[5]
The uses of Watt's steam-engine began to be recognised as
available for manufacturing purposes. It was then found
necessary to invent some method by which continuous rotary motion
should be secured, so as to turn round the moving machinery of
mills. With this object Watt had invented his original
wheel-engine. But no steps were taken to introduce it into
practical use. At length he prepared a model, in which he made
use of a crank connected with the working beam of the engine, so
as to produce the necessary rotary motion.
There was no originality in this application. The crank was one
of the most common of mechanical appliances. It was in daily use
in every spinning wheel, and in every turner's and
knife-grinder's foot-lathe. Watt did not take out a patent for
the crank, not believing it to be patentable. But another person
did so, thereby anticipating Watt in the application of the crank
for producing rotary motion. He had therefore to employ some
other method, and in the new contrivance he had the valuable help
of William Murdock. Watt devised five different methods of
securing rotary motion without using the crank, but eventually he
adopted the "Sun-and-planet motion," the invention of Murdock.
This had the singular property of going twice round for every
stroke of the engine, and might be made to go round much oftener
without additional machinery.
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