patent, but the idea of railroads, instead of common roads,
coming later left the construction of the locomotive to Stephenson.[1]

Watt's last patent bears date June 14, 1785, and was

for certain newly improved methods of constructing furnaces or
fire-places for heating, boiling, or evaporating of water and
other liquids which are applicable to steam engines and other
purposes, and also for heating, melting, and smelting of metals
and their ores, whereby greater effects are produced from the
fuel, and the smoke is in a great measure prevented or consumed.

The principle, "an old one of my own," as Watt says, is in great part
acted upon to-day.

So numerous were the improvements made by Watt at various periods, which
greatly increased the utility of his engine, it would be in vain to
attempt a detailed recital of his endless contrivances, but we may
mention as highly important, the throttle-valve, the governor, the
steam-gauge and the indicator. Muirhead says:

The throttle-valve is worked directly by the engineer to start
or stop the engine, and also to regulate the supply of steam.
Watt describes it as a circular plate of metal, having a spindle
fixed across its diameter, the plate being accurately fitted to
an aperture in a metal ring of some thickness, through the
edgeway of which the spindle is fitted steam-tight, and the ring
fixed between the two flanches of the joint of the steam-pipe
which is next to the cylinder. One end of the spindle, which has
a square upon it, comes through the ring, and has a spanner
fixed upon it, by which it can be turned in either direction.
When the valve is parallel to the outsides of the ring, it shuts
the opening nearly perfectly; but when its plane lies at an
angle to the ring, it admits more or less steam according to the
degree it has opened; consequently the piston is acted upon with
more or less force.

Papin preferred gunpowder as a safer source of power than steam, but
that was before it had been automatically regulated by the "Governor."
The governor has always been the writer's favorite invention, probably
because it was the first he fully understood. It is an application of
the centrifugal principle adapted and mechanically improved. Two heavy
revolving balls swing round an upright rod. The faster the rod revolves
the farther from it the balls swing out. The slower it turns the clos