continuous revolving motion to a shaft provided with a flywheel. He had invented the crank and connecting-rod for this purpose, but it had meanwhile been patented by one Pickard, and Watt, rather than make terms with Pickard, whom he regarded as a plagiarist of Rotative Engine. his own ideas, made use of his sun-and-planet motion until the patent on the crank expired. The reciprocating motion of earlier forms had served only for pumping; by this invention Watt opened up for the steam engine a thousand other channels of usefulness. The engine was still single-acting; the connecting-rod was attached to the far end of the beam, and that carried a counterpoise which served to raise the piston when steam was admitted below it.
Fig. 6.—Watt’s Single- Acting Engine, 1769.
14. In 1782 Watt patented two further improvements of the
first importance, both of which he had invented some years
before. One was the use of double action, that is
to say, the application of steam and vacuum to
each side of the piston alternately. The other
(invented as early as 1769) was the use of steam expansively,
Other Inventions
of Watt.
in other words the plan (now used in all engines that
aim at economy of fuel) of stopping the admission of steam
Fig. 7.—Watt’s Double-Acting
Engine, 1782.
when the piston had
made only a part of its
stroke, and allowing the
rest of the stroke to
be performed by the
expansion of the steam
already in the cylinder.
To let the piston push as
well as pull the end of the
beam Watt devised his
so-called parallel motion,
an arrangement of links
connecting the piston-rod
head with the beam
in such a way as to
guide the rod to move
in a very nearly straight
line. He further added
the throttle valve, for
regulating the rate of
admission of steam, and
the centrifugal governor,
a double conical pendulum,
which controlled
the speed by acting on
the throttle-valve. The stage of development reached at this
time is illustrated by the engine of fig. 7 (from Stuart’s
History of the Steam Engine), which shows the parallel
motion 𝑝𝑝, the governor 𝑔, the throttle-valve 𝑡, and a pair of
steam and exhaust valves at each end of the cylinder. Among
other inventions of Watt were the “indicator,” by which
diagrams showing the relation of the steam pressure in the
cylinder to the movement of the piston are automatically
drawn; a steam tilt-hammer; and also a steam locomotive
for ordinary roads—but this invention was not prosecuted.
In partnership with Matthew Boulton, Watt carried on in Birmingham the manufacture and sale of his engines with the utmost success, and held the field against all rivals in spite of severe assaults on the validity of his patents. Notwithstanding his accurate knowledge of the advantage to be gained by using steam expansively, he continued to employ only low pressures—seldom more than 7 ℔ per sq. in. over that of the atmosphere. His boilers were fed, as Newcomen’s had been, through an open pipe which rose high enough to let the column of water in it balance the pressure of the steam. He gave a definite numerical significance to the term “horse-power” (q.v.) as a mode of rating engines, defining it as the rate at which work is done when 33,000 ℔ are raised one foot in one minute.
15. In the fourth claim in Watt’s first patent the second
sentence describes a non-condensing engine, which would have
required steam of a higher pressure. This, however,
was a line of invention which Watt did not
follow up, perhaps because so early as 1725 a
non-condensing engine had been described by Jacob Leupold
Non-condensing Engine.
in his Theatrum machinarum. Leupold’s proposed engine
Fig. 8.—Leupold’s Non-Condensing Engine, 1725.
is shown in fig. 8, which
makes its action sufficiently
clear. Watt’s aversion to
high-pressure steam was
strong, and its influence on
steam engine practice long
survived the expiry of his
patents. So much indeed
was this the case that the
terms “high-pressure” and
“non-condensing” were for
many years synonymous in
contradistinction to the
“low-pressure” or condensing
engines of Watt. This
nomenclature no longer holds;
in modern practice many
condensing engines use as
high pressures as non-condensing
engines, and by doing
so are able to take advantage
of Watt’s great invention of expansive working to a degree
which was impossible in his own practice.
16. The introduction of the non-condensing and, at that time, relatively high-pressure engine was effected in England by Richard Trevithick and in America by Oliver Evans about 1800. Both Evans and Trevithick applied their engines to propel carriages on roads, and both used for boiler a cylindrical vessel with a cylindrical High-pressure Steam. flue inside—the construction now known as the Cornish boiler. In partnership with William Bull, Trevithick had previously made direct acting pumping-engines, with an inverted cylinder set over and in line with the pump-rod, thus dispensing with the beam that had been a feature in all earlier forms. But in these “Bull” engines, as they were called, a condenser was used, or, rather, the steam was condensed by a jet of cold water in the exhaust-pipe, and Boulton and Watt successfully opposed them as infringing Watt’s patents. To Trevithick belongs the distinguished honour of being the first to use a steam carriage on a railway; in 1804 he built a locomotive in the modern sense, to run on what had formerly been a horse-tramway, in Wales, and it is noteworthy that the
