40 
PROFESSOR T. A. HEARSON ON THE KINEMATICS OF MACHINES. 
The formula for a reversing engine, with Stephenson’s link motion, is 
—o—f>—y—o— u A A u -y 
The object of the mechanism is to get a correct relative movement of the two 
sliding pieces, piston and valve, which are represented by the links adjacent to the 
I’s at the two ends of the formula. 
With this may be compared the formula 
^—v-kr-o —y It- 
for an engine with Joy’s valve gear, which is very differently constructed for the same 
purpose. 
