English: Hand-drawn diagram by James Watt explaining his discovery of the "parallel motion" in a letter to his Sohn: "The idea originated in this manner. On finding double chains, or racks and sectors, very inconvenient for communicating the motion of the piston-rod to the angulat motion of the working-beam, I set to work to try if I could not contrieve some means of performing the same form motions turning upon centres, and after some time it occured to me that AB, CD, being two equal radii revolving on the centres B and C, and connected together by a rod AD, in moving through arches of certain length, the variations from straight line would be nearly equal and opposite, and that the point E would describe a line nearly straight, and that if for convenice the radius CD was only half of AB, 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."