Steam greater than when it was stopped at one-fourth, and yet Engine. the accumulated pressure is not twice as great, being nearly five-thirds. One-fourth of the steam performs nearly three-fifths of the work, and an equal quantity performs more than twice as much work when thus admitted during one-fourth of the motion.

Plate DIIL

71

It is very pleasing to observe so many unlooked-for advantages resulting from an improvement made with the sole view of lessening the waste of steam by condensation. While this purpose is gained, we learn how to husband the steam which is not thus wasted. The engine becomes more manageable, and is more easily adapted to every variation in its task, and all its powers are more easily computed.

The active mind of its ingenious inventor did not stop here: It had always been matter of regret that one-half of the motion was unaccompanied by any work. It was a very obvious thing to Mr Watt, that as the steam admitted above the piston pressed it down, so steam admitted below the piston pressed it up with the same force, provided that a vacuum were made on its upper side. This was easily done, by connecting the lower end of the cylinder with the boiler and the upper end with the condenser.

Fig. 11. is a representation of this construction exact ly copied from Mr Watt's figure accompanying his speDescrip- cification. Here BB is a section of the cylinder, surtion of rounded at a small distance by the case 1111. The secMr Watt's tion of the piston A, and the collar of leathers which steam-enembraces the piston rod, gives a distinct notion of its construction, of the manner in which it is connected with the piston-rod, and how the packing of the piston and collar contributes to make all tight.

gine in its

most improved state.

From the top of the cylinder proceeds the horizontal pipe. Above the letter D is observed the seat of the steam valve, communicating with the box above it. In the middle of this may be observed a dark shaded circle. This is the mouth of the upper branch of the steam pipe coming from the boiler. Beyond D, below the letter N, is the seat of the upper condensing valve. The bottom of the cylinder is made spherical, fitting the piston, so that they may come into entire contact. Another horizontal pipe proceeds from this bottom. Above the letter E is the seat of the lower steam valve, opening into the valve box. This box is at the extremity of another steam pipe marked C, which branches off from the upper horizontal part, and descends obliquely, com

ing forward to the eye. The lower part is represented Steamas cut open, to shew its interior conformation. Beyond Engine. this steam valve, and below the letter F, may be observed the seat of the lower condensing valve. A pipe descends from hence, and at a small distance below unites with another pipe GG, which comes down from the upper condensing valve N. These two eductionpipes thus united go downwards, and open at L into a rectangular box, of which the end is seen at L. This box goes backward from the eye, and at its farther extremity communicates with the air-pump K, whose piston is here represented in section with its butterfly valves. The piston delivers the water and air laterally into another rectangular box M, darkly shaded, which box communicates with the pump I. The piston rods of this and of the air-pump are suspended by chains from a small arch head on the inner arm of the great beam. The lower part of the eduction-pipe, the horizontal box L, the air-pump K, with the communicating box M between it and the pump I, are all immersed in the cold water of the condensing cistern. The box L is made flat, broad, and shallow, in order to increase its surface and accelerate the condensation. But that this may be performed with the greatest expedition, a small pipe H, open below (but occasionally stopped by a plug valve), is inserted laterally into the eduction-pipe G, and then divides into two branches; one of which reaches within a foot or two of the upper valve N, and the other approaches as near to the valve F.

At it is intended by this construction to give the piston a strong impulse in both directions, it will not be proper to suspend its rod by a chain from the great beam; for it must not only pull down that end of the beam, but also push it upwards. It may indeed be suspended by double chains like the pistons of the engines for extinguishing fires; and Mr Watt has accordingly done so in some of his engines. But in his drawing from which this figure is copied, he has communicated the force of the piston to the beam by means of a toothed rack 00, which engages or works in the toothed sector QQ on the end of the beam. The reader will understand, without any farther explanation, how the impulse given to the piston in either direction is thus transmitted to the beam without diminution. The fly XX, with its pinion Y, which also works in the toothed arch QQ, may be supposed to be removed for the present, and will be considered afterwards.

We shall take the present opportunity of describing Mr Watt's method of communicating the force of the steam-engine to any machine of the rotatory kind. VV represents the rim and arms of a very large and heavy metalline fly. On its axis is the concentric toothed wheel U. There is attached to the end of the great beam a strong and stiff rod TT, to the lower end of which a toothed wheel W is firmly fixed by two bolts, so that it cannot turn round. This wheel is of the same size and in the same vertical plane with the wheel U; and an iron link or strap (which cannot be seen here, because it is on the other side of the two wheels) connects the centres of the two wheels, so that the one cannot quit the other. The engine being in the position represented in the figure, suppose the fly to be turned once round by any external force in the direction of the darts. It is plain, that since the toothed wheels cannot quit each other, being kept together by

Steam greater than when it was stopped at one-fourth, and yet Engine. the accumulated pressure is not twice as great, being nearly five-thirds. One-fourth of the steam performs nearly three-fifths of the work, and an equal quantity performs more than twice as much work when thus admitted during one-fourth of the motion.

Plate DIIL.

71

It is very pleasing to observe so many unlooked-for advantages resulting from an improvement made with the sole view of lessening the waste of steam by condensation. While this purpose is gained, we learn how to husband the steam which is not thus wasted. The engine becomes more manageable, and is more easily adapted to every variation in its task, and all its powers are more easily computed.

The active mind of its ingenious inventor did not stop here: It had always been matter of regret that one-half of the motion was unaccompanied by any work. It was a very obvious thing to Mr Watt, that as the steam admitted above the piston pressed it down, so steam admitted below the piston pressed it up with the same force, provided that a vacuum were made on its upper side. This was easily done, by connecting the lower end of the cylinder with the boiler and the upper end with the condenser.

Fig. 11. is a representation of this construction exact ly copied from Mr Watt's figure accompanying his speDescrip- cification. Here BB is a section of the cylinder, surtion of rounded at a small distance by the case 1111. The secMr Watt's tion of the piston A, and the collar of leathers which steam-enembraces the piston rod, gives a distinct notion of its construction, of the manner in which it is connected with the piston-rod, and how the packing of the piston and collar contributes to make all tight.

gine in its

most improved

state.

From the top of the cylinder proceeds the horizontal pipe. Above the letter D is observed the seat of the steam valve, communicating with the box above it. In the middle of this may be observed a dark shaded circle. This is the mouth of the upper branch of the steam pipe coming from the boiler. Beyond D, below the letter Beyond D, below the letter N, is the seat of the upper condensing valve. The bottom of the cylinder is made spherical, fitting the piston, so that they may come into entire contact. Another horizontal pipe proceeds from this bottom. Above the letter E is the seat of the lower steam valve, opening into the valve box. This box is at the extremity of another steam pipe marked C, which branches off from the upper horizontal part, and descends obliquely, com

ing forward to the eye. The lower part is represented Steamas cut open, to shew its interior conformation. Beyond Engine. this steam valve, and below the letter F, may be observed the seat of the lower condensing valve. A pipe descends from hence, and at a small distance below unites with another pipe GG, which comes down from the upper condensing valve N. These two eductionpipes thus united go downwards, and open at L into a rectangular box, of which the end is seen at L. This box goes backward from the eye, and at its farther extremity communicates with the air-pump K, whose piston is here represented in section with its butterfly valves. The piston delivers the water and air laterally into another rectangular box M, darkly shaded, which box communicates with the pump I. The piston-rods of this and of the air-pump are suspended by chains from a small arch head on the inner arm of the great beam. The lower part of the eduction-pipe, the horizontal box L, the air-pump K, with the communicating box M between it and the pump I, are all immersed in the cold water of the condensing cistern. The box L is made flat, broad, and shallow, in order to increase its surface and accelerate the condensation. But that this may be performed with the greatest expedition, a small pipe H, open below (but occasionally stopped by a plug valve), is inserted laterally into the eduction-pipe G, and then divides into two branches; one of which reaches within a foot or two of the upper valve N, and the other approaches as near to the valve F.

At it is intended by this construction to give the piston a strong impulse in both directions, it will not be proper to suspend its rod by a chain from the great beam; for it must not only pull down that end of the beam, but also push it upwards. It may indeed be suspended by double chains like the pistons of the engines for extinguishing fires; and Mr Watt has accordingly done so in some of his engines. But in his drawing from which this figure is copied, he has communicated the force of the piston to the beam by means of a toothed rack 00, which engages or works in the toothed sector QQ on the end of the beam. The reader will understand, without any farther explanation, how the impulse given to the piston in either direction is thus transmitted to the beam without diminution. The fly XX, with its pinion Y, which also works in the toothed arch QQ, may be supposed to be removed for the present, and will be considered afterwards.

We shall take the present opportunity of describing Mr Watt's method of communicating the force of the steam-engine to any machine of the rotatory kind. VV represents the rim and arms of a very large and heavy metalline fly. On its axis is the concentric toothed wheel U. There is attached to the end of the great beam a strong and stiff rod TT, to the lower end of which a toothed wheel W is firmly fixed by two bolts, so that it cannot turn round. This wheel is of the same size and in the same vertical plane with the wheel U; and an iron link or strap (which cannot be seen here, because it is on the other side of the two wheels) connects the centres of the two wheels, so that the one cannot quit the other. The engine being in the position represented in the figure, suppose the fly to be turned once round by any external force in the direction of the darts. It is plain, that since the toothed wheels cannot quit each other, being kept together by