thought-workshop, but more from curiosity than in any spirit of investigation. And yet it would appear from what we have said that here there is a great step further to be made. Let us try.
Watt has left behind for us in a letter some indications of the line of thought which led him directly to the mechanism just alluded to. "The idea," he writes to his son in November 1808, "originated in this manner. On finding double chains, or racks and sectors, very inconvenient for communicating the motion of the piston-rod to the angular motion of the working beam, I set to work to try if I could not contrive some means of performing the same from motions turning upon centres, and after some time it occurred to me that A B and C D being two equal radii revolving on the centres B and C, and connected together by a rod A D, in moving through arches of certain lengths, the variation from the straight line would be nearly equal and opposite, and that the point E would describe a line nearly straight, and that if for convenience the radius C D was only half of A B, by moving the point E nearer to D the same would take place, and from this the construction, afterwards called the parallel motion, was derived. .... Though I am not over anxious after fame, yet I am more proud of the parallel motion than of any other invention I have ever made."
Interesting as this letter is, a closer examination of it reveals a deficiency which perhaps the questioner also may have discovered. We quite appreciate the motives as well as some of the final results of Watt's exertions, but we obtain no indication of a methodical train of ideas leading up to them. Moreover it must be remembered that the description is written twenty-four years
- Facsimile from Watt's letter. See Muirhead's Mechanical Inventions of James Watt, vol. ii. p. 88.