ; so that, by means 
of the levers EK, which at one end turn on these eccentric 
circles, and at the other end turn on joints fixed to the upper 
frame, this frame is made to move ^ of an inch, in the direction 
BA, during one half of the revolution of the axis, and as much 
in the contrary direction, during the other half revolution. 
ee is an axis of the same kind, serving to move the lower 
frame. HH is a windlass, which turns these two axes by means 
of the toothed wheels F ,/, which work in the toothed wheels 
G, g, fastened to the axes EE and ee. TTTTT is the table 
upon which the apparatus stands. 
The wheel F has 90 teeth,/ has 75, and G,^, have each 20 ; 
so that the axis EE makes six revolutions while ee makes five ; 
and, at a medium, these axes make about four revolutions to one 
