If such an arrangement of mechanism as has been here described can be connected with the beam of the steam engine, so that while the point _x′_ is attached to the top of the steam piston, and the space through which it ascends and descends shall be equal to the length of the stroke of that piston, the point _x_ shall be attached to the rod of the air-pump piston, the stroke of the latter being half that of the steam piston, then the points _x′_ and _x_ will guide the motion of the two pistons so as to preserve them in true vertical straight lines. The manner in which these ideas are reduced to practice admits of easy explanation: let the point O be the centre of the great working beam, and let O P′ be the arm of the beam on the side of the steam cylinder. Let P be a pivot upon the beam, at the middle point between its centre O and its extremity P′; and let the links P p, P′ _x′_, and P _p_ be jointed together, as already described. Let the point or pivot _o_ be attached to some part of the fixed framing of the engine or engine house, and let the rod _o p_, equal to half the arm of the beam, be attached by a pivot to the corner of the parallelogram at [Pg201] _p_. Let the end of the steam piston-rod be attached to the corner of the parallelogram _x′_, and let the end of the air-pump be attached to the middle point _x_ of the link P _p_; by which arrangement it is evident that the rectilinear motion of the two piston-rods will be rendered compatible with the alternate circular motions of the points P′ and P on the beam. Among the many mechanical inventions produced by the fertile genius of Watt, there is none which has excited such universal, such unqualified, and such merited admiration as that of the parallel motion. It is indeed impossible, even for an eye unaccustomed to view mechanical combinations, to behold the beam of a steam engine moving the pistons, through the instrumentality of the parallel motion, without an instinctive feeling of pleasure at the unexpected fulfilment of an end by means having so little apparent connection with it. When this feeling was expressed to Watt himself, by those who first beheld the performance of this exquisite mechanism, he exclaimed with his usual vivacity, that he himself, when he first beheld his own contrivance in action, was affected by the same sense of pleasure and surprise at its regularity and precision. He said, that he received from it the same species of enjoyment that usually accompanies the first view of the successful invention of another person.