Mr. Watt established the following proportions for his rotative engines.

The first engines which were made for the breweries in London, were only single-acting cylinders, 24 inches diameter, the length of stroke 6 feet, and they made 18 strokes per minute; these were about 10 horse-power.

A 10 horse double rotative engine, was made with a I7£ inch cylinder, the length of its stroke 4 feet, and to make 25 strokes per minute.

The most standard size of the double-acting rotative engines was called a 20 horse-power, with a cylinder 23J inc. diam., the piston making a 5 feet stroke, and working at the rate of 21^ strokes per minute.

For a 40 horse double engine, the cylinder was 31 £ inches diameter, the piston made a 7 feet stroke, and went at the rate of 17^ strokes per minute.

To calculate the power of any machine in horse-power. Rule.—Multiply the force exerted, in pounds, by the space, in feet, through which that force acts during one minute; and divide the product by 33 000 lbs. The quotient will be the horse-power exerted by the machine.

Example. Suppose the piston of an engine to be impelled by a force, or unbalanced pressure, of 3070 lbs. and that it moves at the rate of 21 ^ double strokes of 5 feet length, or 215 feet motion per minute. Then 3070 lbs. load, x 215 ft. per min. = 660 050 lbs. acting through 1 foot per min., and this -f- 33 000 lbs. = 20 horse-power.

Sliding Rule. ( A Force in pounds. 33 000 lbs. Exam A ''070 lbs. 33 000.

slide inverted. \ 3 Motion ft. per min. Horse-power. rt""P' Q 215 ft. 20 HP.

To calculate the power of a steam-engine in horse-power. Rule. Multiply the square of the diameter of the cylinder, in inches, by the motion of the piston, in feet, per minute; multiply the product, by the effective pressure upon the piston, in pounds, per square inch (exclusive of friction); and divide the latter product, by 42 017; the quotient is the horse-power exerted by the engine.

Example. Suppose a cylinder 23 J inches diameter; the piston urged with a force of 6-94 lbs. per square iuch,and to move 215 feet per minute. Then, 23-7.5 squared = 564 circular inches area of the piston x 215 ft. = 121 260 x 6 94 lbs. per squ. inc. = 842 029 42 000 = 20 horse power.

Sliding f C Mot. ft. per min. x pres. lbs. per sq. inc. Horse-power. ^ C 145-9 20 HP.

Rule. I Tj 205 gage polnE Diam. of cyl. inc. ''D 205 23-75 inc.

Note. If the effective pressure on the piston had been stated in pounds per circular inch, then the divisor would have been 33 OOO; but the pressure being stated per square inch, it is (33 000 -4•7854=) 4 2 017 as above. If an engine expends 33 cubic feet of steam per minute for each horsepower, the effective pressure on the piston will be 6" 944 lbs. per square inch.

Mr. Watt's fifth and last Patent, 1785.

This was for " new methods of constructing furnaces or fire-places, for heating, boiling, or evaporating water; which methods are applicable to steam-engines; and whereby greater effects may be produced from the fuel, and the smoke in a great measure prevented, or consumed." Dated 14 June, 1785. The specification of this patent is printed in the Repertory of Arts, First Series, vol. nr. p. 226.

The specification states, that the smoke which proceeds from the raw coal when it is first thrown into the furnace, must be supplied with a current of fresh air, so as to become mixed therewith; and that mixture of smoke and air, must be caused to pass through, or over that part of the burning fuel which has become intensely red hot, and has already ceased to smoke, by being converted into ignited coaks or cinders. The smoke and grosser part of the flame, being thus brought into close contact with the intensely hot fuel, and being already mixed with a due proportion of fresh or unburnt air, will ignite and burn with a pure flame, free from smoke.

To put this in practice, every passage must be stopped up where air might gain admission into the furnace or flue, and pass into the chimney, leaving only the interstices between the pieces of fresh coals, for the admission of the air; the fresh raw coals must always be supplied at the outside part of the furnace, nearer to the external air, than that part where the intense heat and ignition takes place. In consequence the smoke proceeding from the raw coals will be mixed with, and carried along with, the fresh air which insinuates itself through the interstices between the coals, and will pass, in mixture with that air, through the hottest part of the burning fuel, so as to be consumed; but as a sufficient supply of fresh air would not always find-its way through the interstices of the pieces of coals, an opening must be provided to admit a current of air, at discretion, into the burning part of the fuel, with a stopper to regulate the quantity of air which shall be admitted, according as the combustion is found to require.

Mr. Watt proposed for this plan, to place the furnace in front of the boiler, and without any fire-grate, the fuel being thrown into an upright funnel or hopper, through which the draught and current of air passed downwards, so that the hottest part of the fire was at the bottom. The fresh coals were thrown in at the top of the funnel, which was left open to the air, and therefore the funnel could not be placed beneath the boiler, but the flame and heat was conveyed from the lower part of the funnel through a passage which rose up again, and then passed horizontally beneath the boiler. Or it was proposed to place that funnel or hopper, in an inclined or nearly horizontal direction, so that it might extend beneath the bottom of the boiler, with only its mouth or open end on the outside; and still preserving the principle that the raw fuel shall be supplied at the part nearest to the supply of fresh air, in order that the smoke may be mixed with that air, and be carried into the hot fire with it.

In other cases Mr. Watt proposed to place the fresh fuel, in the usual manner, on a fire-grate situated beneath the boiler, at one end of it, and to fix another small fire-grate beneath the opposite end of the boiler, near the place where the flame and smoke passes from under the boiler, to turn up into the external flue around the same; a fire is to be maintained on this second grate with cokes or cinders which have been previously burnt until they have ceased to emit smoke; this fire will give an intense heat to the smoke of the first fire which is conveyed through or over it, and a suitable supply of fresh air being admitted to mix with that smoke, it will ignite and be consumed.

The plan of introducing the coals into a funnel, or furnace without a firegrate, was not found to answer well in practice, because the coals being subject to cake together in masses, would not always permit a sufficient passage of air through the interstices; and it required too much attention and discretion to regulate the fresh air which was admitted at the air passage, in order to supply that deficiency; when that fresh air entered in too great a body, it would be only partially intermixed with the smoke, which could not therefore be completely consumed. After many trials, Mr. W. returned to the old method of placing a fire-grate beneath the bottom of the boiler, but he altered the construction of the furnace so as to attain in part the object stated in the specification.

This method, which he afterwards adopted generally, he called the smokeless furnace; it is founded upon the same principle as Argand's lamps. The fire-grate and mouth piece of the furnace are laid sloping downwards from the fire-door, at an angle of about 25 to 80 degrees to the horizon, and the mouth-piece is adapted to receive and contain a quantity of coals. The fire is lighted on the sloping grate in the usual way, and a small quantity of air must be admitted through one or two apertures in the fire-door, so as to blow down exactly on the blazing part of the fire, which at first burns principally at the upper part of the grate, and the fresh coals with which the fire is to be supplied when requisite, are laid upon the sloping bottom plate of the mouth-piece, close to the burning fuel, but not upon it. When the fire requires fresh coals, they must not be thrown upon those already on fire, as that would greatly increase the smoke; but the burning coals on the grate, and the unconsumed coals already on the dead plate of the mouthpiece, must be pushed further down the slope on to the grate. With this precaution the fresh coals are gradually dried by lying in the mouth-piece, in the vicinity of the fire; and the current of air which proceeds from the door, mixes with the smoke from the fresh coals, and causes it to pass over the bright burning fuel so as to be consumed. The apertures in the door are so constructed as to admit that quantity of air which is requisite for the consumption of the smoke, but too much air would be prejudicial.

The original plan was somewhat different from this description, which is of the most improved form; it answers the purpose perfectly with free burning coals when properly attended to; but it is difficult to manage with coals which melt and cake together.

The First Establishment of Steam-mills, 1785 to 17US.

Messrs. Boulton and Watt executed their first and greatest work of this kind in London in 1785 and 1788, at the Albion Mills, which was a new establishment for grinding corn, entirely by the power of Mr. Watt's new rotative engines. The mills were contained in an elegant and commodious building erected under the direction of Mr. Samuel Wyatt, by the river side, at the foot of Blackfriars Bridge; and two engines were made for them at Soho, by Messrs. Boulton and Watt, each rated at 50 horse-power, or equal in all to the power of 150 horses working in concert. Each engine worked 10 pairs of millstones, or 20 in the whole mill, of which 12 or more were generally kept at work with the requisite machinery for dressing the flour, and other purposes (a).

The Albion Mills were established by a company, who purchased wheat in the London market, and manufactured it immediately into flour for sale; whereby they avoided the loss of time, and the expense and damage of carrying the corn up the river Thames, to be ground by water-mills, and then bringing back the flour. It was expected that the saving of expense, in time and carriage, would compensate for the cost of fuel consumed by the engines, and afford a profit to the proprietors.

These mills deserve particular notice, as being the first of those numerous establishments which have since arisen in all the manufacturing districts of

(a) The following account of this work is given in a letter to Mr. Smeaton by a friend, in August, 1785.

"The new steam flour-mill now building near Blackfriars Bridge, is an immense undertaking, which is carried on with great spirit and judgment; they expect to begin to work before Christmas, with one engine, and 10 pair of stones, 6 of them to work at once. The wheels and axes are all cast iron; there are two fly-wheels of 18 feet diameter, to be turned by a spear suspended from the great lever of the engine, and each of the fly-wheels is to work 5 pair of millstones of 4 feet 4 inches diameter."

By another letter dated 17th March 1788: "I have this day been through the Albion Mills with Mr. Boulton. Five bushels of Newcastle coals are said to work the engine an hour, and those 5 bushels will grind 50 bushels of wheat per hour, upon C pair of stones. It is reckoned that one horse-power is equal to the grinding of one bushel of wheat per hour, and therefore the engine is equal to 50 horse-power. The steam cylinder is 34 inches diameter."

England, Scotland, and Wales, entirely out of the advantages of Mr. Watt's engines; the unlimited command of power thus attained, enables a manufactory to be placed at once in the vicinity of the market for the purchase of its materials, and for the sale of its produce, instead of carrying the materials to a water-fall.

The Albion Mills were said to be the means of sensibly reducing the price of flour in the metropolis, whilst they continued at work, for they occasioned a greater competition amongst the millers and meal-men than had ever existed before; but nevertheless great prejudices were excited against the company amongst the lower class of people, to whom it was represented as a monopoly highly injurious to the public. These mills were destroyed by fire in 1791 (a), and it has been suspected that it was not occasioned by accident; the satisfaction of the populace was afterwards expressed by songs in the streets of London.

The engines at the Albion Mills were constructed with great care, and contained all Mr. Watt's improvements combined into the most advantageous form; the invention of the double acting steam-engine for turning mills may be considered as completed at that time, nor has any important improvement in the operation of the steam-engine been made since, except Mr. Woolf's method of working with less fuel, by high pressure steam used expansively. All the modern improvements of Mr. Watt's engine have been confined to the methods of putting the parts together, in more substantial and durable forms.

The mill-work and machinery of the Albion Mills was executed under the direction of Mr. John Rennie, who had then commenced business in London, as a millwright and engineer, and who finally succeeded Mr. Smeaton in most of his public appointments. In place of wooden wheels, which are always subject to change of figure, wheels and axes of cast iron were employed, with their teeth, accurately formed, and proportioned in strength to the strain they had to bear; many other parts of the machinery, which had been usually made of wood before, were then made of cast iron, in improved forms.

Mr. Smeaton was the first who began to use cast iron in mill-work, several years before (see p. 275), and he made larger and more powerful mills than his predecessors; but the founder's art was so imperfect in his time, that he was obliged to proceed with the utmost caution in the use of cast iron, and experienced some vexatious failures (see p. 305). Mr. Rennie had greater advantages, and was enabled to apply cast iron in machinery with more success; and as the treatment of that material has become more known, it has been more and more used, up to the present time, when nothing but stone and metal is used in the construction of mills and engines, either for the fixed framing, or for the moving parts.

The practicability and advantages of Mr. Watt's double acting rotative-steamengines, being completely established by the performance of those at the Albion

(a) Extract from the Annual Register, vol. 33. "On the 3d of March, 1791, soon after 6o'clock in the morning, a fire broke out in the Albion Mills, and raged with such fury, that in about half an hour, that extensive edifice with an immense stock of fiour and grain, was reduced to ashes; the house and offices of the superintendant only escaping by the thickness of the party wall. It was low water when the fire was first discovered, and before the fire-engines were collected, their assistance was ineffectual, for the flames burst out on every side with such force, and heat, that it was impossible to approach any part, until the roof fell in. The wind blew across the wide street, or rather place, at the foot of Blackfriars Bridge; and the heat of the fire was so great as to scorch the houses on the opposite side of that street. The insurances in 5 different fire offices amounted to 26 000/. on the premises, and 35 000/. on the stock; 4 000 sacks of corn were destroyed."

Mills, they began to be generally adopted, and large manufactories were in a few years founded in towns, to work entirely by the new power (a).

After the rotative engine had thus been brought to a standard form, Mr. Watt made no further alterations during the remainder of the term of his patent, except such modifications as became necessary in manufacturing engines of all sizes from 4 horse-power to 100 horse-power. A great number of rotative-engines were made by Messrs. Boulton and Watt at their manufactory at Soho near Birmingham, and were fixed up in every part of the kingdom, and many were exported. They were called patent engines, and were looked upon as standards of perfection in mechanical works: many of them are still in use, and their performance is fully equal to that of any modern engines which operate on the same principle. It is now time to describe the construction of the complete patent engine in detail.

DESCRIPTION OF MR. WATT's ROTATIVE STEAM ENGINE FOR TURNING MILLS, CONSTRUCTED BY MESSRS. BOULTON AND WATT, AT SOHO, FROM 1787 TO 1800.

Plate XI. is a general elevation of the whole engine, and its boiler.

Plate XII. contains a cross section, and an end elevation of the engine; also a plan, and a longitudinal section of the boiler: all these are drawn to a scale of a quarter of an inch to a foot, and represent an engine of 10 horse power.

Plate XIII. contains a front view, sections and plan of the cylinder, with its valves and working gear, and coudenser and air pump: all on a scale of half an inch to a foot, and some enlarged sections of the valves. The same letters and characters of reference, are employed in all the different figures.

(a) At Manchester a cotton-mill was built on Sliude Hill in 1783, by Messrs. Arkwright and Simpson, with an atmospheric-engine; and some years afterwards it was replaced by one of Messrs. Boulton and Watt's patent engines.

At Glasgow Messrs. Scott and Stevenson began spinning cotton, by steam-power, in 1792.

At Leeds a large woollen manufactory was built by Mr. Gott in 1793, with a 40 horse-engine by Boulton and Watt; it continued in use till 18'J5, when it was replaced by an 80 horse-engine.

Mr. Marshall began the spinning of flax by steam-power at Leeds, about 1793; an engine on Savery's principle was first tried (see p. 123), and then a 28 horse-engine of Boulton and Watt. This establishment has since been extended by other engines, until it has become the largest manufacturing establishment in the kingdom by steam-power.

The spinning of worsted was begun by steam, at Arnold near Nottingham, by Messrs. Davison and Hawksley; they first had an engine on Savery's principle, and afterwards a large atmosphericengine, with a crank and fly-wheel, constructed by Mr. Thompson, with two cylinders combined to produce a double action.

At Warwick a worsted spinning-mill was begun about 1793 by Mr. Parkes, with a 26 horseengine by Messrs. Boulton and Watt.

At Birmingham a large rolling-mill was set up with steam-power, about 1792; and another at Liverpool, with an atmospheric-engine.

At Sheffield a steam-rolling-mill was built, about the same time: and soon after, a mill for grinding cutlery; with two engines of 40 horse-power each.

In London all the large breweries and distilleries were in a few years furnished with engines, from Soho, except one or two with atmospheric-engines from Manchester. An iron forge was built at Rotherhithe, about 1787. for making up scraps of old iron into bars. Also a paper-mill with an atmospheric-engine. A fulling-mill and logwood-mill was set up in the Borough in 1792, at a large dyehouse for woollen cloth, with a 20 horse-engine. *An oil mill was begun in the Borough soon after; and then a mill for grinding apothecaries' drugs; also a mill for calendering, glazing, and packing cloths for exportation.

These first establishments in each place, were greatly multiplied and extended in the course of 10 years, particularly tbc steam-cotton-mills at Manchester and Glasgow, and steam com mills in every large town; the extension has been still more rapid since the expiration of Mr. Watt's patent in 1800, when several new establishments were formed for making steam-engines.

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