The cylinder in Mr. Watt's engines is very accurately bored, and surrounded at a small distance with another cylinder fur nished with a bottom and lid. The inter val between the cylinders communicates with the boilers by a large pipe, open at both ends, so that it is always filled with steam, and thereby keeps the inner cylin der at the same temperature as that of the steam, and prevents any condensa tion. The inner cylinder has a bottom and piston as usual ; and as it does riot reach quite up to the lid of the outer cy linder, the steam in the interval has al ways free access to the upper side of the piston. The lid of the outer cylinder has a hole in its middle; and the piston rod, which is truly cylindrical, moves up and down through that hole, which is kept steam tight by a collar of oakum screwed down upon it. At the bottom of the in ner cylinder there are two regulating valves, one of which admits the steam to pass from the interval into the inner cy linder below the piston, or shuts it out at pleasure : the other opens or shuts the end of a pipe, which leads to the conden ser. The condenser consists of one or more pumps, furnished with clacks and buckets, which are worked by chains fas tened to the great working beam of the engine. The pipe which comes from the cylinder is joined to the bottom of these pumps, and the whole condenser stands Immersed in a cistern of cold water, sup plied by the engine. The place of the cistern is either within the house, or un der the floor, between the cylinder and the lever wall ; or without the house, be tween that wall and the engine shaft. The condenser being exhausted of sir, and both cylinders filled with steam, the regulating valve which admits the steam into the inner cylinder is shut, and the other regulator which communicates with the condenser is opened, and the steam rushes into the vacuum of the condenser with violence : but there it comes into contact with the cold sides of the pumps and pipes, and meets a jet of cold water, which was opened at the same time with the exhaustion regulator: these instantly deprive it of its heat, and reduce it to water; and the vacuum remaining per fect, more steam continues to rush in, and is condensed, till the inner cylinder is ex hausted. Then the steam which is above the piston, ceasing to he counteracted by that which was below it, acts upon the piston with its whole elasticity, and forces it to descend to the bottom of the cylin der, and so raises the buckets of the pumps, which are hung to the other end of the beam. The exhaustion regulator is now shut, and the steam one opened again ; which, by letting in the steam, allows the piston to be pulled up by the superior weight of the pump rods, and then the engine is ready for another stroke.
The peculiar advantages that result from this construction are, that the cylin der, being surrounded with the steam from the boiler, is kept always uniformly as hot as the steam itself, and is, there fore, incapable of destroying any part of the steam which should fill it : and again, the condenser being kept always as cold as water can be procured, the steam is perfectly condensed, and does not op pose the descent of the piston ; which is, therefore, forced down by the full power of the steam from the boiler, which is greater than that of the atmos phere.
A steam-engine of the best construction, with a thirty inch cylinder, acts with the force of 40 horses ; and since it acts with out intermission, will perform the work of 120 horses, or 600 men ; each square inch of the piston being nearly equivalent to a labourer. The consumption of about 841bs. of good pit-coal will raise 48,000 cubical feet of water 10 feet high, which is equivalent to more than the daily labour of eight men : the value of this quantity of coal is seldom so much as that of the work of a single labourer for a day ; but the expense of the machinery generally renders a steam-engine somewhat more than half as expensive as the number of horses for which it is substituted. We shall now proceed to a more particular description.
Plates I. and II. Steam Engine, are draw ings of a steam-engine of six horses pow er, built by Mr. John Dixion, Maid Lane, Southwark, and used by him in his steam engine manufactory, for turning lathes, &c. Plate 1. is a general elevation of the whole engine; and Plate II. is a section on a larger scale of the cylinder, steam pipes, condenser, &c. ; b b, Plate I. is the cylinder ; the internal structure will be described hereafter ; a is the piston rod connected to the great working beam, B, by a system of levers called a parallel motion, the property of which is, that the rectilinear motion of the piston rod is pre served, though the end of the beam de scribes an arc of a circle ; at the other end of the beam, B, the connecting rod, D, is jointed at its lower end : it is also joint ed to the crank, E, upon the axis of the great fly-wheel. When, by the expansive
force of the steam, the piston rod, a, is caused to ascend, it raises one end of the great beam and depresses the other, and by the connecting rod, D, turns the crank and fly-wheel round. As soon as the piston rod arrives at the top of its stroke, it receives a new impulse, which brings it down again, and consequently raises the connected rod, D, and crank, E. The use of the fly-wheel is to acquire an im petus, at the time when the crank is hori., zontal, and at which time the connecting rod exerts all the force of the engine upon the crank, to turn it round; this mo mentum causes the wheel to turn, and the rest of the machinery, when the crank is at the top or bottom of its motion, be ing then in a line with the connecting rod, it has no tendency to turn it round. To describe the manner in which the power is given to the piston rod, a, we must turn to Plate II. where bbbb is the cylinder of cast iron, and truly bored ; it is closed at the top by an iron lid, screw ed on by a flanch at the top; in the cen tre of the lid is a contrivance called a stuffing box, and is for holding a packing of hemp, through which the piston rod, a a, passes perfectly air and steam tight, d dis the piston packed with hemp, round between its circumference and the inside of the cylinder, so that it can move up and down in the cylinder easily, without allowing any steam to pass by it; the pis ton is fitted to the rod, a, and fast keyed in ; the cylinder has a flanch or project ing ring round it, a little below the mid dle, by which it is held into a jacket, c c c, which is constantly supplied with steam from the boiler, A ; Plate I. of the engine of the pipe, e; f f, Plate II. is a pipe, cast at the same time with the cy linder, leading from the top of it, and by a crooked passage to the cock, E ; g g is another similar passage from the bottom of the cylinder (which is closed, except this pipe has an iron bottom screwed to it) to the cock, and enters the cock dia. metrically opposite to the other passage ; Ii is a passage bringing steam from the jacket, c c c ; by means of a short pipe not seen in the figure, being behind the cy linder, cast at the same time with the cylinder and joining at the bottom to the flanch, by which the cylinder is screwed into the jacket, c c c; the bore of this short pipe is however continued through the Ranch, and opens into the jacket, and when they are screwed together, the steam has then free access from the boi. ler through the jacket into the short pipe, and from thence into the passage, /4 which goes horizontal towards it ; the short pipe has a thin circular vane in it, turning on a pivot across the centre of it, which comes through the pipe, and has a handle on it; by turning this handle, the spindle and vane are turned also ; when the vane is set, so that its plane is perpendicular to the axis of the pipe, it nearly fills the circular passage, and allows very little steam to pass by it; but when the vane is turned edgewise, it presents very little surface, and the steam passes by without obstruction to the cock, E ; p p p is a pipe conveying the steam away from the engine to the condenser, which is an iron cylinder, F F, immersed in cold water contained in a cast iron cistern, G G ; the condenser, F F, has a passage from the bottom of it, leading to a pump, H H, called the air-pump, and in which is a valve, i, shutting towards the conden ter, and preventing any passage from the air-pump to the condenser, though the valve will easily open and allow a passage the other way. The air-pump has a bucket, k, sliding up and down in it ; the bucket has a hole through it, covered by a valve, 4 which opens upwards, and pre vents the descent of any fluid which may be above it ; I is a square iron cistern, screwed on the top of the air-pump, it has a stuffing box in the bottom over the centre of the pump, through which the bucket rod of the air-pump moves freely, yet perfectly airtight; the bottom of the cistern, I, has also two valves, in m, in it, over the air-pump ; K is a handle fixed upon a spindle, on which is a rack turning a cog-wheel upon the end of the cock, E ; this rack and wheel are taken away in the section, but are plainly seen in Plate I.; L M are two pins fixed upon the rod of the air-pump, and taking the handle, K, as they move up and down, and thus turning the cock, E; n an is a lever fixed on the spindle of the handle, K ; its ends stop against the ends of a crooked iron, which is screwed to the iron frame, sup porting the bearings for the spindle of the rack, so that the motion allowed thereby to the handle, K, and the rack, will turn the cock one-fourth of a whole turn, but no more; N is a cock communi cating (when open) from the jacket, c c, to the pipe, p p, and thereby to the con denser.