Steam Engine, 113 
similar toothed wheel I, is fastened by straps to a rod com- 
ing from the end of the beam, so that it cannot turn round 
on its axis, but must rise and fall with the motion of the 
sreat beam. 
o 
A bar of iron connects the centres of these two small 
toothed wheels, so that they cannot quit each other. 
When, therefore, the beam raises the wheel I, it must 
move round the circumference of the wheel H, and turn 
it together with the fly ; and it will be evident, upon con- 
sideration, that the fly, driven in this manner, will make 
two revolutions for every one of the wheel L This mode 
of moving the fly, is preferable to a crank ; as it goes with 
twice the velocity. This contrivance is called the sun 
and planet wheel, from the resemblance of the motion to 
that of those luminaries. 
The valves of this steam-engine are all puppet- valves, 
as these are found least liable to be out of order. 
The mode of operation in Mr. Watt’s engine, is as fol- 
lows : 
Suppose the piston at the top of the cylinder, in the si- 
tuation represented in the plate, and the lower part of the 
cylinder filled with steam. By means of the handle E, 
open the steam- valve a, and the eduction- valve d, the 
levers of which are connected together ; there being now 
a communication between the cylinder and the condenser, 
the steam instantly rushes into the condenser, leaving the 
cylinder empty ; whilst at the same time the steam from 
the boiler, entering by the steam- valve a, presses upon 
the piston and forces it down. As soon as the piston has 
arrived at the bottom, the steam- valve c, and the educ- 
tion valve b, are opened, whilst the valves a and d are 
slant the steam therefore immediately rushes through 
the eduction-valve b, into the condenser, whilst the piston 
is forced up again by the steam, which is now admitted 
I by the steam- valve c. 
|- >oL II. 
P 
