STEAM-Engine

STEAM-Engine, an engine for raising water by means of fire.

The earliest account to be met with of the invention of this engine is in a book published by the marquis of Worcester in the year 1663, where he proposed the raising of great quantities of water by the force of fire, or by turning water into steam; and mentions an engine of that kind of his own contrivance which could raise a continual stream like a fountain 40 feet high, by the means of two cocks which were alternately and successively turned by a man to admit the steam, and to re-fill the vessel with cold water, the fire being continually kept up: for which reason this nobleman is generally looked upon as the first inventor of this engine; and though his method of applying the force of steam was certainly much different from the present, yet the water was raised by the same original power, which is the expansion of water into steam by fire. However, this invention not meeting with encouragement, probably owing to the confused state in which the public affairs were about that time, it was neglected, and lay dormant several years until one captain Savery having read the marquis of Worcester's books, several years afterwards, tried many experiments upon the force and power of steam; and at last luckily hit upon a method of applying it to raise water. This he had no sooner done, than he bought up and destroyed all the marquis's books that could be got, and then claimed the honour of this invention to himself, and obtained a patent for it, pretending to have discovered this secret of nature by such an accident as by experiment was found could not give him any such idea. He contrived an engine which, after many experiments, he brought to some degree of perfection, so as to raise water in small quantities: but he could not succeed in raising water a great height and in large quantities for the draining of mines; to effect which by his method, the steam required to be boiled to such a strength as would have
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burst all his vessels; so that he was obliged to be content with raising water a small height or in small quantities. The largest engine that he ever erected, was for the York-buildings company in London, for the supplying the inhabitants of the Strand and that neighbourhood with water. A draught and description of one of these engines is in Harris's Lexicon Technicum.

At the same time that captain Savery was employed in perfecting his engine, Dr Papinus of Marburg was contriving one on the same principles, which he describes in a small book published in 1707, intitled Ars nova ad aquam ignis adminiculo efficacissime elevandum. Captain Savery's engine, however, was much completer than that proposed by Dr Papinus.

About the same time also one Monsieur Amontons of Paris was engaged in the same pursuit: but his method of applying the force of steam was different from those before-mentioned; for he intended it to drive or turn a wheel, which he called a fire-mill, which was to work pumps for raising water: but he never brought it to perfection. Each of these three gentlemen claimed the originality of the invention; but it is more than probable they all took the hint from the book published by the marquis of Worcester many years before.

In this imperfect state it continued without farther improvements until the year 1705, when Mr Newcommon and Mr Calley of Dartmouth in Southampton-shire made several experiments to bring it to work with a piston and beam, as now used; in which, after much pains taken, they succeeded, and obtained a patent for the sole use of this invention, for 14 years. The first proposal they made for draining of mines by this engine was in the year 1711; but they were very coldly received by many persons in the south of England, who did not understand the nature of it. In 1712, they came to an agreement with the owners of a colliery at Griff in Warwickshire, where they erected an engine with a cylinder of 22 inches diameter. At first they were under great difficulties in many things; but by the assistance of some good workmen they got all the parts put together in such a manner as to answer their intention tolerably well: and this was the first engine of the kind erected in England. There was at first one man to attend the steam-cock, and another to attend the injection-cock; but they afterwards contrived a method of opening and shutting them by some small machinery connected with the working-beam. The next engine erected by these patentees was at a colliery in the county of Durham, about the year 1718, where one Mr Beighton was concerned; who, not approving of the intricate manner of opening and shutting the cocks by strings and catches as in the former engine, substituted the hanging-beam for that purpose as at present used, and likewise made some improvement in the pipes, valves, and some other parts of the engine.

In a few years afterwards these engines came to be better understood than they had been; and their advantages, especially in draining of mines, became more apparent: and from the great number of them erected, they received additional improvements from different hands, until they arrived at their present degree of perfection.

This engine, as now improved, is the most curious and compound machine of all those inventions which have been owing to modern philosophy; and is not only applicable to the raising of large quantities of water out of mines, and for the supplying of towns, &c. but to many other of the most necessary purposes for mankind.

The principles on which it acts are truly philosophical; and when all the parts of the machine are proportioned to each other agreeable to these principles, it never fails answering the intention of the engineer.

1. It has been proved in Pneumatics, that the pressure of the atmosphere upon a square inch at the earth's surface, is about 14.8 pounds avoirdupoise at a mean. And,

2. If a vacuum is made by any means in a cylinder, Theory; which has a moveable piston suspended at one end of a lever equally divided, the air will endeavour to rush in, and will press down the piston, with a force proportionable to the area of the surface, and will raise an equal weight at the other end of the lever.

3. Water may be rarefied near 14,000 times by being reduced into steam, the particles whereof are so strongly repellent, as to drive away air of the common density, only by a heat sufficient to keep the water in a boiling state: by increasing the heat, the steam may be rendered much stronger; but this requires great strength in the vessels to support it. This steam may again be condensed into its former state by a jet of cold water dispersed among it; so that 14,000 cubic inches of steam admitted into a cylinder, may be reduced into the space of one cubic inch of water only, by which means a vacuum is partly obtained.

4. Though the pressure of the atmosphere be about 14.8 pounds upon every square inch, yet on account of the friction of the several parts, the resistance from some air which is unavoidably admitted with the jet of cold water, and from some remainder of steam in the cylinder, the vacuum is very imperfect, and the piston does not descend with a force exceeding eight or nine pounds upon every square inch of its surface.

5. The gallon of water of 282 cubic inches weighs 10.2 pounds avoirdupoise, or a cubic foot 62.5 pounds. The piston being pressed by the atmosphere with a force proportionable to its area in inches, multiplied by about eight or nine pounds, depresseth that end of the lever, and raiseth a column of water in the pumps of equal weight at the other end, by means of the pump-rods suspended to it. When the steam is again admitted, the piston rises and the pump-rods sink; and when that steam is condensed, the pump-rods again lift; and so alternately as long as the engine works.

It has been observed above, that the piston does not descend with a force exceeding eight or nine pounds upon every square inch of its surface; but by reason of accidental frictions and alterations in the density of the air, it will be the safest method, in calculating the power of the cylinder, to allow something less than eight pounds for the pressure of the atmosphere upon every square inch, viz. seven pounds ten ounces; and it being allowed that the gallon of water of 282 cubic inches weighs 10.2 pounds avoirdupoise, from these premisses the dimensions of the cylinder, pumps, &c. for any fire-engine, may be deduced as follows:

Suppose $c$ = the cylinder's diameter in inches.

$p$ = the pump's ditto.

$f$ = the depth of the pit in fathoms.

$g$ = the gallons drawn by a stroke of six feet or one fathom.

$h$ = the hogheads drawn per hour.

$s$ = the number of strokes per minute.

Then $c^2\; \backslash times\; 7.64$ = area of the cylinder, which multiplied by 7.64 pound, the air's pressure on a square inch, we have $6c^2$ for the power of any cylinder in pounds avoirdupois.

And, $\backslash frac\{p^2\; \backslash times\; 7.64\; \backslash times\; 72\}\{282\}\; =\; g$, the gallons contained in one fathom of any pump = $2.005p^2$; which multiplied by $f$ fathoms, we have $2.005pf$ for the gallons contained in any number of fathoms of any pump.

Also, $2.005p^2\; \backslash times\; 10.2$ pound, the weight of one gallon, we have $2.045p^2f$ = the weight in pounds of the column of water which is to be raised by the power of the cylinder.

But as a sufficient allowance was made in the power of the cylinder, by estimating it at 7.64 pound only, instead of 8 pound, the fraction of .0451 in the weight of the column of water may be safely omitted; whence we shall have $6c^2\; =\; 2p^2f$ by the latter equation; and by the same mode $2p^2\; =\; \backslash frac\{p^3\}\{5\}\; =\; g$ by the former.

Or if, instead of six pounds for the pressure of the air on each circular inch of the cylinder, it be supposed a pound, we shall then have $ac^2\; =\; 2p^2f$; and substituting $5g$ in the place of $p^2$, it will be, $ac^2\; =\; 10gf$. And farther, $\backslash frac\{63h\}\{60s\}\; =\; g$; whence $\backslash frac\{1.05h\}\{s\}\; =\; \backslash frac\{p^2\}\{5\}$.

From a comparison of these equations, the following theorems are derived, which will determine the size of the cylinder and pumps of any steam-engine capable of drawing a certain quantity of water from any assigned depth, with the pressure of the atmosphere on each circular inch of the cylinder's area.

In this Table there are four particular values of each letter, which render it more extensively useful than it would have been upon a less scale, because sometimes one value is more convenient for finding the unknown letter or quantity than another; as will be seen in the following examples, in which, to avoid repetitions, it is supposed that the pressure of the atmosphere is six pounds upon a circular inch of the piston, and that the engine goes at the rate of 12 strokes, 6 feet long each, in a minute.

1. Required the size of the cylinder to work a pump of 12 inches diameter, 30 fathoms deep. Per theorem 1. $c\; =\; \backslash sqrt\{\backslash frac\{p^2\; f\}\{3\}\}$; therefore $\backslash sqrt\{\backslash frac\{12\; \backslash times\; 12\; \backslash times\; 30\}\{3\}\}\; =\; c\; =\; 38$ inches.

2. Required the size of the pump that a cylinder of 38 inches diameter can work at 30 fathoms deep. Per theorem 2. $p\; =\; \backslash sqrt\{\backslash frac\{3c^2\}\{f\}\}$; therefore $\backslash sqrt\{\backslash frac\{38\; \backslash times\; 38\; \backslash times\; 3\}\{30\}\}\; =\; p\; =\; 12$ inches.

3. Required the depth that a 36-inch cylinder will work a pump of 10 inches diameter. Per theorem 3. $f\; =\; \backslash frac\{3c^2\}\{p^2\}$; therefore $\backslash frac\{36\; \backslash times\; 36\; \backslash times\; 3\}\{10\; \backslash times\; 10\}\; =\; f\; =\; 39$ fathoms.

4. Required the number of gallons drawn at a six-feet stroke per last-mentioned cylinder and pumps. Per theorem 4. $g\; =\; \backslash frac\{p^2\}\{5\}$; therefore $\backslash frac\{10\; \backslash times\; 10\}\{5\}\; =\; 20$ gal. = $g$.

5. Required the pressure of the atmosphere on a cylinder of 36 inches, which works a pump 10 inches diameter, 39 fathoms deep. Per theorem 5. $a\; =\; \backslash frac\{2p^2\; f\}\{c^2\}$; therefore $\backslash frac\{10\; \backslash times\; 10\; \backslash times\; 39\; \backslash times\; 2\}\{36\; \backslash times\; 36\}\; =\; a\; =\; 6$ pounds.

6. Required the hogheads delivered per hour by a pump of 16 inches diameter, at 12 strokes per minute. Per theorem 6. $h\; =\; \backslash frac\{sp^2\}\{5.25\}$; therefore $\backslash frac\{16\; \backslash times\; 16\; \backslash times\; 12\}\{5.25\}\; =\; h\; =\; 585$ hogheads.

7. Required the number of strokes per minute an engine must make to raise 585 hogheads per hour by a 16-inch pump. Per theorem 7. $s\; =\; \backslash frac\{5.25h\}\{p^2\}$; therefore $\backslash frac\{585\; \backslash times\; 5.25\}\{16\; \backslash times\; 16\}\; =\; s\; =\; 12$ strokes.

By these examples it is evident that the quantity sought is discoverable (by the help of the theorems) by one operation only, which without them might have taken several. But it often happens in practice, that an engine has to draw several pumps of different diameters from different depths; in which case the operations will be somewhat different from those above, as will be seen in the following example.

8. Let it be required to find the diameters of the cylinder and pumps to draw 520 hogheads per hour from 30 fathoms deep, 450 hogheads per hour from 20 fathoms deep, and 80 hogheads per hour for the jackhead from 10 fathoms deep, allowing the engine to make 12 six-feet strokes per minute, and the air's pressure 6 pounds upon each circular inch of the piston.

Per theor. 2. $p\; =\; \backslash sqrt\{\backslash frac\{5\; \backslash cdot\; 25h\}\{s\}\}$; therefore $\backslash sqrt\{\backslash frac\{520\; \backslash times\; 5\; \backslash cdot\; 25\}\{12\}\}\; =\; 15$, the first pump = x.

Per ditto, $\backslash sqrt\{\backslash frac\{450\; \backslash times\; 5\; \backslash cdot\; 25\}\{12\}\}\; =\; 14$, the 2d pump = y.

Per ditto, $\backslash sqrt\{\backslash frac\{80\; \backslash times\; 5\; \backslash cdot\; 25\}\{12\}\}\; =\; 6$, the 3d pump = z.

Then, per theor. 1. $c\; =\; \backslash sqrt\{\backslash frac\{p^2\; f\}\{3\}\}$, if the water was to be raised in one column from a certain depth; but it being in three columns of various dimensions, it is evident from the question, that the power of the cylinder must be a counterpoise to the weight of all these columns; and putting x, y, z, for the three pumps, instead of p, the equation will be $c\; =\; \backslash sqrt\{\backslash frac\{x^2\; f\; +\; y^2\; f\; +\; z^2\; f\}\{3\}\}$, which is $\backslash sqrt\{\backslash frac\{11030\}\{3\}\}\; =\; 60.6$ inches, the cylinder's diameter. If there had been a greater number of pumps, the size of the cylinder might have been found in the same manner, by substituting the sum of their squares instead of $p^2$ in the theorem.

It is the practice of some engineers to allow a longer stroke than six feet; and, although the advantages of it are rather problematical, if that be supposed, for instance, a z feet stroke; then, instead of $p\; =\; \backslash sqrt\{5g\}$, and $g\; =\; \backslash frac\{p^2\}\{5\}$ in the table of theorems, we shall have $p\; =\; \backslash sqrt\{\backslash frac\{5g\; \backslash times\; 6\}\{z\}\}$, and $g\; =\; \backslash frac\{p^2\; z\}\{5\; \backslash times\; 6\}$.

On the following page is given a Table, calculated from the foregoing theorems, of the powers of cylinders from 30 to 70 inches diameter; and the diameter and lengths of pumps which those cylinders are capable of working, from a six-inch bore to that of 20 inches, together with the quantity of water drawn per stroke and per hour; allowing the engine to make 12 strokes of 6 feet per minute.

The first column on the left is the diameter of the cylinders from 30 inches to 70. The first line of numbers at the top is the diameters of pumps from 6 to 20 inches; and the numbers in the common angle of meeting are the fathoms in depth which the cylinder is capable of working with any of these pumps. The right-hand column gives the power of the cylinder in pounds, to which it stands opposite, likewise the weight of the columns of water in the same line. Thus the first sum, 5400 pounds, is the power of a 30-inch cylinder; it is also the weight of water contained in 75 fathoms of 6-inch pumps, 55 fathoms of 7 inches diameter, 42 fathoms of 8 inches, 33 fathoms of 9 inches, &c.

The quantity in gallons drawn by a six-feet stroke,

or the quantity contained in one fathom of any of the pumps, is expressed in the lowest line but one; any quantity in this line multiplied by 10.2 will give the weight in pounds of one fathom, if wanted. The lowest line of figures is the number of hogheads drawn in an hour by each pump respectively, when the engine goes 12 strokes per minute.

1. Required the size of the cylinder to work a pump of 12 inches diameter 30 fathom deep.

Under 12 inches, the diameter of the pump, find the fathoms 30, (or the nearest number to it); and on the same line in the first column is 38 inches, the diameter of the cylinder.

2. Required the size of the pump that a cylinder of 38 inches diameter can work at 30 fathoms deep.

Find 38, the cylinder's diameter in the first column; then in the line from it look for 30 fathoms, or the nearest number to it; and where that is found, the diameter of the pump will be found above: thus 30 fathoms has 12 inches for the pump's diameter.

3. Required the depth that a 36-inch cylinder will work a pump of 10 inches diameter.

Find 36 in the side-column, and 10 in the upper line; and in the common angle of meeting is 39, the fathoms required.

4. Required the number of gallons drawn per stroke by the last-mentioned cylinder and pumps.

Under the diameter 10 of the pump, and in the lowest line but one, is 20, the number of gallons drawn per stroke.

5. Required the hogheads delivered per hour by a 12-inch pump.

Under 12, the pump's diameter, in the lowest line of figures, is 328, the hogheads delivered per hour.

6. Required the diameter of the cylinder and pumps to draw 520 hogheads per hour from 30 fathoms deep, 450 hogheads per hour from 20 fathoms deep, and 80 hogheads per hour from 10 fathoms deep.

In this question take the nearest numbers in the Table: thus 513 hogheads per hour is the nearest to 520, and above it find the fathoms 29, which is nearest to 30; then at the head of that column is 15 inches for the size of the largest pump, and in the right-hand column, opposite 29, is 12,696 lb. the weight of that water; viz. Pump, 15 inches, Weight, 12,696. In the same manner find pump 14 inch. 7776. Also pump 6 inch. 800.

The sum is the weight of the whole, 21272.

Then, by looking in the last column amongst the powers of cylinders, the nearest number to 21,272 is 21,600; and opposite to it, in the first column, is 60 inches, the diameter of a cylinder capable of working those three pumps.

TABLE of the Power and Effects of STEAM-ENGINES, allowing 12 strokes of 6 feet long each per minute, and the Pressure of the Air 7 lb. 10 oz. per square Inch.

We shall now describe the several parts of an engine, and exemplify the application of the foregoing principles in the construction of one of the completest of the modern engines. See Plate CCLXXIII. fig. 1.

A, Represents the fire-place under the boiler, for the boiling of the water, and the ash-hole below it.

B, The boiler, filled with water about three feet above the bottom, made of iron plates.

C, The steam-pipe through which the steam passes from the boiler into the receiver.

D, The receiver, a close iron vessel, in which is the regulator or steam-cock, which opens and shuts the hole of communication each stroke.

E, The communication-pipe betwixt the receiver and the cylinder; it rises 5 or 6 inches up, in the inside of the cylinder bottom, to prevent the injected water from descending into the receiver.

F, The cylinder, of cast iron, about 10 feet long, bored smooth in the inside; it has a broad flanch in the middle on the outside, by which it is supported when hung in the cylinder-beams.

G, The piston, made to fit the cylinder exactly: it has a flanch rising 4 or 5 inches upon its upper surface, betwixt which and the side of the cylinder a quantity of junk or oakham is stuffed, and kept down by weights, to prevent the entrance of air or water and the escaping of steam.

H, The chain and piston shank by which it is connected to the working-beam.

II, The working-beam or lever: it is made of two or more large logs of timber, bent together at each end, and kept at the distance of 8 or 9 inches from each other in the middle by the gudgeon, as represented in the Plate. The arch-heads, II, at the ends, are for giving a perpendicular direction to the chains of the piston and pump-rods.

K, The pump-rod which works in the sucking-pump L, and draws the water from the bottom of the pit to the surface.

M, A cistern, into which the water drawn out of the pit is conducted by a trough, so as to keep it always full; and the superfluous water is carried off by another trough.

N, The jack-head pump, which is a sucking-pump wrought by a small lever or working-beam, by means of a chain connected to the great beam or lever near the arch g at the inner end, and the pump-rod at the outer end. This pump commonly stands near the corner of the front of the house, and raises the column of water up to the cistern O, into which it is conducted by a trough.

O, The jack-head cistern for supplying the injection, which is always kept full by the pump N: it is fixed so high as to give the jet a sufficient velocity into the cylinder when the cock is opened. This cistern has a pipe on the opposite side for conveying away the superfluous water.

PP, The injection-pipe, of 3 or 4 inches diameter, which turns up in a curve at the lower end, and enters the cylinder bottom: it has a thin plate of iron upon the end a, with 3 or 4 adjutage holes in it, to permit the jet of cold water from the jack-head cistern to fly up against the piston and condense the steam each stroke, when the injection-cock is open. e, A valve upon the upper end of the injection-pipe within the

cistern, which is shut when the engine is not working, to prevent any waste of the water. f, A small pipe which branches off from the injection-pipe, and has a small cock to supply the piston with a little water to keep it air-tight.

Q, The working plug, suspended by a chain to the arch g of the working-beam. It is usually a heavy piece of timber, with a slit vertically down its middle, and holes bored horizontally through it, to receive pins for the purpose of opening and shutting the injection and steam cocks as it ascends and descends by the motion of the working-beam.

h, The handle of the steam-cock or regulator. It is fixed to the regulator by a spindle which comes up through the top of the receiver. The regulator is a circular plate of brass or cast iron, which is moved horizontally by the handle h, and opens or shuts the communication at the lower end of the pipe E within the receiver. It is represented in the Plate by a circular dotted line.

i, The spanner, which is a long rod or plate of iron for communicating motion to the handle of the regulator; to which it is fixed by means of a slit in the latter, and some pins put through to fasten it.

k, The vibrating lever, called the Y, having the weight k at one end and two legs at the other end. It is fixed to an horizontal axis, moveable about its centre-pins or pivots m n, by means of the two shanks o p fixed to the same axis, which are alternately thrown backwards and forwards by means of two pins in the working plug; one pin on the outside depressing the shank o, throws the loaded end k of the Y from the cylinder into the position represented in the plate, and causes the leg l to strike against the end of the spanner; which forcing back the handle of the regulator or steam-cock, opens the communication, and permits the steam to fly into the cylinder. The piston immediately rising by the admission of the steam, the working-beam II rises; which also raises the working-plug, and another pin which goes through the slit raises the shank p, which throws the end k of the Y towards the cylinder, and, striking the end of the spanner, forces it forward, and shuts the regulator or steam-cock.

q r, The lever for opening and shutting the injection-cock, called the F. It has two tots from its centre, which take betwixt them the key of the injection-cock. When the working-plug has ascended nearly to its greatest height, and shut the regulator, a pin catches the end q of the F and raises it up, which opens the injection-cock, admits a jet of cold water to fly into the cylinder, and, condensing the steam, makes a vacuum; then the pressure of the atmosphere bringing down the piston in the cylinder, and also the plug-frame, another pin fixed therein catches the end of the lever in its descent, and, by pressing it down, shuts the injection-cock, at the same time the regulator is opened to admit steam, and so on alternately; when the regulator is shut the injection is open, and when the former is open the latter is shut.

R, The hot well. A small cistern made of planks, which receives all the waste water from the cylinder.

S, The sink-pit, to convey away the water which is injected into the cylinder each stroke. Its upper end is even with the inside of the cylinder bottom, its lower end has a lid or cover moveable on a hinge, which

which serves as a valve to let out the injected water, and shuts close each stroke of the engine, to prevent the water being forced up again when the vacuum is made.

T, The feeding pipe, to supply the boiler with water from the hot well. It has a cock to let in a large or small quantity of water as occasion requires, to make up for what is evaporated; it goes nearly down to the boiler bottom.

U, Two gage-cocks, the one larger than the other, to try when a proper quantity of water is in the boiler: upon opening the cocks, if one give steam and the other water, it is right; if they both give steam, there is too little water in the boiler; and if they both give water, there is too much.

W, A plate which is screwed on to a hole in the side of the boiler, to allow a passage into the boiler for the convenience of cleaning or repairing it.

X, The steam-clack or puppet-valve, which is a brass valve on the top of a pipe opening into the boiler, to let off the steam when it is too strong. It is loaded with lead, at the rate of one pound to an inch square; and when the steam is nearly strong enough to keep it open, it will do for the working of the engine.

f, The snifting valve, by which the air is discharged from the cylinder each stroke which was admitted with the injection, and would otherwise obstruct the due operation of the engine.

t, The cylinder-beams; which are strong joints going through the house for supporting the cylinder.

v, The cylinder-cap of lead, soldered on the top of the cylinder, to prevent the water upon the piston from flashing over when it rises too high.

w, The waste-pipe which conducts the superfluous water from the top of the cylinder to the hot-well.

xx, Iron bars, called the catch-pins, fixed horizontally through each arch-head, to prevent the beam descending too low in case the chain should break.

y, Two strong wooden springs to weaken the blow given by the catch-pins when the stroke is too long.

z, Two friction-wheels, on which the gudgeon or centre of the great beam is hung; they are the third or fourth part of a circle, and move a little each way as the beam vibrates. Their use is to diminish the friction of the axis, which, in so heavy a lever, would otherwise be very great.

When this engine is to be set to work, the boiler must be filled about three or four feet deep with water, and a large fire made under it; and when the steam is found to be of a sufficient strength by the puppet-clack, then by thrusting back the spanner, which opens the regulator or steam-cock, the steam is admitted into the cylinder, which raises the piston to the top of the cylinder, and forces out all the air at the snifting valve; then by turning the key of the injection-cock, a jet of cold water is admitted into the cylinder, which condenses the steam and makes a vacuum; and the atmosphere then pressing upon the piston, forces it down to the lower part of the cylinder, and makes a stroke by raising the column of water at the other end of the beam. After two or three strokes are made in this manner, by a man opening and shutting the cocks to try if they be right, then the pins may be put into the pin-holes in the working plug,

and the engine left to turn the cocks of itself; which it will do with greater exactness than any man can pretend to do.

There are in some engines different methods of shutting and opening the cocks than in the one above described, but perhaps none better adapted to the purpose; and as the principles on which they all act are originally the same, any difference in the mechanical construction of the small machinery will have no influence of consequence upon the total effect of the grand machine.

The furnace or fire-place should not have the bars so close as to prevent the free admission of fresh air to the fire, nor so open as to permit the coals to fall through them; for which purpose two inches or thereabouts is sufficient for the distance betwixt the bars. The size of the furnace depends upon the size of the boiler; but in every case the ash-hole ought to be capacious to admit the air, and the greater its height the better. If the flame is conducted in a flue or chimney round the outside of the boiler, or in a pipe round the inside of it, it ought to be gradually diminished from the entrance at the furnace to its egress at the chimney; and the section of the chimney at that place should not exceed the section of the flue or pipe, and should also be somewhat less at the chimney-top.

The boiler or vessel in which the water is rarefied by the force of fire, may be made of iron-plates, cast iron, or such other materials as can withstand the effects of the fire, and the elastic force of the steam. It may be considered as consisting of two parts; the upper part which is exposed to the steam, and the under part which is exposed to the fire. The form of the latter should be such as to receive the full force of the fire in the most advantageous manner, so that a certain quantity of fuel may have the greatest possible effect in heating and evaporating the water; which is best done by making the sides cylindrical, and the bottom a little concave, and then conducting the flame by an iron flue or pipe round the inside of the boiler beneath the surface of the water, before it reach the chimney. For, by this means, after the fire in the furnace has heated the water by its effect on the bottom, the flame heats it again by the pipe being wholly included in the water, and having every part of its surface in contact with it; which is preferable to carrying it in a flue or chimney round the outside of the boiler, as a third or a half of the surface of the flame only could be in contact with the boiler, the other being spent upon the brick-work. This cylindrical lower part may be less in its diameter than the upper part, and may contain from four to six feet perpendicular height of water in it.

The upper part of the boiler is best made hemispherical, for resisting the elasticity of the steam; yet any other form may do, provided it be of sufficient strength for the purpose. The quick going of the engine depends much on the capaciousness of the boiler-top; for if it be too small, it requires the steam to be heated to a great degree, to increase its elastic force so much as to work the engine. If the top is so capacious as to contain eight or ten times the quantity of steam used each stroke, it will require no more fire to preserve its elasticity than is sufficient to keep the water in a proper state of boiling; this, therefore, is the

the best size for a boiler-top. If the diameter of the cylinder be $c$, and works a six-foot stroke, and the diameter of the boiler supposed $b$, then $200c^2\; =\; b^2$, or

The effect of the injection in condensing the steam in the cylinder, depends upon the height of the reservoir and the diameter of the adjutage. If the engine makes a six-feet stroke, then the jackhead cistern should be twelve feet perpendicular above the bottom of the cylinder or the adjutage. The size of the adjutage may be from one to two inches in diameter; or if the cylinder be very large, it is proper to have three or four holes rather than one large one, in order that the jet may be dispersed the more effectually over the whole area of the cylinder. The injection-pipe, or pipe of conduct, should be so large as to supply the injection freely with water: if the injection-pipe be called $p$, and the diameter of the adjutage $a$, then

Mr James Watt of Birmingham having lately obtained a patent for an improvement of the steam-engine above-described, we shall give an account of this improvement, and point out in what the advantage of it consists.

The cylinder, or steam-vessel, A, of this engine, (see Plate CCLXXIII. fig. 2.) is shut at bottom and open at top, as usual; and is included in an outer cylinder or case BB, of wood or metal, covered with materials which transmit heat slowly. This case is at a little distance from the cylinder, and is shut at both ends; the cover C has a hole in it through which the piston-rod E slides; and near the bottom is another hole F, by which the steam from the boiler has always free entrance into this case or outer cylinder, and by the interfice GG between the two cylinders has access to the upper side of the piston HH. To the bottom of the inner cylinder A is joined a pipe I, with a cock or valve K, which is opened and shut when necessary, and forms a passage to another vessel L called a condenser, made of thin metal. This vessel is immersed in a cistern M full of cold water, and is contrived in such a manner as to expose a very great surface externally to the water, and internally to the steam. It is also made air-tight, and has pumps N wrought by the engine, which keep it always exhausted of air and water.

Both the cylinders A and BB being filled with steam, the passage K is opened from the inner one to L the condenser, into which the steam rushes with violence by its elasticity, because that vessel is exhausted; but it no sooner enters it, than coming into contact with the cold matter of the condenser, it is reduced to water, and the vacuum still remaining, the steam continues to rush in until the inner cylinder A below the piston is left empty. The steam which is above the piston ceasing to be counteracted by that which was below it, acts upon the piston HH, and forces it to descend to the bottom of the cylinder, and so raises the bucket of the pump by means of the lever. The passage K between the inner cylinder and the condenser is then shut, and another passage O is opened, which permits the steam to pass from the outer cylinder, or from the boiler into the inner cylinder under the piston; and then the superior weight

of the bucket and pump-rods pulls down the outer end of the lever or great beam, and raises the piston, which is suspended to the inner end of the same beam. Steatites.

The advantages that accrue from this construction are, first, that the cylinder being surrounded with the steam from the boiler, is kept always uniformly as hot as the steam itself, and is therefore incapable of destroying any part of the steam which should fill it, as the common engines do. Secondly, the condenser being kept always as cold as water can be procured, and colder than the point at which it boils in vacuo, the steam is perfectly condensed and does not oppose the descent of the piston; which is therefore forced down by the full power of the steam from the boiler, which is somewhat greater than that of the atmosphere.

In the common fire-engines, when they are loaded to seven pounds upon the inch, and are of a middle size, the quantity of steam which is condensed in restoring to the cylinder the heat which it had been deprived of by the former injection of cold water, is about one full of the cylinder, besides what really it required to fill that vessel; so that twice the full of the cylinder is employed to make it raise a column of water equal to about seven pounds for each square inch of the piston: or, to take it more simply, a cubic foot of steam raises a cubic foot of water about eight feet high, besides overcoming the friction of the engine, and the resistance of the water to motion.

In the improved engine, about one full and a fourth of the cylinder is required to fill it, because the steam is one-fourth more dense than in the common engine. This engine raises a load equal to 12 pounds and a half upon the square inch of the piston; and each cubic foot of steam of the density of the atmosphere, raises one cubic foot of water 22 feet high.