96 
Steam Engine. 
by the engineer there employed. This did not disheail- 
en Mr. Watt ; and, without troubling himself with set- 
ting aside a patent which, so long as it continued attach- 
ed to the common atmospheric engine, could do him little 
harm, he set about other modes of effecting the same 
thing; and, in 1781, took out a patent for several new 
methods of applying the vibrating or reciprocating motion 
of steam engines to produce a continued rotative motion 
round an axis ; one of which was that beautiful contri- 
vance of the revolving motion of one wheel round another. 
These and the crank were indifferently used in his en- 
gines, without any molestation on the part of the pirati- 
cal patentee. 
This, however, was only a part of what Mr. Watt saw 
to be necessary, in order to perfect this application of the 
steam engine. The steam had hitherto been used only 
to press do^vn the piston, which was returned by a weight 
at the opposite end of the beam, so that the power of the 
steam may be said to have been inactive during that pe- 
riod. Mr. Watt remedied this, by applying the power of 
the steam to press the piston down, as well as to press it 
up, thus forming alternately a vacuum above and below 
the piston. This he called the double engine ; and, in 
fact, it doubled the power exerted within the same cylin- 
der. He had long had in his mind the idea of this im- 
provement ; and had even produced a drawing of it to 
the house of commons in 1774, at the time he procured 
the act to prolong his original patent ; but the first he ex- 
ecuted was, we believe, at Soho in the year 1781 or 1782,- 
and the first public exhibition of it at the Albion Mills a ^ 
few years later. i 
About the same period, finding double chains or racks 
and sectors very inconvenient for communicating the mo- 
tion of the piston rod to the angular motion of the beam, 
he invented and applied tvhat has been called the parallel 
