It is evident that, in the arrangement now described, the condenser must be in constant action: while the piston is descending the condenser must draw off the steam below it, and while it is ascending, it must draw off the steam above it. As steam, therefore, must be constantly drawn into the condenser, the jet of cold water which condenses the steam must be kept constantly playing. This jet, therefore, will not be worked by the valve alternately opening and closing, as in the single engine, but will be worked by a cock, the opening of which will be adjusted according to the quantity of cold water necessary to condense the steam. When the steam is used at a low pressure, and, therefore, in a less compressed state, less condensing water would be necessary than when it is used at a higher pressure and in a more compressed state. In the one case, therefore, the condensing cock would be less open than in the other. Again, the quantity of condensing water must vary with the speed of the engine, because the greater the speed of the engine, the more rapidly will the steam flow from the cylinder into the condenser; and, as the same quantity of steam requires the same quantity of condensing water, the supply of the condensing water must be proportional to the speed of the engine. In the double-acting engine, then, the jet cock is regulated by a lever or index which moves upon a graduated arch, and which is regulated by the engineer according to the manner in which the engine works.
This change in the action of the steam upon the piston rendered it necessary to make a corresponding change in the mechanism by which the piston-rod was connected with the beam. In the single acting engine, the piston-rod pulled the end of the beam down during the descent, and was pulled up by it in the ascent. The connection by which this action was transmitted between the beam and piston was, as we have seen, a flexible chain passing from the end of the piston and playing upon the arch-head of the beam. Now, where the mechanical action to be transmitted is a pull, and not a push, a flexible chain, or cord, or strap, is always sufficient; but if a push or thrust is required to be transmitted, then the flexibility of the medium of mechanical communication afforded by a chain, renders it inapplicable. In the double-acting engine, during the descent, the piston-rod still pulls the beam down, and so far a chain connecting the piston-rod with the beam would be sufficient to transmit the action of the one to the other; but in the ascent the beam no longer pulls up the piston-rod, but is pushed up by it. A chain from the piston-rod to the arch-head, as described in the single acting engine, would fail to transmit this force. If such a chain were used with the double engine, where there is no counter weight on the opposite end of the beam, the consequence would be, that in the ascent of the piston the chain would slacken, and the beam would still remain depressed. It is therefore necessary that some other mechanical connection be contrived between the piston-rod and the beam, of such a nature that in the descent the piston-rod may pull the beam down, and may push it up in the ascent.
Watt first proposed to effect this by attaching to the end of the piston-rod a straight rack, faced with teeth, which should work in corresponding teeth raised on the arch-head of the beam as represented in [fig. 13]. If his improved steam engines required no further precision of operation and construction than the atmospheric engines, this might have been sufficient; but in these engines it was indispensably necessary that the piston-rod should be guided with a smooth and even motion through the stuffing-box in the top of the cylinder, otherwise any shake or irregularity would cause it to work loose in the stuffing-box, and either to admit the air, or to let the steam escape. In fact, it was necessary to turn these piston-rods very accurately in the lathe, so that they may work with sufficient precision in the cylinder. Under these circumstances, the motion of the rack and toothed arch-head were inadmissible, since it was impossible by such means to impart to the piston-rod that smooth and equable motion which was requisite. Another contrivance which occurred to Watt was, to attach to the top of the piston-rod a bar which should extend above the beam, and to use two chains or straps, one extending from the top of the bar to the lower end of the arch-head, and the other from the bottom of the bar to the upper end of the arch-head. By such means the latter strap would pull the beam down when the piston would descend, and the former would pull the beam up when the piston would ascend. These contrivances, however, were superseded by the celebrated mechanism, since called the Parallel Motion, one of the most ingenious mechanical combinations connected with the history of the steam engine.
It will be observed that the object was to connect by some inflexible means the end of the piston-rod with the extremity of the beam, and so to contrive the mechanism, that while the end of the beam would move alternately up and down in a circle, the end of the piston-rod connected with the beam should move exactly up and down in a straight line. If the end of the piston-rod were fastened upon the end of the beam by a pivot without any other connexion, it is evident that, being moved up and down in the arch of a circle, it would be bent to the left and the right alternately, and would consequently either be broken, or would work loose in the stuffing box. Instead of connecting the end of the rod immediately with the end of the beam by a pivot, Watt proposed to connect them by certain moveable rods so arranged that, as the end of the beam would move up and down in the circular arch, the rods would so accommodate themselves to that motion, that the end connected with the piston-rod should not be disturbed from its rectilinear course.
To accomplish this, he conceived the notion of connecting three rods in the following manner:—A B and C D ([fig. 14].), are two rods or levers turning on fixed pivots or centres at A and C. A third rod, B D, is connected with them by pivots placed at their extremities, B and D, and the lengths of the rods are so adjusted that when A B and C D are horizontal, B D shall be perpendicular or vertical, and that A B and C D shall be of equal lengths. Now, let a pencil be imagined to be placed at P, exactly in the middle of the rod B D: if the rod A B be caused to move up and down like the beam of the steam engine in the arch represented in the figure, it is clear, from the mode of their connexion, that the rod C D will be moved up and down in the other arch. Now Watt conceived that, under such circumstances, the pencil P would be moved up and down in a perpendicular straight line.
However difficult the first conception of this mechanism may have been, it is easy to perceive why the desired effect will be produced by it. When the rod A B rises to the upper extremity of the arch, the point B departs a little to the right; at the same time, the point D is moved a little to the left. Now the extremities of the rod B D being thus at the same time, carried slightly in opposite directions, the pencil in the middle of it will ascend directly upwards; the one extremity of the rod having a tendency to draw it as much to the right as the other has to draw it to the left. In the same manner, when the rod A B moves to the lower extremity of the arch, the rod C D will be likewise moved to the lower extremity of its arch. The point B is thus transferred a little to the right, and the point D to the left; and, for the same reason as before, the point P in the middle will move neither to the right nor to the left, but straight downwards.[17]
Now Watt conceived that his object would be attained if he could contrive to make the beam perform the part of A B in [fig. 14]., and to connect with it other two rods, C D and D B, attaching the end of the piston to the middle of the rod D B. The practical application of this principle required some modification, but is as elegant as the notion itself is ingenious.
The apparatus adopted for carrying it into effect is represented on the arm which works the piston in [fig. 15]. The beam, moving on its axis C, every point in its arm moves in the arc of a circle of which C is the centre. Let B be the point which divides the arm A C into equal parts, A B and B C; and let D E be a straight rod equal in length to C B, and playing on the fixed centre or pivot D. The end E of this rod is connected by a straight bar, E B, with the point B, by pivots at B and E on which the rod B E plays freely. If the beam be supposed to move alternately on its axis C, the point B will move up and down in a circular arc, of which C is the centre, and at the same time the point E will move in an equal circular arc round the point D as a centre. According to what we have just explained, the middle point F of the rod B E will move up and down in a straight line.
Also, let a rod, A G, equal in length to B E, be attached to the end A of the beam by a pivot on which it moves freely, and let its extremity G be connected with E by a rod, G E, equal in length to A B, and playing on pivots at G and E.
By this arrangement the joint A G being always parallel to B E, the three points C, A, and G will be in circumstances precisely similar to the points C, B, and F, except that the system C A G will be on a scale of double the magnitude of C B F: C A being twice C B, and A G twice B F, it is clear, then, that whatever course the point F may follow, the point G must follow a similar line,[18] but will move twice as fast. But, since the point F has been already shown to move up and down in a straight line, the point G must also move up and down in a straight line, but of double the length.[19]