STEAM ENGINE

of the cylinder, but in marine engines they are made somewhat larger.

Proportions of Paddle-wheels.-The diameter for the paddle-wheel of a steam vessel depends mainly upon the variation of immersion which the ship is required to undergo. In river steamers, where there is little variation of immersion, the wheels should be small in diameter. If a line be taken in the length of each float, so that the mean pressure of the water against the float is the same above the line as below it, then this line will constitute a centre of pressure, and in determining the velocity of the wheel it is the velocity of the centre of pressure which should be regarded. In all cases the centre of pressure must move more rapidly through the water than the vessel, and the difference between the velocity of the wheel and the velocity of the vessel is termed the slip of the wheel. The slip which usually occurs in steam vessels is one-third or onefourth of the velocity of the wheel; and an imaginary circle described upon the arms of the wheel at that point at which the velocity is the same as that of the ship, is termed the rolling circle. The rolling circle should fall above the level of the water. As a general rule, the larger the paddle floats, the more efficient will be the performance of the wheel, as is more fully explained in the article on the SCREW PROPELLER. In radial wheels, the usual practice is to introduce one float for every foot of the wheel's diameter, so that the floats are about three feet apart. In feathering wheels the floats are usually set twice this distance asunder.

blowing off be sufficiently practised if the boiler is worked with salt water, and that the furnaces are fed with coal in a regular manner--the coal being distributed evenly over the grate without allowing any holes to exist in the fire. The boilers should be covered with felt to prevent the radiation of the heat, and over the felt sheet lead should be spread, soldered at the corners, so as to prevent any drip of water from the deck having an injurious operation on the boiler. The steam pipes should be covered with felt, and then wrapped round with canvas. The cylinders should also be covered with felt, round which may be placed staves of wood, which should be hooped like a cask. The felt proper for this purpose is thick and soft, and is made expressly for the retention of heat. The steam should be used with a considerable pressure, and be worked expansively; but very little advantage will be derived from working. expansively in steam vessels, unless the cylinders be clothed very effectually, so as to prevent the dispersion of the heat. Condensers, which condense the steam without mixing the resulting water with water from the sea, have now come into extended operation; for the only impediment to the use of steam of a considerably higher pressure in steam vessels, is the liability of the boiler to become incrusted with salt, when it might become red hot in some part, and perhaps burst; and this risk the use of fresh water in the boilers would prevent. A species of condenser, called Hall's condenser, which operated on the principle of a still, was at one time in pretty extensive use in steam vessels; but it was unskilfully applied in most Relations of Power and Speed.-With any cases, and the main benefit-the ability to use given type of vessel, and any given power of steam of a higher pressure was not simultaengine, it is possible to predict, with very con- neously sought to be attained. In Hall's considerable accuracy, the speed which a steam denser, the steam on escaping from the cylinder vessel will attain. First, a coefficient must passed into a multitude of copper pipes, imbe found, which, when introduced into the com- merged in a cistern of cold water, and the putation, will give a result answering to that steam being thus reconverted into water was derived from experiment, and this coefficient returned by a pump to the boiler. In another will vary with the shape of the vessel. In the species of condenser, suggested by Symington, case of sea-going vessels of good average form, a jet of cold water was to be employed, as is the coefficient is 800; and in the case of very the present practice; but the fresh water with sharp river vessels it is 1,000. Multiply this which the vessel started, instead of being discoefficient by the number of actual horse- charged into the sea, was transmitted through power exerted by the engine [HORSE-POWER], a number of pipes which were kept cool by the and divide the product by the number of square sea water, and was returned to the condenser feet in the transverse section of that part of after having suffered sufficient refrigeration. the vessel lying beneath the water. Extract The urgent demand for speed in steam vessels the cube root of the quotient, and this will be necessitates the employment of a large amount the speed which the vessel will attain in statute of power, which in its turn involves a large miles per hour. It is possible to make steam vessels too sharp to attain a minimum resistance, since the increase of length consequent upon increased sharpness involves a larger amount of rubbing surface in the bottom of the vessel, and the loss due to the increased friction may more than counterbalance the benefit accruing from the finer lines.

Circumstances conducive to the Efficiency of Marine Engines. Supposing the boiler to be properly proportioned, it is necessary to see that the flues or tubes are kept clean, that

consumption of fuel. To reduce the consumption of fuel, without reducing the power, engines operating more expansively must be employed; but steam of a higher pressure is necessary for the satisfactory operation of such engines, and fresh water in the boilers is conducive to safety where a considerable pressure is employed.

Recent Improvements in the Steam Engine.— Up to the end of 1866 the chief improvements introduced into the steam engine, besides surface condensation in the case of marine

STEAM ENGINE

engines, are the use of superheated steam and the substitution of steel for iron in many of the parts. At a temperature answering to a pressure of forty pounds per square inch, salt water deposits sulphate of lime, not from concentration such as that which causes salt to be deposited by a saline solution, but from the mere application of a high degree of heat; and boilers working at a pressure of over forty pounds with sea water cannot be preserved from incrustation by any amount of blowing off. The insides of boilers using surface condensers, however, have been found to be much corroded by the galvanic action of the copper pipes in the condensers unless the pipes are tinned on the side exposed to the sea water; and all marine boilers, when new, should be worked with sea water at first, so as to deposit a thin enamel of scale within them. The ⚫ steam is usually superheated by carrying it through or among pipes exposed to the heat of the smoke escaping to the chimney, and in practice it is found that if it is heated to a higher temperature than 315° Fahr. it will burn the hemp packings of the stuffing boxes, corrode the valve faces, and hinder the proper lubrication of the piston. Tallow subjected to a high temperature within the cylinder, will sometimes carbonise the piston, and convert it into a substance resembling plumbago. The crank shafts of screw engines are now very frequently formed of steel which is of about twice the strength of common wrought iron. Many screw engines, moreover, have the momentum of the moving parts balanced by counterweights upon the cranks, an improvement introduced by Mr. Bourne in 1853. The counterweights enable engines to work at a high degree of speed without jolting.

For further information on the steam engine, see the Treatise on the Steam Engine, by the Artizan Club; A Catechism of the Steam Engine, by J. Bourne, C.E.; or Handbook of the Steam Engine, by J. Bourne, C.E.

of a locomotive, but smaller; there being a fire-box and a barrel containing small horizontal tubes for the transmission of the smoke to the chimney. But in a few cases the barrel of the boiler is placed in a vertical position with a cylindrical fire-grate and fire-box; and the fire-box, which is made of great height, has a number of tubes, closed at the lower ends, hanging from its top. These tubes being filled with water, and being acted upon by the flame, generate the steam. To enable a circulation of water to be maintained within these tubes, a thin tube of smaller diameter, and not reaching quite to the bottom, is introduced within each, and the water descends through the internal tube, and ascends through the surrounding annulus. In this case the cylinder is usually attached to the side of the boiler, and is inverted so as to work down to a shaft placed beneath. The wheels of portable engines are generally of iron, and the fore wheels are connected to the boiler by a ball-and-socket joint at the middle of the axle, so as to prevent the engine from at any time resting on three wheels. Usually the engine is drawn from place to place by horses; but in some cases the engine is made to move itself, by imparting motion to the wheels. Further information respecting agricultural engines of modern construction may be obtained in Bourne's Catechism of the Steam Engine, 11th edition.

Steam Engine, Substitutes for the. It would be impossible here to recapitulate the expedients which have at different times been propounded for superseding the steam engine. The most promising are electricity or galvanism and hot air. The best forms of the galvanic battery are constructed with zine surfaces, which are gradually consumed by oxidation. But a pound of coal consumed in a steam engine will produce twice the power generated in an electro-motive engine; and as the cost per pound of the coal is very much less than that of the zine, it is most Steam Engine, Agricultural. There are unlikely that galvanism will supersede steam, two forms of agricultural steam engine. The unless a carbon battery can be constructed first is a vertical or horizontal engine of simple in which coal will be consumed instead of construction, resembling the smaller classes of zinc. The hot-air engine is a contrivance engines used to drive factories. The second-of greater promise; and Ericsson, in America, called a portable engine-more nearly resembles has constructed many hot-air engines, or, a locomotive, and consists of a tubular boiler as he calls them, caloric engines, which are set upon wheels, to enable it to be drawn from place to place, with a cylinder and its connections usually laid on the top of the boiler and giving motion to the fly-wheel shaft, which is carried across the top of the boiler by suitable brackets. These engines have now been very widely introduced for agricultural and miscellaneous purposes, some of the principal makers turning out nearly 1,000 engines per annum, or something under twenty per week. They are used for driving thrashing machines, for pumping water, for sawing timber, &c. and in some cases also the waste steam is used for steaming food for cattle. They average from eight to ten horses power each. For the most part, agricultural portable engines have a boiler resembling that

working successfully in different parts of the world. But as in these engines a cylinder and piston are employed, the temperature of the air cannot be made very high, and unless high temperatures are employed the air engine will not be more economical in fuel than the steam engine, although for some purposes it will be more convenient, inasmuch as the boiler is dispensed with. The most promising expedient of all at the present time is an air engine employing very high temperatures. Such an engine cannot directly employ a cylinder and piston, though it may act on some fluid, through the medium of which the power may be transmitted, or it may consist of a reaction engine, like a Barker's

STEAM FIRE ENGINE

mill, moving in water. In the case of steam vessels, propulsion may be effected by spouting out steam and smoke below the water at the stern. Various plans have been propounded for propelling in this manner, but none of them have yet been practically successful. That the steam engine will be superseded by a form of air engine using high temperatures, is highly probable; but an electro-motive engine would be preferable, provided that the power could be obtained from coal instead of from zinc.

Steam Fire Engine. An arrangement of pumps worked by steam for extinguishing fires, by projecting upon them a continuous stream of water from a suitable nozzle or spout-pipe. The first steam fire engine we owe, in common with many other things, to the ingenuity of Ericsson, the eminent Swedish engineer, who subsequently to the completion of his locomotive, the Novelty, in 1830, constructed the first steam fire engine which was used with good effect in the fire which destroyed the Argyll Rooms about that time. In 1832, he

constructed, for the king of Prussia, the steam fire engine called the Comet, which had two horizontal cylinders of 12 inches diameter, and 14 inches stroke, and two pumps 10 inches diameter, and of the same stroke. This engine was set on wheels like those of an ordinary fire engine, and the flow of water was equalised by the aid of a great globular air vessel set behind the driver's seat. This engine weighed 4 tons. It raised its steam in from 13 to 20 minutes, and it threw 336 gallons of water per minute, or about 90 tons per hour.

Notwithstanding the success of Ericsson's early engines, steam fire engines did not come into extended use in this country for more than thirty years afterwards, and then various specimens of such engines came to us from America, which were inferior in constructive excellence to those which Ericsson had long before produced. Latterly, numerous steam fire engines have been constructed by Messrs. Merryweather and Son, Messrs. Shand, Mason, and Co., and by various other makers; and numerous competitive trials have been made to esta

blish the relative efficiency of the engines of in others two, but Messrs. Shand, Mason, and the different makers. In the engines of both Co.'s engines are formed with a crank, whereas Messrs. Shand, Mason, and Co., and in those Messrs. Merryweather and Son's engines are of Messrs. Merryweather and Son, the boilers made without one, and are, therefore, not rotaare vertical; but in the former, the smoke is tive engines. On the whole, the rotative form conducted through a number of vertical tubes of engine appears to be best for pumping as to the chimney, whereas, in the latter, the form well as for other purposes, as it can be driven of boiler known as the Field boiler, is adopted, faster, and enables the piston to be brought in which a number of tubes filled with water closer to the ends of the cylinder at the terhang from the top of the fire-box, but terminate mination of the stroke, thereby saving steam. in close ends above the level of the fire-grate. The form of pump commonly used is the bucket In these tubes smaller internal tubes are in- and plunger pump, first introduced by Mr. troduced, to enable the water to circulate by David Thomson, in the Richmond water-works descending the internal tube and ascending in 1845, and which resembles a common sucthrough the annulus left between the two. In tion pump, with a very thick rod, which acts as some engines one cylinder is employed, and a plunger, and the pump consequently forces

STEAM GAUGE

STEAM GUN

Steam Gun. A contrivance by which projectiles used in warlike operations may be discharged by the expansive force of steam. This invention is due to Mr. Jacob Perkins.

both in the ascent and descent of the bucket, | amount of uncoiling, as shown by a suitable but draws only during the ascent of the bucket. hand, indicates the amount of pressure. In some cases the cylinders and pumps are vertical, and in others horizontal. The figure on p. 591 represents one of Messrs. Shand, Mason, and Co.'s horizontal engines. In an experiment made in 1864 with an engine of this kind with a single cylinder and small fly-wheel, the cylinder being 7 inches diameter and 8 inches stroke, and the pressure of steam 145 lbs. per square inch in the boiler, and 128.16 lbs. in the cylinder, a jet of water 1 inch diameter was projected under a water pressure of 125 lbs. per square inch, the engine making 165 revolutions per minute, with 5 lbs. per square inch of back pressure resisting the piston. In this case the engine exerted 32 indicated horsepower; and as the total weight of the engine was only 32 cwt., the weight was only one cwt. per indicated horse-power. In these engines a small piston, which is pressed against a spring by the water which is being forced out of the jet pipe, governs the speed of the engine by moving suitably the throttle valve in the steam pipe. In April 1866, Messrs. Shand, Mason, and Co. delivered to the Metropolitan Fire Brigade seven of their vertical steam fire engines, those previously in use in that force having been found to act in a most satisfactory manner. They also about the same time sent out three engines to Bombay, one of which was tested in London before being shipped. Steam of 60 lbs. pressure was raised from cold water in 8 minutes, which threw a jet of water an inch in diameter 150 feet high, and 228 feet horizontal. In the fire brigade stations, it is usual to keep the water in the boilers of the fire engines warm by keeping a small jet of gas alight in the furnace, and by this simple expedient the steam is almost sure to be up before the engine can reach even the nearest fire.

Steam Gauge. An instrument intended to measure the pressure of the steam in the boiler. Steam gauges are of different kinds. One that has been much employed is the mercurial steam gauge, which consists of a small U tube of iron which is filled with mercury about half-way up, and the top of one leg communicates with the boiler, while a small wooden float projects above the top of the other leg and points to the graduations on a scale divided into inches, the float pointing to 0 when the steam is not up. If, now, steam be raised in the boiler, the mercury will be depressed in one leg and raised in the other, and every rise of an inch on the scale indicates a pressure equal to that due to two inches of mercury, or very nearly a pound pressure per square inch. Another form of steam gauge is a conical glass tube containing air, against which mercury or some other liquid is forced by the steam; another form, now much used, is that known as Bourdon's gauge, which consists substantially of a coiled flat elastic tube, into which steam enters, and the steam causes the tube to assume more nearly the cylindrical form, and simultaneously to uncoil. The

If a strong close iron vessel, having two valves, one opening inwards and the other outwards-the latter being loaded with some definite pressure-be completely filled with water, such a vessel may be heated to the temperature corresponding to the pressure with which the valve is loaded without causing any portion of the water in it to be converted into steam. To render the effect more easily understood, let us suppose that the valve is loaded with a pressure of fifty atmospheres. The temperature of water evaporated under that pressure being 510° [STEAM], the vessel may be raised to any temperature not exceeding 500°, without having any of the water contained in it converted into steam. If the temperature to which the water is raised be 500°, and a cubic inch of water at common temperatures be forced into the vessel through the valve which opens inwards, water being sensibly incompressible, a cubic inch of water at 500° will be forced out at the valve which opens outwards. This water, being no longer subjected to the pressure which kept it in the liquid state, will suddenly expand and flash into steam, which at first will have a pressure of fifty atmospheres, but as it expands will have its pressure diminished in nearly the same proportion as the volume into which it swells shall be increased. Since, however, 500° is not sufficient heat to enable the whole of the water thus ejected to pass into steam [STEAM], that part of it which will assume the vaporous form will take the requisite amount of latent heat from the sensible heat of that portion which remains in the liquid state. As this latter portion will still retain a considerable temperature, it may be conducted to a vessel containing the feed for the heated vessel just mentioned, whence it will be again forced into that vessel.

Such was the principle of Mr. Perkins' generators; by which term he denominated those close vessels in which water was raised to a high temperature without being converted into steam.

Now, if the valve through which the heated water is ejected be supposed to be in communication with the barrel of a gun or piece of ordnance, in the same manner as the barrel of an air gun is in communication with the hollow metallic ball in which the air is compressed, a projectile may be discharged by the expansive force of the water ejected from the valve, precisely as the ball of an air gun is projected by the expansive force of the compressed air.

As the water may be ejected from the valve either in a constant stream or by a rapid succession of jets, the projectiles may be discharged from the barrel as rapidly as it is possible for them to be brought under the action of the

STEAM HAMMER

steam; and since the heating of the barrel tends only to increase the elastic force of the steam, there appears to be no other practical limit to the action of such an engine of offence except that which may be imposed on the heating power applied to the generator.

begun the construction of such a hammer immediately on his return, it so happened that the French hammer was at work before the English one. In Watt's proposed steam hammer the cylinder was at one end of a wooden beam, while the hammer was at the other, and the hammer, Of the abstract practicability of applying instead of moving vertically as modern steam steam in this manner as an offensive engine, hammers do, moved in the arc of a circle, being there can be no doubt. Both theory and fitted with a wooden shank like the old forge experiment conspire to establish it; but of and tilt hammers when moved by cams upon the comparative efficacy, convenience, and a revolving shaft. But in Nasmyth's arrangeeconomy of it, compared with gunpowder, ment the cylinder is erected over the anvil, many doubts will present themselves to all and the piston rod which passes through the who duly reflect on the circumstances by bottom of the cylinder has the hammer fixed to which the innovations are surrounded. The its lower extremity, the hammer being directed necessity of using a steam generator of any vertically by suitable guides. In Condie's kind obviously limits the application of such arrangement, the piston is stationary and the an instrument to particular cases, and even cylinder moves, the hammer being attached in those special cases the necessity of employ- to the bottom of the cylinder; and the piston ing a generator or boiler which shall reconcile rod, which is a stationary cylindrical pipe, a ready conduction of the heat to the water, serves to convey the steam to and from the with great strength and solidity in the heating vessel, must continue to be regarded as a weighty difficulty. Without a very high pressure of steam the projectile cannot acquire an adequate velocity with any ordinary length of bore; and the use of steam of a very high pressure is dangerous and inconvenient. By greatly increasing the length of the gun, the same effect may be obtained with a smaller pressure; but this expedient is also inadmissible. Hence the steam gun could never come into effect for any but very small calibres; and the places where small guns come into use are precisely those in which it would be most difficult to employ a steam boiler, viz. in the field, in boat service, &c.

Steam Hammer. A heavy hammer, moved by a steam engine, employed chiefly for forging masses of iron and steel, but also for crushing quartz in gold mining, and for other purposes in the arts. Hammers lifted by cams upon a revolving shaft, deriving its motion either from a water wheel or a steam engine, have long been used in certain processes of the iron manufacture. But in the steam hammer! there is no intermediate mechanism intervening between the engine and the hammer, and the force and number of the blows are regulated by suitably governing, by a proper valve, the flow of steam to the engine.

The first proposal to apply a steam engine to work a hammer direct was made by Mr. Watt in 1784, but it was not until upwards of half a century later that the problem was practically gone into and solved by Mr. James Nasmyth, then of Patricroft near Manchester. The first steam hammer actually set to work was erected by M. Bourdon in France. But M. Bourdon confessedly obtained his ideas on the subject at Mr. Nasmyth's works, which had been visited by M. Bourdon when he was in England, and to whom the plans of the steam hammer were shown along with the plans of other tools and machines then in progress. But the completion of the steam hammer having been delayed by the progress of other work, and M. Bourdon having

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Fig. 1.

Fig. 2.

Light Steam Hammer.

cylinder. Figs. 1 and 2 are a front and side view of Condie's 3 cwt. hammer, and figs. 3 and 4 a front and side view of Condie's 6 cwt. hammer.

It will be observed that in the lighter hammers the standard is single and the hammer is overhung, while in the heavier hammers the standard is double and the moving cylinder is guided between the two parts. In the early hammers the weight was raised by the pressure of the steam, and the hammer descended by gravity alone; but in all modern hammers the steam presses the hammer down as well as raises it up. Many of the modern hammers are of great size, and of these the standards are generally formed of wrought iron. It is found that hammers of great weight are quite indispensable for consolidating and giving soundness to the Bessemer steel, and it is for this purpose that the very heavy hammers are chiefly employed. In Neilson's radial hammer the cylinder Q Q