tidious mechanical taste of Watt. He soon perceived that all which he proposed to accomplish by the application of two cylinders and pistons working alternately, could be attained with greater simplicity and effect by a single cylinder, if he could devise means by which the piston might be impelled by steam upwards as well as downwards. To accomplish this, it was only necessary to throw the lower end of the cylinder into alternate communication with the boiler, while the upper end would be put into communiIcation with the condenser. If, for example, during the descent of the piston, the upper end of the cylinder communicated with the boiler, and the lower end with the condenser; and, on the other hand, during the ascent of the piston, the lower end communicated with the boiler, and the upper end with the condenser; then the piston would be driven continually, whether upwards or downwards, by the power of steam acting against a vacuum. Watt obtained his third patent for this contrivance, on the 12th of March, 1782. This change in the principle of the machine involved several other changes in the details of its mechanism. B (116.) It was necessary, in the first place, to provide means for admitting and withdrawing the steam at either end of the Fig. 33. cylinder. For this purpose let B and B' fig.33.) be two steam-boxes, в the upper, and B' the lower, communicating respectively with the top and bottom of the cylinder by proper passages D D'. Let two valves be placed in в, one, s, above the passage D, and the other, c, below it; and in like manner two other valves in the lower valve-box, B', one, s', above the passage D', and the other, c', below it. Above the valve s in the upper steam-box is an opening at which the steam-pipe from the boiler enters, and below the valve c is another opening, at which enters the exhausting-pipe leading to the condenser. In like manner, above the valve s' in the lower steam-box enters a steam-pipe leading from the boiler, and below the valve c' enters an exhausting-pipe leading to B' the condenser. It is evident, therefore, that steam can always be admitted above the piston by opening the valve s, and below it by opening the valve s'; and, in like manner, steam can be withdrawn from the cylinder above the piston, and allowed to pass to the condenser, by opening the valve c, and from below it by opening the valve c'. Fg. 34. B D Supposing the piston P to be at the top of the cylinder, and the cylinder below the piston to be filled with pure steam, let the valves s and c' be opened, the valves c and s' being closed as represented in fig. 34. Steam from the boiler will, therefore, flow in through the open valve s, and will press the piston downwards, while the steam that has filled the cylinder below the piston will pass through the open valve c' into the exhausting-pipe leading to the condenser, and being condensed will leave the cylinder below the piston a vacuum. The piston will, therefore, be pressed downwards by the action of the steam above it, as in the single-acting engine. Having arrived at the bottom of the cylinder, let the valves s and c' be both closed, and the valves s' and c be opened, as represented in fig. 34. Steam will now be admitted through the open valve s' and through the passage D' below the piston, while the steam which has just driven the piston downwards, filling the cylinder above the piston, will be drawn off through the open valve c, and the exhausting-pipe, into the condenser, leaving the cylinder above the piston a vacuum. The piston will, therefore, be pressed upwards by the action of the steam below it, against the vacuum above it, and will ascend with the same force as that with which it had descended. This alternate action of the piston upwards and downwards may evidently be continued by opening and closing the valves alternately in pairs. Whenever the piston is at the top of the cylinder, as represented in fig. 33., the valves s and c', that is, the upper steam-valve and the lower exhaustingvalve, are opened, and the valves c and s', that is, the upper exhausting-valve and the lower steam-valve, are closed; and when the piston has arrived at the bottom of the cylinder, as represented in fig. 34., the valves c and s', that is, the upper exhausting-valve and the lower steam-valve, are opened, and the valves s and c', that is, the upper steam-valve and the lower exhausting-valve, are closed. If these valves, as has been here supposed, be opened and closed at the moments at which the piston reaches the top and bottom of the cylinder, it is evident that they may be all worked by a single lever connected with them by proper mechanism. When the piston arrives at the top of the cylinder, this lever would be made to open the valves s and c', and at the same time to close the valves s' and c; and when it arrives at the bottom of the cylinder, it would be made to close the valves s and c', and to open the valves s' and c. If, however, it be desired to cut off the steam before the arrival of the piston at the termination of its stroke, whether upwards or downwards, then the steam-valves must be closed before the arrival of the piston at the end of its stroke; and as the exhausting-valve ought to be left open until the stroke is completed, these valves ought to be moved at different times. In that case separate levers should be provided for the different valves. We shall, however, return again to the subject of the valves which regulate the admission of steam to the cylinder and its escape to the condenser. (117.) It will be remembered that in the single-acting engine the process of condensation was suspended while the piston ascended in the cylinder, and therefore the play of the jet of cold water in the condenser was stopped during this interval. In the double-acting engine, however, the flow of steam from the cylinder to the condenser is continued, whether the piston ascends or descends, and therefore a constant condensation of steam must be produced. The condensing jet, therefore, does not in this case, as in the former, play with intervals of intermission. A constant jet of cold water must be maintained in the condenser. It will presently appear that in the double-acting engine applied to manufactures, the motion of the piston was subject to more or less variation of speed, and the quantity of steam admitted to the cylinder was subject to a corresponding change. The quantity of steam, therefore, drawn into the condenser was subject to variation, and required a considerable change in the quantity of cold water admitted through the jet to condense it. To regulate this, the valve or cock by which the water was admitted into the condenser was worked in the double-acting engine by a lever furnished with an index, by which the quantity of condensing water admitted into the condenser could be regulated. This index played upon a graduated arch, by which the engine-man was enabled to regulate the supply. METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE DOUBLEACTING ENGINE.- RACK AND SECTOR. - PARALLEL MOTION. CONNECTING ROD AND CRANK.-FLY-WHEEL. THROTTLE-VALVE.GOVERNOR CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING ENGINE ECCENTRIC. COCKS AND VALVES. SINGLE-CLACK VALVE. CONICAL VALVES. - SLIDE VALVES. -DOUBLE-CLACK VALVE. (118.) IN the single-acting engine, the force of the piston acted on the beam only during its descent; and this force was transmitted from the piston to the beam, as we have seen, by a flexible chain, extended from the end of the piston-rod, |