invented the machinery called the parallel joint, the construction of which will be easily understood from

1

the figure.

Emma. How are the valves opened and shut?

Father. Long levers o and p are attached to them, which are moved up and down by the piston-rod of the air-pump EF. In order to communicate a rotatory motion to any machinery by the motion of the beam, Mr. Watt makes use of a large fly-wheel x, on the axis of which is a small concentric toothed wheel H; a similar toothed wheel I is fastened to a rod T coming from the end of the beam, so that it cannot turn on its axis, but must rise and fall with the emotion of the great beam.

A bar of iron connects the centres of the two small toothed wheels; when, therefore, the beam raises the wheel 1, it must move round the circumference of the wheel H, and with it turn the fly-wheel x; which will make two revolutions while the wheel I goes round it once. These are called the Sun and Planet wheels; H, like the sun, turns only on its axis, while I revolves about it as the planets revolve round the sun.

If to the centre of the fly-wheel any machinery were fixed, the motion of the great beam RS would keep it in constant work.

Charles. Will you describe the operation of the engine?

Father. Suppose the piston at the top of the cylinder, as it is represented in the plate, and the lower

part of the cylinder, filled with steam. By means of the pump-rod E F, the steam valve a and the eduction valve d will be opened together, the branches from which being connected at o. There being now a communication at d between the cylinder and condenser, the steam is forced from the former into the latter, leaving the lower part of the cylinder empty, while the steam from the boiler entering by the 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 eduction valve b are opened, while those at a and d are shut; the steam, therefore, immediately rushes through the eduction valve b into the condenser, while the piston is forced up

again by the steam, which is now admitted by the valve c.

Hence, you observe, that the steam is condensed, in a separate, vessel, for the purpose of forming a vacuum under the piston: the force of steam is also introduced above the piston to depress it, an operation that was formerly done by the pressure of the atmosphere.

CONVERSATION XVII.

Of the Steam-Engine.

CHARLES. I do not understand how the two sets of valves act, which your described yesterday, as the steam and eduction valves.

Father. If you look to Fig. 36; Plate IV, there is a different view of this part of the machine, unconnected with the rest: s is part of the pipe which brings the steam from the boiler, a represents the valve, which, being opened, admits the steam into the upper part of the cylinder, forcing down the piston.

VOL. IV.