THE STEAM-ENGINE. 



445 



crank in the same year, by Mr. James Pickard, of Birmingham. It will 

 presently appear, however, that the suggestion of this application of the crank 

 was derived from the proceedings of Watt, who was at the same time engaged 

 in similar experiments. 



The single-acting steam-engine, as constructed by Watt, was not adapted 

 to produce continuous uniform motion of rotation, for the following reasons : 



First. The effect required was that of a uniformly-acting force. The 

 steam-engine, on the other hand, supplied an intermitting force. Its operation 

 was continued during the descending motion of the piston, but it was suspended 

 during the ascent of the piston. To produce the continued effect now required, 

 either its principle of operation should be altered, or some expedient should be 

 devised for maintaining the motion of the revolving shaft during the ascent of 

 the piston, and the consequent suspension of the moving power. 



Secondly. The action of the steam-engine was rectilinear. It was a power 

 which acted in a straight line, viz., in the direction of the cylinder. The 

 motion, however, required to be produced, was a circular motion a motion of 

 rotation around the axis or shaft of the mill. 



The steps by which Watt proceeded to accomplish these objects have been 

 recorded by himself as follows, in his notes upon Dr. Robinson's article on the 

 steam-engine : 



" I had very early turned my mind to the producing of continued motion 

 round an axis ; and it will be seen, by reference to my first specification in 

 1769, that I there described a steam-wheel, moved by the force of steam, acting 

 in a circular channel against a valve on one side, and against a column of 

 mercury, or some other fluid metal, on the other side. This was executed 

 upon a scale of about six feet diameter at Soho, and worked repeatedly, but 

 was given up, as several practical objections were found to operate against it ; 

 similar objections lay against other rotative engines, which had been contrived 

 by myself and others, as well as to the engines producing rotatory motions by 

 means of ratchet-wheels. 



" Having made my single reciprocating engines very regular in their move- 

 ments, I considered how to produce rotative motions from them in the best 

 manner ; and among various schemes which were subjected to trial, or which 

 passed through my mind, none appeared so likely to answer the purpose as the 

 application of the crank, in the manner of the common turning lathe ; but as 

 the rotative motion is produced in that machine by impulse given to the crank 

 in the descent of the foot only, it requires to be continued in its ascent by the 

 energy of the wheel, which acts as a fly ; being unwilling to load ray engines 

 with a fly-wheel heavy enough to continue the motion during the ascent of the 

 piston (or with a fly-wheel heavy enough to equalize the motion, even if a 

 counterweight were employed to act during that ascent), I proposed to employ 

 two engines, acting upon two cranks fixed on the same axis, at an angle of 

 120 to one another, and a weight placed upon the circumference of the fly- 

 wheel at the same angle to each of the cranks, by which means the motion 

 might be rendered nearly equal, and only a very light fly-wheel would be 

 requisite. 



" This had occurred to me very early ; but my attention being fully employed 

 in making and erecting engines for raising water, it remained in pstto until 

 about the year 1778 or 1779, when Mr. Wasbrough erected one of his ratchet- 

 wheel engines at Birmingham, the frequent breakages and irregularities of 

 which recalled the subject to my mind, and I proceeded to make a model of 

 my method, which answered my expectations ; but having neglected to take 

 out a patent, the invention was communicated by a workman employed to 

 make the model, to some of the people about Mr. Wasbrough's engine, and a* 



