to the compound engine, consisting of
two engines, primary and secondary, working steam expansively
independently or in concert, with strokes alternate or simultaneous. The
compound engine was first thought of by Watt about 1767. He laid a large
drawing of it on parchment before parliament when soliciting an
extension of his first patent. The reason he did not proceed to
construct it was "the difficulty he had encountered in teaching others
the construction and use of the single engine, and in overcoming
prejudices"; the patent of 1782 was only taken out because he found
himself "beset with a host of plagiaries and pirates."

One of the earliest of these double-acting engines was erected at the
Albion Mills, London, in 1786. Watt writes:

The mention of Albion Mills induces me to say a few words
respecting an establishment so unjustly calumniated in its day,
and the premature destruction of which, by fire, in 1791, was,
not improbably, imputed to design. So far from being, as
misrepresented, a monopoly injurious to the public, it was the
means of considerably reducing the price of flour while it
continued at work.

The "double-acting" engine was followed by the "compound" engine, of
which Watt says:

A new compound engine, or method of connecting together the
cylinders and condensers of two or more distinct engines, so as
to make the steam which has been employed to press on the piston
of the first, act expansively upon the piston of the second,
etc., and thus derive an additional power to act either
alternately or co-jointly with that of the first cylinder.

We have here, in all substantial respects, the modern engine of to-day.

Two fine improvements have been made since Watt's time: first, the
piston-rings of Cartwright, which effectively removed one of Watt's most
serious difficulties, the escape of steam, even though the best packing
he could devise were used--the chief reason he could not use
high-pressure steam. In our day, the use of this is rapidly extending,
as is that of superheated steam. Packing the piston was an elaborate
operation even after Watt's day.

It was not because Watt did not know as well as any of our present
experts the advantages of high pressures, that he did not use them, but
simply because of the mechanical difficulties then attending their
adoption. He was always in advance of mechanical practicalities rather
than behind, and a