In the comparison of carriages propelled by steam with carriages drawn

by horses, there is no respect in which the advantage of the former

is so apparent as the safety afforded to the passenger. Steam power is

under the most perfect control, and a carriage thus propelled is

capable of being guided with the most admirable precision. It is also

capable of being stopped almost suddenly, whatever be its speed; it is

capable of being turned within a space considerably less than that

which would be necessary for four-horse coaches. In turning sharp

corners, there is no danger, with the most ordinary care on the part

of the conductor. On the other hand, horse-power, as is well known, is

under very imperfect control, especially when horses are used, adapted

to that speed which at present is generally considered necessary for

the purpose of travelling. "The danger of being run away with and

overturned," says Mr. _Farey_, in his evidence before the House of

Commons, "is greatly diminished in a steam coach. It is very difficult

to control four such horses as can draw a heavy stage coach ten miles

an hour, in case they are frightened or choose to run away; and, for

such quick travelling, they must be kept in that state of courage that

they are always inclined to run away, particularly down hill, and at

sharp turns in the road. Steam power has very little corresponding

danger, being perfectly controllable, and capable of having its power

reversed to retard in going down hill. It must be carelessness that

would occasion the overturning of a steam carriage. The chance of

breaking down has been hitherto considerable, but it will not be more

than in stage coaches when the work is truly proportioned and properly

executed. The risk from explosion of the boiler is the only new cause

of danger, and that I consider not equivalent to the danger from

horses."

That the risk of accident from explosion is very slight indeed, if any

such risk exists, may be proved from the fact that the boilers used on

the Liverpool and Manchester railroad being much larger, and, in

proportion, inferior in strength to those of Mr. _Gurney_, and other

steam carriage projectors, have never yet been productive of any

injurious consequences by explosion, although they have frequently

burst. I have stood close to a locomotive on the railroad when the

boiler burst. The effect was that the water passed through the tubes

into the fire and extinguished it, but no other consequence ensued.

In fig. 65. is represented the appearance of a locomotive of Mr.

_Gurney's_ drawing after it a carriage for passengers.

[Illustration: Fig. 65.]

(109.) One of the greatest difficulties which locomotives upon a

turnpike road have to encounter is the ascent of very steep hills, for

it is agreed upon all hands that hills of very moderate inclinations

present no difficulty which may not be easily overcome, even in the

present state of our knowledge. The fact of Mr. _Gurney_ having

propelled his carriage up Old Highgate Hill, when the apparatus was in

a a much more imperfect state than that to which it has now attained,

establishes the mere question of the possibility of overcoming the

difficulty; but it remains still to be decided whether the

inconvenience caused by providing means of meeting the exigency of

very steep hills may not be greater than the advantage of being able

to surmount them can compensate for: and Mr. _Farey_, whose authority

upon subjects of this kind is entitled to the highest respect, thinks

that it is upon the whole more advantageous to provide, at very steep

hills, post horses to assist the steam carriage up them, than to incur

the inconvenience of providing the necessary power and strength of

machinery for occasions which at best but rarely occur. If the

question merely referred to the command of motive power, it appears

to me that Mr. _Gurney's_ boiler would be amply sufficient to supply

all that could be required for any hills which occur upon turnpike

roads; but it is not to be forgotten, that not merely an ample supply

of motive power, but also a strength and weight in the machinery

proportionate to the power to be exerted, is indispensably necessary.

The strength and weight necessary to ascend a very steep hill will be

considerably greater than that which is necessary for a level road, or

for hills of moderate inclinations; and it follows that if we ascend

those steep hills by the unaided power of the locomotive, we must load

the engine with all the weight of machinery requisite for such

emergencies, such additional weight being altogether unnecessary, and

therefore a serious impediment upon all other parts of the road,

inasmuch as it must exclude an equivalent weight of goods or

passengers, which might otherwise be transported, and thereby in fact

diminish proportionally the efficiency of the machine. It is right,

however, to observe, that this is a point upon which a difference of

opinion is entertained by persons equally competent to form a

judgment, and that some consider that it is practicable to construct

an engine without inconvenient weight which will ascend all the hills

which occur upon turnpike roads.

However this may be, the difficulty is one which the improved system

of roads in England renders of a comparatively trifling nature. If

horses were resorted to as the means of assistance up such hills as

the engine would be incapable of surmounting, such aid would not be

requisite more than twice or thrice upon the mail-coach road between

London and Holyhead; and the same may be said of the roads connecting

the greatest points of intercourse in the kingdom. Such hills as the

ascent at Pentonville upon the New Road, the ascent in St. James's

Street, the ascent from Waterloo Place to the County Fire Office, the

ascent at Highgate Archway, present no difficulty whatever. It is only

Old Highgate Hill, and hills of a similar kind, which would ever

require a supply of horses in aid of the engine. I therefore incline

to agree with Mr. _Farey_, that, at least for the present, it will be

more expedient to construct carriages adapted to surmount moderate

hills only, and to provide post horses in aid of the extreme

emergencies to which I have just alluded.

(110.) In the boiler to be used in the steam carriage projected by Mr.

_Walter Hancock_, the subdivision of the water is accomplished by

dividing a case or box by a number of thin plates of metal like a

galvanic battery, the water being allowed to flow between every

alternate pair of plates, at E, fig. 66., and the intermediate spaces

H forming the flue through which the flame and hot air are propelled.

[Illustration: Fig. 66.]

In fact, a number of thin plates of water are exposed on both sides to

the most intense action of flame and heated air; so that steam of a

high-pressure is produced in great abundance and with considerable

rapidity. The plates forming the boiler are bolted together by strong

iron ties, extending across the boiler, at right angles to the

plates, as represented in the figure. The distance between the plates

is two inches.

There are ten flat chambers of this kind for water, and intermediately

between them ten flues. Under the flues is the fire-place, or grate;

containing six square feet of fuel in vivid combustion. The chambers

are all filled to about two thirds of their depth with water, and the

other third is left for steam. The water-chambers, throughout the

whole series, communicate with each other both at top and bottom, and

are held together by two large bolts. By releasing these bolts, at any

time, the chambers fall asunder; and by screwing them up they may be

all made tight again. The water is supplied to the boiler by a

forcing-pump, and the steam issues from the centre of one of the flues

at the top.

These boilers are constructed to bear a pressure of 400 or 500 lbs. on

the square inch; but the average pressure of the steam on the safety

valve is from 60 to 100. There are 100 square feet of surface in

contact with the water exposed to the fire. The stages which such an

engine performs are eight miles, at the end of which a fresh supply of

fuel and water are taken in. It requires about two bushels of coke for

each stage.

The steam carriage of Mr. _Hancock_ differs from that of Mr. _Gurney_

in this,--that in the former the passengers and engine are all placed

on the same carriage. The boiler is placed behind the carriage; and

there is an engine-house between the boiler and the passengers, who

are placed on the fore part of the vehicle; so that all the machinery

is behind them. The carriages are adapted to carry 14 passengers, and

weigh, exclusive of their load, about 3-1/2 tons, the tires of the

wheels being about 3-1/2 inches in breadth. Mr. _Hancock_ states, that

the construction of his boiler is of such a nature, that, even in the

case of bursting, no danger is to be apprehended, nor any other

inconvenience than the stoppage of the carriage. He states that, while

travelling about 9 miles an hour, and working with a pressure of

about 100 lbs. on the square inch, loaded with 13 passengers, the

carriage was suddenly stopped. At first the cause of the accident was

not apparent; but, on opening one of the cocks of the boiler, it was

found that it contained neither steam nor water. Further examination

proved that the boiler had burst. On unscrewing the bolts, it was

found there were several large holes in the plates of the

water-chamber, through which the water had flowed on the fire; but

neither noise nor explosion, nor any dangerous consequences ensued.

This boiler has some obvious defects. It is evident that thin flat

plates are the form which, mechanically considered, is least

favourable to strength; nor does it appear that any material advantage

is gained to compensate for this by the magnitude of the surface

exposed to the action of the fire. It is a great defect that a part of

the surface of each of the plates is exposed to the action of the fire

while it is out of contact with the water; in fact, in the upper part

of the spaces marked E, fig. 66., steam only is contained. It has been

observed by engineers, and usually shown by experiment, that if steam

be heated on the surface of water it will be decomposed, and its

elasticity destroyed; this is not the only evil connected with the

arrangement, for on this part of the metal, nevertheless, the fire

acts,--with less intensity, it is true, than on that part which

contains the water,--but still with sufficient intensity to destroy

the metal. Mr. _Hancock_ appears to have attempted to remedy this

defect by occasionally inverting the position of the flat chambers,

placing that which at one time was at the bottom at the top, and _vice

versâ_. This may equalise the wear produced by the action of the fire

upon the metal out of contact with water, but still the wear on the

whole will not be less rapid. There appears to be no provision or

space for separating the steam from the water with which it is

charged; in fact, there are no means in this engine of discharging the

function of Mr. _Gurney's_ separator. This will be found to produce

considerable waste and loss of power in practice.

The bars upon which the fire rests are of solid metal; and such is the

intense heat to which they are subject, that, in an engine constantly

at work, it is unlikely that they will last, without being renewed,

more than about a week, if so much. The draft is maintained in this

engine by means of a revolving fan worked by the engine. This,

perhaps, is one of the greatest defects as compared with other

locomotives. The quantity of power requisite to work this bellows, and

of which the engine is robbed, is very great. This defect is so fatal,

that I consider it is quite impossible that the ingenious inventor can

persevere in the use of it. Mr. _Hancock_ has abandoned the use of the

cranks upon his working axle, and has substituted an endless chain and

rag-wheels. This also appears to me defective, and a source by which

considerable power is lost. On the other hand, however, the weakness

of the axle which is always produced by cranks is avoided.

(111.) Mr. _Nathaniel Ogle_ of Southampton obtained a patent for a

locomotive carriage, and worked it for some time experimentally; but

as his operations do not appear to have been continued, I suppose he

was unsuccessful in fulfilling those conditions, without which the

machine could not be worked with economy and profit. In his evidence

before a committee of the House of Commons, he has thus described his

contrivance:--

"The base of the boiler and the summit are composed of cross pieces,

cylindrical within and square without; there are holes bored through

these cross pieces, and inserted through the whole is an air tube. The

inner hole of the lower surface, and the under hole of the upper

surface, are rather larger than the other ones. Round the air tube is

placed a small cylinder, the collar of which fits round the larger

aperture on the inner surface of the lower frame, and the under

surface of the upper frame-work. These are both drawn together by

screws from the top; these cross pieces are united by connecting

pieces, the whole strongly bolted together; so that we obtain, in one

tenth of the space, and with one tenth of the weight, the same heating

surface and power as is now obtained in other and low-pressure

boilers, with incalculably greater safety. Our present experimental

boiler contains 250 superficial feet of heating surface in the space

of 3 feet 8 inches high, 3 feet long, and 2 feet 4 inches broad, and

weighs about 8 cwt. We supply the two cylinders with steam,

communicating by their pistons with a crank axle, to the ends of which

either one or both wheels are affixed, as may be required. One wheel

is found to be sufficient, except under very difficult circumstances,

and when the elevation is about one foot in six, to impel the vehicle

forward.

"The cylinders of which the boiler is composed are so small as to bear

a greater pressure than could be produced by the quantity of fire

beneath the boiler; and if any one of these cylinders should be

injured by violence, or any other way, it would become merely a safety

valve to the rest. We never, with the greatest pressure, burst, rent,

or injured our boiler; and it has not once required cleaning, after

having been in use twelve months."

(112.) Dr. _Church_ of Birmingham has obtained a succession of patents

for contrivances connected with a locomotive engine for stone roads;

and a company, consisting of a considerable number of individuals,

possessing sufficient capital, has been formed in Birmingham for

carrying into effect his designs, and working carriages on his

principle. The present boiler of Dr. _Church_ is formed of copper. The

water is contained between two sheets of copper, united together by

copper nails, in a manner resembling the way in which the cloth

forming the top of a mattress or cushion is united with the cloth

which forms the bottom of it, except that the nails or pins, which

bind the sheets of copper, are much closer together. The water, in

fact, seems to be "quilted" or "padded," in between two sheets of

thin copper. This double sheet of copper is formed into an oblong

rectangular box, the interior of which is the fire-place and ash-pit,

and over the end of which is the steam-chest. The great extent of

surface exposed to the immediate action of the fire causes steam to be

produced with great rapidity.

An obvious defect which such a boiler presents is the difficulty of

removing from it any deposite or incrustation, which may collect

between the sheets of copper so closely and intricately connected. Dr.

Church proposes to effect this, when it is required, by the use of an

acid, which will combine readily with the incrustation, and by which

the boiler may therefore be washed. This method of cleansing boilers

was recommended by Dr. Wollaston to Mr. Gurney, who informed me,

however, that he found that it was not practicable in the way in which

boilers must commonly be used.

I apprehend, also, that the spaces between the sheets of copper, in

Dr. Church's boiler just described, will hardly permit the steam

bubbles which will be formed to escape with sufficient facility into

the steam-chest; and being retained in that part of the boiler which

is exposed to the action of the fire, the metal will be liable to

receive an undue temperature.

I have, however, seen this engine working, and its performance was

very satisfactory.

(113.) Various other projects for steam carriages on common roads are

in various degrees of advancement, among which may be mentioned those

of Messrs. Maudslay and Field, Col. Macerone, and Mr. Russell of

Edinburgh; but our limits compel us to omit any detailed account of

them.

CHAPTER XII.

STEAM NAVIGATION.

Propulsion by paddle-wheels. -- Manner of driving them. -- Marine

Engine. -- Its form and arrangement. -- Proportion of its

cylinder. -- Injury to boilers by deposites and incrustation. --

Not effectually removed by _blowing out_. -- Mr. Samuel Hall's

condenser. -- Its advantages. -- Originally suggested by Watt. --

Hall's _steam saver_. -- Howard's vapour engine. -- Morgan's

paddle-wheels. -- Limits of steam navigation. -- Proportion of

tonnage to power. -- Average speed. -- Consumption of fuel. --

Iron steamers. -- American steam raft. -- Steam navigation to

India. -- By Egypt and the Red Sea to Bombay. -- By same route to

Calcutta. -- By Syria and the Euphrates to Bombay. -- Steam

communication with the United States from the west coast of

Ireland to St. John's, Halifax, and New York.

(114.) Among the various ways in which the steam engine has ministered

to the social progress of our race, none is more important and

interesting than the aid it has afforded to navigation. Before it lent

its giant powers to that art, locomotion over the waters of the deep

was attended with a degree of danger and uncertainty, which seemed so

necessary and so inevitable, that, as a common proverb, it became the

type and representative of everything else which was precarious and

perilous. The application, however, of steam to navigation has rescued

the mariner from much of the perils of the winds and waves; and even

in its actual state, apart from the improvements which it is still

likely to receive, it has rendered all voyages of moderate length as

safe and as regular as journeys over land. We are even now upon the

brink of such improvements as will probably so extend the powers of

the steam engine, as to render it available as the means of connecting

the most distant points of the earth.

The manner in which the steam engine is commonly applied to propel

vessels must be so familiar as to require but short explanation. A

pair of wheels, like common under-shot water-wheels, bearing on their

rims a number of flat boards called _paddle boards_, are placed one at

each side of the vessel, in such a position that when the vessel is

immersed to her ordinary depth the lowest paddle boards shall be

submerged. These wheels are fixed upon a shaft, which is made to

revolve by cranks placed upon it, in the same manner as the fly-wheel

of a common steam engine is turned. It is now the invariable custom to

place in steam vessels two engines, each of which works a crank: these

two cranks are placed at right angles to each other, in the same

manner as the cranks already described upon the working axles of

locomotive engines. When either crank is at its dead point, the other

is in full activity, so that the necessity for a fly wheel is

superseded. The engines may be either condensing or high-pressure

engines; but in Europe the low-pressure condensing engine has been

invariably used for nautical purposes. In the United States, where

steam navigation had its origin, and where it was, until a recent

period, much more extensively practised than in Europe, less objection

was felt to the use of high-pressure engines; and their limited bulk,

their small original cost, and simplicity of structure, strongly

recommended them, more especially for the purposes of river

navigation.

(_g._) The original type of nearly all the engines used in steam

navigation was that constructed at Soho by Watt and Bolton for Mr.

Fulton, and first used by him upon the Hudson river. This had the beam

below the piston-rod as in the English boat-engines, but the cylinder

above deck, as in the American. From this primitive form, the two

nations have diverged in opposite directions. The Americans,

navigating rivers, and having speed for their principal object, have

not hesitated to keep the cylinder above deck, and have lengthened the

stroke of the piston in order to make the power cut on a more

advantageous point of the wheel. Compactness has been gained by the

suppression of the working beam.

On the other hand, the English, having the safe navigation of stormy

seas as their more important object, have shortened the cylinder in

order that the piston-rod may work wholly under the deck, and the

arrangement of Fulton's working beam has been retained by them. In

this way there can be no doubt, that they have lost the power of

obtaining equal speed from a given expenditure of power, and those

conversant in the practice and theory of stowing ships may well doubt

whether security is not also sacrificed.--A. E.

The arrangement of the parts of the maritime engine differs in some

respects, from that of the land engine. Want of room renders greater

compactness necessary; and in order to diminish the height of the

machine, the working beam is transferred from above the cylinder to

below it. In fact, there are two beams, one at each side of the

engine, which are connected by a parallel motion with the piston, the

rods of the parallel motion extending from the lower part of the

engine to the top of the piston-rod. The working end of the beam is

connected with the crank by a connecting rod, presented upwards

instead of downwards, as in the land engine. The proportion of the

length and diameters of the cylinders differ from those of land

engines for a like reason: to save height, short cylinders with large

diameters are used. Thus, in an engine of 200 horse-power, the length

of the cylinder is sometimes 60 inches, and its diameter 53 inches:

the valves and the gearing which work them, the air-pump, condenser,

and other parts of the machine, do not differ materially from those

already described in the land engines.

(_h._) The action of machinery may be rendered more equable by using

two engines, each of half the power, instead of a single one. If one

of these be working with its maximum force when the other is changing

the direction of its motion, the result of their joint action will be

a force nearly constant. Such a combination was invented by Mr.

Francis Ogden, and has been used in several steam boats constructed

under his directions. It would however be far more valuable in other

cases, particularly where great uniformity in the velocity is

indispensable.

This method has now become almost universal in the engines used in the

English steam boats, each of which has usually two, both applied to

the same shaft, and therefore capable of being used singly or together

to turn the paddle wheels. In the American steam boats, although two

engines have been often applied, each usually acts upon no more than

one of the wheels. We can see no other good reason for this, than that

our engineers do not wish to be thought to copy Mr. Ogden.--A. E.

The nature of the work which the marine engine has to perform, is

such, that great regularity of action is neither necessary nor

possible. The agitation of the surface of the sea will cause the

immersion of the paddle-wheels to vary very much, and the resistance

to the engine will undergo a corresponding change: the governor, and

other parts of the apparatus already described, contrived for

imparting to the engine that extreme regularity which is indispensable

in its application to manufactures, are therefore here omitted; and

nothing is introduced except what is necessary to maintain the engine

in its full working power.

It is evident that it must be a matter of considerable importance to

reduce the space occupied by the machinery on board a vessel to the

least possible dimensions. The marine boilers, therefore, are

constructed so as to yield the necessary quantity of steam with the

smallest practical dimensions. With this view a much more extensive

surface in proportion to the size of the boiler is exposed to the

action of the fire. In fact, the flues which carry off the heated air

to the chimney are conducted through the boiler, so as to act upon

the water on every side in thin oblong shells, which traverse the

boiler backward and forward repeatedly, until finally they terminate

in the chimney. By this arrangement the original expense of the

boilers is very considerably increased; but, on the other hand, their

steam-producing power is also greatly augmented; and from experiments

lately made by Mr. Watt at Birmingham, it appears that they work with

an economy of fuel compared with common land boilers in the proportion

of about two to three. Thus they have the additional advantage of

saving the tonnage as well as the expense of one third of the fuel.

One of the most formidable difficulties which has been encountered in

applying the steam engine to the purposes of navigation has arisen

from the necessity of supplying the boiler with sea water, instead of

pure fresh water. This water (also used for the purpose of

condensation) being injected into the condenser and mixed with the

condensed steam, is conducted as feeding water into the boiler.

The salt contained in the sea water, not being evaporated, remains in

the boiler. In fact, it is separated from the water in the same manner

as by the process of distillation. As the evaporation in the boiler is

continued, the proportion of salt contained in the water is,

therefore, constantly increased, until a greater proportion is

accumulated than the water is capable of holding in solution; a

deposition of salt then commences, and is lodged in the cavities at

the bottom of the boiler. The continuance of this process, it is

evident, would at length fill the boiler with salt.

But besides this, under some circumstances, a deposition of lime[35]

is made, and a hard incrustation is formed on the inner surface of the

boiler. In some situations, also, sand and mud are received into the

boiler, being suspended in the water pumped in for feeding it. All

these substances, whether deposited in a loose form in the lower parts

of the boiler or collected in a crust on its inner surface, form

obstructions to the passage of heat from the fire to the water. The

crust thus formed is not unfrequently an inch or more in thickness,

and so hard that good chisels are broken in removing it. The heat more

or less intercepted by these substances collects in the metal of the

boiler, and raises it to a temperature far exceeding that of the water

within. It may even, if the incrustation be great, be sufficient to

render the boiler red-hot. These circumstances occasion the rapid wear

of the boiler, and endanger its safety by softening it.

[Footnote 35: Ten thousand grains of pure sea water contain muriate of

soda 220 grs., sulphate of soda 33 grs., muriate of magnesia 42 grs.,

and muriate of lime 8 grs.]

The remedy which has generally been adopted to remove or diminish

these injurious effects consists in allowing a stream of hot water

continually to flow from the boiler, and supplying from the feed-pipe

a corresponding portion of cold water. While the hot water which flows

from the boiler in this case contains, besides its just proportion of

salt, that portion which has been liberated from the water converted

into vapour, the cold water which is supplied through the feed-pipe

contains less than its just proportion of salt, since it is composed

of the natural sea water, mixed with the condensed steam, which latter

contains no salt. In this manner, the proportion of the salt in the

boiler may be prevented from accumulating; but this is attended with

considerable inconvenience and loss. It is evident that the discharge

of the hot water, and the introduction of so considerable a quantity

of cold water, entails upon the machine a great waste of fuel, and,

consequently, renders it necessary that the vessel should be supplied

with a much larger quantity of coals than are merely necessary for

propelling it. In long voyages, where this inconvenience is most felt,

this is a circumstance of obvious importance. But besides the waste of

fuel, the speed of the vessel is diminished by the rate of evaporation

in the boiler being checked by the constant stream of cold water

flowing into it. This process of discharging the water, which is

called _blowing out_, is only practised occasionally. In the Admiralty

steamers, the engineers are ordered to blow out every two hours. But

it is more usual to do so only once a day.

This method, however, of blowing out furnishes but a partial remedy

for the evils we have alluded to: a loose deposite will perhaps be

removed by such means, but an incrustation, more or less according to

the circumstances and quality of the water, will be formed; besides

which, the temptation to work the vessel with efficiency for the

moment influences the engine men to neglect blowing out; and it is

found that this class of persons can rarely be relied upon to resort

to this remedy with that constancy and regularity which are essential

for the due preservation of the boilers. The class of steam vessels

which, at present, are exposed to the greatest injury from these

causes are the sea-going steamers employed by the Admiralty; and we

find, by a report made by Messrs. Lloyd and Kingston to the Admiralty,

in August, 1834, that it is admitted that the method of blowing out

is, even when daily attended to, ineffectual. "The water in the

boiler," these gentlemen observe, "is kept from exceeding a certain

degree of saltness, by periodically blowing a portion of it into the

sea; but whatever care is taken, in long voyages especially, salt will

accumulate, and sometimes in great quantities and of great hardness,

so that it is with difficulty it can be removed. Boilers are thus

often injured as much in a few months as they would otherwise be in as

many years. The other evil necessarily resulting from this state of

things is, besides the rapid destruction of the boilers, a great waste

of fuel, occasioned by the difficulty with which the heat passes

through the incrustation on the inside, by the leaks which are thereby

caused, and by the practice of blowing out periodically, as before

mentioned, a considerable portion of the boiling water."

It would be impracticable to carry on board the vessel a sufficient

quantity of pure fresh water to work the engine exclusively by its

means. To accomplish this, it would be necessary to have a sufficient

supply of cold water to keep the condensing cistern cold, to supply

the jet in the condenser, and to have a reservoir in which the warm

water coming from the waste pipe of the cold cistern might be allowed

to cool. Engineers have therefore directed their attention to some

method by which the steam may be condensed without a jet, and after

condensation be preserved for the purpose of feeding the boiler. If

this could be accomplished, it would not be necessary to provide a

greater quantity of pure water than would be sufficient to make up the

small portion of waste which might proceed from leakage and from other

causes; and it is evident that this portion might always be readily

obtained by the distillation of sea water, which might be effected by

a small vessel exposed to the same fire which acts upon the boiler.

(115.) Mr. _Samuel Hall_ of _Basford_, near Nottingham, has taken out

patents for a new form of condenser, contrived for the attainment of

these ends, besides some other improvements in the engine.

The condenser of Mr. _Hall_ consists of a great number of narrow tubes

immersed in a cistern of cold water: the steam as it passes from the

cylinder, after having worked the piston, enters these tubes, and is

immediately condensed by their cold surfaces. It flows in the form of

water from their remote extremities, and is drawn off by the air-pump,

and conducted in the usual way to a cistern from which the boiler is

fed. In the marine engines constructed under Mr. _Hall_'s patents, the

tubes of the condenser being in an upright or vertical position, the

steam flows from the cylinder into the upper part of the condenser,

which is a low flat chamber, in the bottom of which is inserted the

upper extremities of the tubes, through which the steam passes

downwards, and as it passes is condensed. It flows thence into a

similar chamber below, from whence it is drawn off by the air-pump.

It is evident that at sea an unlimited supply of cold water may be

obtained to keep the condensing cistern cold, so that a perfect

condensation may always be effected by these tubes, if they be made

sufficiently small. The water formed by the condensed steam will be

pure distilled water; and if the boiler be originally filled with

water which does not hold in solution any earthy or other matter which

might be deposited or encrusted, it may be worked for any length of

time without injury. The small quantity of waste from leakage is

supplied in Mr. Hall's engine by a simple apparatus in which a

sufficient quantity of sea water may be distilled.

The following are the advantages, as stated by Mr. Hall, to be gained

by his condenser:--

1. A saving of fuel, amounting in some cases to so much as a third of

the ordinary consumption.

2. The preservation of the boilers from the destruction produced in

common engines by the corrosive action of sea or other impure water,

and by encrustations of earthy matter.

3. The saving of the time spent in cleaning the boilers.

4. A considerable increase of power, owing to the cleanness of the

boilers; the absence of injected water to be pumped out of a vacuum;

the greater perfection of the vacuum; the better preservation of the

piston and valves of the air-pump; and (by another contrivance of his)

the more perfect lubrication of the parts of the engine.

5. The water in the boiler being constantly maintained at the same

height by self-acting arrangement.

6. The size of a boiler exerting a given power, being much smaller

than the common kind, owing to its more perfect action.

Messrs. _Lloyd_ and _Kingston_ were employed by government to examine

and report the effects of Mr. Hall's boilers, and they stated in their

report, already referred to, that the result is so successful as to

leave nothing to be wished for. Among the advantages which they

enumerate are the increased durability of the engines; the prevention

of accidents through carelessness, or otherwise, arising from the

condenser and air-pump becoming choked with injection water; and the

additional security against the boilers being burnt in consequence of

the water being suffered to get too low. But the greatest advantages,

compared with which they consider all others to be of secondary

importance, are the increased durability of the boilers and the saving

of fuel.

About 16 engines, built either wholly upon Mr. Hall's principle or

having his condenser attached to them, have now (October, 1835) been

working in different parts of England, and on board different vessels

for various periods, from three years to three months; and it appears

from the concurrent testimony of the proprietors and managers of them,

that they are attended with all the advantages which the patentee

engaged for. The part of the contrivance the performance of which

would have appeared most doubtful would have been the maintenance of a

sufficiently good vacuum in the condenser, in the absence of the usual

method of condensation by the injection of cold water; nevertheless it

appears that a better vacuum is sustained in these engines than in the

ordinary engines which condense by jet. The barometer-gauge varies

from 29 to 29-1/2 inches, and in some cases comes up to 30 inches,

according to the state of the barometer: this is a vacuum very nearly

perfect, and indeed may be said to be so for all practical purposes.

The _Prince Llewellyn_ and the _Air_ steam packets, belonging to the

St. George Steam Packet Company, have worked such a pair of these

engines for about a year. The _City of London_ steam packet, the

property of the General Steam Navigation Company, has been furnished

with two fifty-horse engines, and has worked them during the same

period. In all cases the boilers have been found perfectly free from

scale or incrustation; and the deposite is either absolutely nothing

or very trifling, requiring the boiler to be swept about once in half

a year, and sometimes not so often. The trial which has been made of

these engines in the navy has proved satisfactory, so far as it has

been carried. The Lords of the Admiralty have lately ordered a pair of

seventy-horse engines to be constructed on this principle for a vessel

now (October, 1835) in process of construction;[36] and another vessel

in all respects similar, except having copper boilers, is likewise

ordered; so that a just comparison may be made. It would, however,

have been more fair if both vessels had been provided with iron

boilers, since copper does not receive incrustation as readily as

iron.

[Footnote 36: This order has, as Mr. Hall informs me, been given

without requiring any guarantee as to the performance of the engines.]

It would seem that the advantages of these boilers in the vessels of

the St. George Steam Packet Company were regarded by the directors as

sufficiently evident, since, after more than a year's experience, they

are about to place a pair of ninety-horse engines of this kind in a

new and powerful steamer called the _Hercules_.

Engines furnished with Mr. Hall's apparatus have not yet, so far as I

am informed, been tried with reference to the power exerted by the

consumption of a given quantity of fuel. The mere fact of a good

vacuum being sustained in the condenser cannot by itself be regarded

as a conclusive proof of the efficiency of the engine, without the

water or air introduced by a condensing jet. Mr. Hall, nevertheless,

uses as large an air-pump as that of an ordinary condensing engine,

and recommends even a larger one. For what purpose, it may be asked,

is such an appendage introduced? If there be nothing to be removed but

the condensed steam, a very small pump ought to be sufficient. It is

not wonderful that a good vacuum is sustained in the condenser, if the

power expended on the air-pump is employed in _pumping away

uncondensed steam_. Such a contrivance would be merely a deception,

giving an apparent but no real advantage to the engine.

Having mentioned these advantages, which are said to arise from Mr.

Hall's condenser, it is right to state that it is in fact a

reproduction of an early invention of Mr. Watt. There is in possession

of James Watt, esquire, a drawing of a condenser laid before

parliament in 1776, in which the same method of condensing without a

jet is proposed. Mr. Watt, however, finding that he could not procure

by that means so sudden or so perfect a vacuum as by injection,

abandoned it. I believe he also found that the tubes of the condenser

became furred with a deposite which impeded the process of

condensation. It would seem, however, that Mr. Hall has found means to

obviate these effects. It is right to add, that Mr. Hall, in his

specification, distinctly disclaims all claim to the method of

condensing by tubes without jet.

There is another part of Mr. Hall's contrivance which merits notice.

In all engines, a considerable quantity of steam is allowed to escape

from the safety valve. Whenever the vessel stops, the steam, which

would otherwise be taken from the boiler by the cylinders, passes out

through this valve into the atmosphere. Also, whenever the cylinders

work at under-power, and do not consume the steam as fast as it is

produced by the boiler, the surplus steam escapes through the valve.

Now, according to the principle of Mr. Hall's method, it is necessary

to save the water which thus escapes in vapour, since otherwise the

pure water of the boiler would be more rapidly wasted. Mr. Hall

accordingly places a safety valve of peculiar construction in

communication with a tube which leads to the condenser, so that

whenever, either by stopping the engine or diminishing its working

power, steam accumulates in the boiler, its increased pressure opens

the safety valve, and it passes through this pipe to the condenser,

where it is reconverted into water, and pumped off by the air-pump

into the cistern from which the boiler is fed.

The attainment of an object so advantageous as to extend the powers

of steam navigation, and to render the performance of voyages of any

length practicable, so far as the efficiency of the machinery is

concerned, has naturally stimulated the inventive genius of the

country. The preservation of the boiler by the prevention of deposite

and incrustation is an object of paramount importance; and its

attainment necessarily involves, to a certain degree, another

condition on which the extension of steam voyages must depend, viz.

the economy of fuel. In proportion as the economy of fuel is

increased, in the same proportion will the limit to which steam

navigation may be carried be extended.

(116.) A patent has been obtained by Mr. Thomas Howard of London for a

form of engine possessing much novelty and ingenuity, and having

pretensions to the attainment of a very extraordinary economy of fuel,

in addition to those advantages which have been already explained as

attending Mr. Hall's engines. In these engines, as in Mr. Hall's, the

steam is constantly reproduced from the same water, so that pure or

distilled water may be used; but Mr. Howard dispenses with the use of

a boiler altogether. The steam also with which he works is in a state

essentially different from the steam used in ordinary engines. In

these, the vapour is raised directly from the water in a boiling

state, and it contains as much water as it is capable of holding at

its temperature. Thus, at the temperature of 212°, a cubic foot of

steam used in common engines will contain about a cubic inch of water;

but in the contrivance of Mr. Howard, a considerable quantity of heat

is imparted to the steam before it passes into the cylinder in

addition to what is necessary to maintain it in the vaporous form.

A quantity of mercury is placed in a shallow wrought-iron vessel over

a coke fire, by which it is maintained at the temperature of from 400°

to 500°. The surface exposed to the fire is three fourths of a square

foot for each horse-power. The upper surface of the mercury is covered

by a very thin plate of iron, which rests in contact with it, and

which is so contrived as to present about four times as much surface

as that exposed beneath to the fire. Adjacent to this a vessel of

water is placed, kept heated nearly to the boiling-point, which

communicates by a nozzle and valve with the chamber or vessel

immediately above the mercury. At intervals corresponding to the

motion of the piston, a small quantity of water is injected from this

vessel, and thrown upon the plate of iron which rests upon the hot

mercury: from this it receives the heat necessary not only to convert

it into steam, but to expand that steam, and raise it to a temperature

above the temperature it would receive if raised in immediate contact

with water. In fact, the steam thus produced will have a temperature

not corresponding to its pressure, but considerably above that point,

and it will therefore be in circumstances under which it will part

with more or less of its heat, and allow its temperature to be lowered

without being even partially condensed, whereas steam used in the

ordinary steam engines must be more or less condensed by the slightest

diminution of its temperature. The quantity of liquid injected into

the steam chamber must be regulated by the power at which the engine

is intended to work. The fire is supplied with air by a blowing

machine, which is subject to exact regulation. The steam, produced in

the manner already explained, passes into a chamber which surrounds

the working cylinder; and this chamber itself is enclosed by another

space, through which the air from the furnace must pass before it

reaches the flue. In this way it imparts its redundant heat to the

steam which is about to work the cylinder, and raises it to a

temperature of about 400°; the pressure, however, not exceeding 25

lbs. per square inch. The arrangement of valves for the admission of

the steam to the cylinder is such as to cause the steam to act

expansively.

The vacuum on the opposite side of the piston is maintained by

condensation in the following manner:--The condenser is a copper

vessel placed in a cistern constantly supplied with cold water, and

the steam flows to it from the cylinder by an eduction pipe in the

usual way: a jet is admitted to it from an adjacent vessel, which,

before the engine commences work, is filled with distilled water; the

condensing water and condensed steam are pumped from the condenser by

air-pumps of the usual construction, but smaller, inasmuch as there is

no air to be withdrawn, as in common engines. The warm water thus

pumped out of the condenser is driven into a copper pipe or worm,

which is carried with many coils through a cistern of cold water, so

that when it arrives at the end of this pipe it is reduced to the

common temperature of the atmosphere. The pipe is then conducted into

the vessel of distilled water already mentioned, and the water flowing

from it continually replaces the water which flows into the condenser

through the condensing jet. The condensing water being purged of air,

a very small air-pump is sufficient; since it has only to exhaust the

condenser and tubes at starting, and to remove whatever air may enter

by casual leakage. The patentee states that the condensation takes

place as rapidly and as perfectly as in the best steam engine, and it

is evident that this method of condensation is applicable even where

the mercurial generator already described may not be employed. The

vessel from which the water is injected into the mercurial generator

is likewise fed by the air-pump connected with the condenser. There is

another pipe besides the copper worm already described, which is

carried from the hot well to this vessel, and the water is of course

returned through it without being cooled. This vessel is likewise

sufficiently exposed to the action of the fire to maintain it at a

temperature somewhat below the boiling point.

An apparatus of this construction was in the spring of the present

year (1835) placed in the Admiralty steamer called the COMET, in

connexion with a pair of 40-horse engines. The patentee states that

these engines were ill adapted to the contrivance; nevertheless, the

vessel was successfully worked in the Thames for 800 miles: she also

performed a voyage from Falmouth to Lisbon, but was prevented from

returning by an accident which occurred to the machinery near the

latter port. In this experimental voyage, the consumption of fuel is

stated never to have exceeded a third of her former consumption, when

worked by Bolton and Watt's engines; the former consumption of coals

being about 800 lbs. per hour, and the consumption with Mr. Howard's

engine being under 250 lbs. of coke per hour.

After this failure (which, however, was admitted to be one of accident

and not of principle) the government did not consider itself justified

in bestowing further time or incurring greater expense in trying this

engine. Mr. Howard, however, has himself built a new vessel, in which

he is about to place a pair of forty-horse engines. This vessel is now

(December, 1835) nearly ready, and will bring the question to issue by

a fair experiment. The advantages of the contrivance as enumerated by

the patentee are:--

_First_, The small space and weight occupied by the machinery, arising

from the absence of a boiler.

_Secondly_, The diminished consumption of fuel.

_Thirdly_, The reduced size of the flues.

_Fourthly_, The removal of the injurious effects arising from deposite

and incrustation.

_Fifthly_, The absence of smoke.

Some of these improvements, if realized, will be attended with

important advantages in steam navigation. Steamers of a given tonnage

and power will have more disposable space for lading and fuel, and in

short voyages may carry greater freight, or an increased number of

passengers; or by taking a larger quantity of fuel,[37] may make

greater runs than are now attainable; or, finally, with the same

tonnage and the same lading, they may be supplied with more powerful

machinery.

[Footnote 37: The fuel used in this form of engine is coke, and not

coal. A ton of coke occupies the same space as two tons of coal; the

saving of tonnage, therefore, by the increased economy of fuel will

not be in so great a proportion as the saving of fuel. A quantity of

fuel of equivalent power will occupy about half the present space, but

the displacement or immersion which it produces will be only one

fourth of its present effect.]

(117.) To obtain from the moving power its full amount of mechanical

effect in propelling the vessel, it would be necessary that its force

should propel, by constantly acting against the water in a horizontal

direction, and with a motion contrary to the course of the vessel. No

system of mechanical propellers has, however, yet been contrived

capable of perfectly attaining this end. Patents have been granted for

many ingenious mechanical combinations to impart to the propelling

surfaces such angles as appeared to the respective contrivers most

advantageous. In most of these, however, the mechanical complexity has

formed a fatal objection. No part of the machinery of a steam vessel

is so liable to become deranged at sea as the paddle-wheels; and,

therefore, such simplicity of construction as is compatible with those

repairs which are possible on such emergencies is quite essential for

safe practical use.

[Illustration: Fig. 67.]

The ordinary paddle-wheel, as I have already stated, is a wheel

revolving upon a shaft driven by the engine, and carrying upon its

circumference a number of flat boards, called paddle boards, which are

secured by nuts or braces in a fixed position; and that position is

such that the planes of the paddle boards diverge nearly from the

centre of the shaft on which the wheel turns. The consequence of this

arrangement is that each paddle board can only act in that direction

which is most advantageous for the propulsion of the vessel when it

arrives near the lowest point of the wheel. In figure 67. let o be

the shaft on which the common paddle wheel revolves; the position of

the paddle boards are represented at A, B, C, &c.; X, Y represents the

water line, the course of the vessel being supposed to be from X to Y;

the arrows represent the direction in which the paddle-wheel revolves.

The wheel is immersed to the depth of the lowest paddle board, since a

less degree of immersion would render a portion of the surface of each

paddle board mechanically useless. In the position A the whole force

of the paddle board is efficient for propelling the vessel; but, as

the paddle enters the water in the position H, its action upon the

water, not being horizontal, is only partially effective for

propulsion: a part of the force which drives the paddle is expended in

depressing the water, and the remainder in driving it contrary to the

course of the vessel, and, therefore, by its reaction producing a

certain propelling effect. The tendency, however, of the paddle

entering the water at H, is to form a hollow or trough, which the

water, by its ordinary property, has a continual tendency to fill up.

After passing the lowest point A, as the paddle approaches the

position B, where it emerges from the water, its action again becomes

oblique, a part only having a propelling effect, and the remainder

having a tendency to raise the water, and throw up a wave and spray

behind the paddle-wheel. It is evident that the more deeply the

paddle-wheel becomes immersed the greater will be the proportion of

the propelling power thus wasted in elevating and depressing the

water; and, if the wheel were immersed to its axis, the whole force of

the paddle boards, on entering and leaving the water, would be lost,

no part of it having a tendency to propel. If a still deeper immersion

takes place, the paddle boards above the axis would have a tendency to

retard the course of the vessel. When the vessel is, therefore, in

proper trim, the immersion should not exceed nor fall short of the

depth of the lowest paddle; but for various reasons it is impossible

in practice to maintain this fixed immersion: the agitation of the

surface of the sea, causing the vessel to roll, will necessarily

produce a great variation in the immersion of the paddle-wheels, one

becoming frequently immersed to its axle, while the other is raised

altogether out of the water. Also the draught of water of the vessel

is liable to change, by the variation in her cargo: this will

necessarily happen in steamers which take long voyages. At starting

they are heavily laden with fuel, which as they proceed is gradually

consumed, whereby the vessel is lightened; and it does not appear that

it is practicable to use sea water as ballast to restore the proper

degree of immersion.

(118.) Among the contrivances which have been proposed for remedying

these defects of the common paddle wheel by introducing paddle boards

capable of shifting their position as they revolve with the

circumference of the wheel, the only one which has been adopted to any

considerable extent in practice is that which is commonly known as

Morgan's Paddle Wheel. The original patent for this contrivance was

granted to Elijah Galloway, and sold by him to Mr. William Morgan.

Subsequently to the purchase some improvements in its structure and

arrangements were introduced, and it is now extensively adopted by

Government in the Admiralty steamers. It was first tried on board His

Majesty's steamer the CONFIANCE; and after several successful

experiments was ordered by the Lords of the Admiralty to be introduced

on board the FLAMER, the FIREBRAND, the COLUMBIA, the SPITFIRE, the

LIGHTNING, a large war steamer called the MEDEA,[38] the TARTARUS, the

BLAZER, &c. It has been tried by Government in several well-conducted

experiments, where two vessels of precisely the same model, supplied

with similar engines of equal power, and propelled, one by Morgan's

paddle-wheels, and the other by the common paddle-wheels; when it was

found that the advantage of the former, whether in smooth or in rough

water, was quite apparent. One of the commanders in these experiments

(Lieutenant Belson) states that the improvement in the speed of the

CONFIANCE, after being supplied with these wheels, was proportionately

greater in a sea way than in smooth water; that their action was not

impeded by the waves, since the variation of the velocity of the

engine did not exceed one or two revolutions per minute: the vessel's

way was never stopped, and there was no sensible increase of vibration

on the paddle boxes during the gale. Another commander reported that

on a comparison of the CONFIANCE and a similar and equally powerful

vessel, the CARRON, the CONFIANCE performed in fifty-four hours the

voyage which occupied the CARRON eighty-four hours in running.

Independently of the great saving of fuel effected (namely, ten

bushels per hour[39]), or the time saved in running the same

distance, other advantages have been secured by the modification in

question. On a comparison of the respective logs of the two vessels,

it appeared that the CONFIANCE had gained by the alteration in her

wheels an increase of speed amounting to 2 knots on 7 in smooth water,

and 2-1/2 knots on 4 to 4-1/2 knots in rough weather; that the action

of the paddles did not bring up the engine or retard their velocity in

a head sea; that in rolling their action assisted in righting the

vessel; and that the wear and strain, as well on the vessel as on the

engines, were materially reduced. With respect to the durability of

these wheels, the commander of the FLAMER reported in January, 1834,

that in six weeks of the most tempestuous weather they found them to

act remarkably well, without even a single float being shifted.[40]

[Footnote 38: This splendid ship is 860 tons burden, with engines of

220 horse power.]

[Footnote 39: See Report quoted in Mechanic's Magazine, vol. xxii. p.

275. This saving cannot amount to less than 40 per cent. upon the

whole consumption of fuel; it certainly is considerably beyond what I

should have conceived to be possible; but I have no reason to doubt

the accuracy of the Report. I estimate the former consumption at 10

pounds per horse power per hour, which on 220 horse-power would be

2200 pounds, of which 840 pounds = 10 bushels, would be about

4-10ths.]

[Footnote 40: See a more detailed account of these reports in the

Mechanic's Magazine, vol. xxii. p. 274., from which I have taken the

drawing of this paddle-wheel; and also see Report of the Committee of

the House of Commons on Steam Navigation to India, Evidence of William

Morgan, p. 95.]

This paddle-wheel is represented in figure 68. The contrivance may be

shortly stated to consist in causing the wheel which bears the paddles

to revolve on one centre, and the radial arms which move the paddles

to revolve on another centre. Let A B C D E F G H I be the polygonal

circumference of the paddle-wheel, formed of straight bars, securely

connected together at the extremities of the spokes or radii of the

wheel which turns on the shaft which is worked by the engine; the

centre of this wheel being at O. So far this wheel is similar to the

common paddle-wheel; but the paddle boards are not, as in the common

wheel, fixed at A B C, &c., so as to be always directed to the centre

O, but are so placed that they are capable of turning on axles which

are always horizontal, so that they can take any angle with respect to

the water which may be given to them. From the centres, or the line

joining the pivots on which these paddle boards turn, there proceed

short arms K, firmly fixed to the paddle boards at an angle of about

120°. On a motion given to this arm K, it will therefore give a

corresponding angular motion to the paddle board, so as to make it

turn on its pivots. At the extremities of the several arms marked K is

a pin or pivot, to which the extremities of the radial arms L are

severally attached, so that the angle between each radial arm L and

the short paddle arm K is capable of being changed by any motion

imparted to L; the radial arms L are connected at the other end with a

centre P, round which they are capable of revolving. Now since the

points A B C, &c., which are the pivots on which the paddle boards

turn, are moved in the circumference of a circle, of which the centre

is O, they are always at the same distance from that point;

consequently they will continually vary their distance from the other

centre P. Thus, when a paddle board arrives at that point of its

revolution at which the centre P lies precisely between it and the

centre O, its distance from P is less than in any other position. As

it departs from that point, its distance from the centre P gradually

increases until it arrives at the opposite point of its revolution,

where the centre O is exactly between it and the centre P; then the

distance of the paddle board from the centre P is greatest. This

constant change of distance between each paddle board and the centre P

is accommodated by the variation of the angle between the radial arm L

and the short paddle board arm K; as the paddle board approaches the

centre P this gradually diminishes; and as the distance of the paddle

board from P increases, the angle is likewise augmented. This change

in the magnitude of the angle, which thus accommodates the varying

position of the paddle board with respect to the centre P, will be

observed in the figure. The paddle board D is nearest to P; and it

will be observed that the angle contained between L and K is there

very acute; at E the angle between L and K increases, but is still

acute; at F it increases to a right angle; at G it becomes obtuse; and

at K, where it is most distant from the centre P, it becomes most

obtuse. It again diminishes at I, and becomes a right angle between A

and B. Now this continual shifting of the direction of the short arm K

is necessarily accompanied by an equivalent change of position in the

paddle board to which it is attached; and the position of the second

centre P is, or may be, so adjusted that this paddle board, as it

enters the water and emerges from it, shall be such as shall be most

advantageous for propelling the vessel, and therefore attended with

less of that vibration which arises chiefly from the alternate

depression and elevation of the water, owing to the oblique action of

the paddle boards.[41]

[Footnote 41: A paddle-wheel resembling this has lately been

constructed by Messrs. Seawards. It has been charged by Mr. Morgan as

being a colourable invasion of his patent, and the dispute has been

brought into the courts of law.]

(_i_) The relative value of the two wheels, namely, the common

paddle-wheel, and that of Morgan has been investigated by Professor

Barlow of the Military School, at Woolwich, and the results published

in a paper of much ability in the Philosophical Transactions for 1834.

By this paper it appears, that, when the paddles are not wholly

immersed, the wheel of Morgan has no important advantage over the

other, and only acquires one when the wheel wallows. But the most

important of his inferences is that the common paddle is least

efficient when in a vertical position, contrary to the usual opinion.

From this we have a right to infer that the search for a form of wheel

which shall always keep the paddle vertical is one whose success need

not be attended with any important consequence. The superior qualities

of Morgan's wheel when the paddles are deeply immersed is ascribed by

Barlow to the lessening of the shock sustained by the common

paddle-wheel when it strikes the water. This being the case, the

triple wheel of Stevens is probably superior to that of Morgan in its

efficiency, while it has the advantage of being far simpler and less

liable to be put out of order.--A. E.

(119.) To form an approximate estimate of the limit of the present

powers of steam navigation, it will be necessary to consider the

mutual relation of the capacity or tonnage of the vessel; the

magnitude, weight, and power of the machinery; the available stowage

for fuel; and the average speed attainable in all weathers, as well as

the general purposes to which the vessel is to be appropriated,

whether for the transport of goods and merchandise, or merely of

despatches and passengers. That portion of the capacity of the vessel

which is appropriated to the moving power consists of the space

occupied by the machinery and the space occupied by the fuel; the

magnitude of the latter will necessarily depend upon the length of the

voyage which the vessel must make without receiving a fresh supply of

coals. If the voyage be short, this space may be proportionally

limited, and a greater portion of room will be left for the machinery.

If, on the contrary, the voyage be longer, a greater stock of coals

will be necessary, and a less space will remain for the machinery.

More powerful vessels, therefore, in proportion to their tonnage, may

be used for short than for long voyages.

[Illustration: Pl. XII.]

Taking an average of fifty-one voyages made by the Admiralty steamers,

from Falmouth to Corfu and back during four years ending June, 1834,

it was found that the average rate of steaming, exclusive of

stoppages, was 7-1/4 miles per hour, taken in a direct line between

the places, and without allowing for the necessary deviations in the

course of the vessel. The vessels which performed this voyage varied

from 350 to 700 tons burthen by measurement, and were provided with

engines varying from 100 horse to 200 horse-power, with stowage for

coals varying from 80 to 240 tons. The proportion of the power to the

tonnage varied from 1 horse to 3 tons to 1 horse to 4 tons; thus, the

MESSENGER had a power of 200 horses, and measured 730 tons; the

FLAMER had a power of 120 horses, and measured 500 tons; the COLUMBIA

had 120 horses, and measured 360 tons.

In general, it may be assumed that for the shortest class of trips,

such as those of the Margate steamers, and the packets between

Liverpool or Holyhead and Dublin, the proportion of the power to the

tonnage should be that of 1 horse-power to every 2 tons by measure;

while for the longest voyages the proportion would be reduced to 1

horse to 4 tons, voyages of intermediate lengths having every variety

of intermediate proportion.

Steamers thus proportioned in their power and tonnage may then, on an

average of weathers, be expected to make 7-1/4 miles an hour while

steaming, which is equivalent to 174 miles per day of twenty-four

hours. But, in very long voyages, it rarely happens that a steamer can

work constantly without interruption. Besides stress of weather, in

which she must sometimes lie-to, she is liable to occasional

derangements of her machinery, and more especially of her paddles. In

almost every long voyage hitherto attempted, some time has been lost

in occasional repairs of this nature while at sea. We shall perhaps,

therefore, for long voyages, arrive at a more correct estimate of the

daily run of a steamer by taking it at 160 miles.[42]

[Footnote 42: The American reader will hardly be able to refrain from

a smile at this estimate of Dr. Lardner of the speed of steamboats,

founded upon the most improved practice of Europe at the close of the

year 1835. The boats on the Hudson River have for years past averaged

a speed of 15 miles per hour, and the Lexington, which was constructed

for a navigation part of which is performed in the open ocean, could

probably keep up a speed of the same amount, except in severe storms.

With boats, constructed on the principles of those which navigate Long

Island Sound and the Chesapeake, we should not fear to assume 12 miles

per hour, or, upwards of 280 miles per day as their average rate of

crossing the ocean.--A. E.]

By a series of carefully conducted experiments on the consumption of

coals, under marine boilers and common land boilers, which have been

lately made at the works of Mr. Watt, near Birmingham, it has been

proved that the consumption of fuel under marine boilers is less than

under land boilers, in the proportion of 2 to 3 very nearly. On the

other hand, I have ascertained from general observation throughout the

manufacturing districts in the North of England, that the average

consumption of coals under land boilers of all powers above the very

smallest class is at the rate of 15 pounds of coals per horse-power

per hour. From this result, the accuracy of which may be fully relied

upon, combined with the result of the experiments just mentioned at

Soho, we may conclude that the average consumption of marine boilers

will be at the rate of 10 lbs. of coal per horse power per hour. Mr.

Field, of the firm of Maudslay and Field, in his evidence before a

Select Committee of the House of Commons on Steam Navigation to India,

has stated from his observation, and from experiments made at

different periods, that the consumption is only 8 lbs. per horse-power

per hour. In the evidence of Mr. William Morgan, however, before the

same committee, the actual consumption of fuel on board the

Mediterranean packets is estimated at 16 cwt. per hour for engines of

200 horse power, and 8-1/4 cwt. for engines of 100 horse-power. From

my own observation, which has been rather extensive both with respect

to land and marine boilers, I feel assured that 10 lbs. per hour more

nearly represents the practical consumption than the lower estimate of

Mr. Field. We may then assume the daily consumption of coal by marine

boilers, allowing them to work upon an average for 22 hours, the

remainder of the time being left for casual stoppages, at 220 lbs. of

coal per horse-power, or very nearly 1 ton for every ten horses'

power. In short voyages, where there will be no stoppage, the daily

consumption will a little exceed this; but the distance traversed will

be proportionally greater.

When the proportion of the power to the tonnage remains unaltered,

the speed of the vessel does not materially change. We may therefore

assume that 10 pounds of coal per horse power will carry a sea-going

steamer adapted for long voyages 7-1/4 miles direct distance; and

therefore to carry her 100 miles will require 138 pounds, or the

1/16th part of a ton nearly. Now, the Mediterranean steamers are

capable of taking a quantity of fuel at the rate of 1-1/4 tons per

horse power; but the proportion of their power to their tonnage is

greater than that which would probably be adapted for longer runs. We

shall, therefore, perhaps be warranted in assuming that it is

practicable to construct a steamer capable of taking 1-1/2 tons of

fuel per horse-power. At the rate of consumption just mentioned, this

would be sufficient to carry her 2400 miles in average weather; but as

an allowance of fuel must always be made for emergencies, we cannot

suppose it possible for her to encounter this extreme run. Allowing,

then, spare fuel to the extent of a quarter of a ton per horse-power,

we should have as an extreme limit of a steamer's practicable voyage,

without receiving a relay of coals, a run of about 2000 miles.

(120.) This computation is founded upon results obtained from the use

chiefly of the North of England coal. It has, however, been stated in

evidence before the select committee above mentioned, that the

_Llangennech_ coal of Wales is considerably more powerful. Captain

Wilson, who commanded the HUGH LINDSAY steamer in India, has stated

that this coal is more powerful than Newcastle, in the proportion of 9

to 6-1/2.[43] Some of the commanders of the Mediterranean packets have

likewise stated that the strength of this coal is greater than that of

Newcastle in the proportion of 16 to 11.[44] So far then as relates to

this coal, the above estimate must be modified, by reducing the

consumption nearly in the proportion of 3 to 2.

[Footnote 43: Vide Report of Select Committee on Steam Navigation, p.

152.]

[Footnote 44: Ibid. p. 7.]

The class of vessels best fitted for undertaking long voyages,

without relays of coal, would be one from about 800 to 1000 tons

measurement furnished with engines from 200 to 250 horse-power.[45]

Such vessels could take a supply of from 300 to 400 tons of coals,

which being consumed at the rate of from 20 to 25 tons per day, would

last about fifteen days.

[Footnote 45: Engines in steam vessels generally work considerably

above their nominal power. The power, however, to which we uniformly

refer is the nominal power, or that power at which they would work

with steam of the ordinary pressure.]

Applying these results, however, to particular cases, it will be

necessary to remember that they are average calculations, and must be

subject to such modifications as the circumstances may suggest in the

particular instances: thus, if a voyage is contemplated under

circumstances in which an adverse wind generally prevails, less than

the average speed must be allowed, or, what is the same, a greater

consumption of fuel for a given distance. Against a strong head wind,

in which a sailing vessel would double-reef her top-sails, even a

powerful steamer cannot make more than from 2 to 3 miles an hour,

especially if she has a head sea to encounter.

(121.) In considering the general economy of fuel, it may be right to

state, that the results of experience obtained in the steam navigation

of our channels, and particularly in the case of the Post Office

packets on the Liverpool station, have clearly established the fact,

that by increasing the ratio of the power to the tonnage, an actual

saving of fuel in a given distance is effected, while at the same time

the speed of the vessel is increased. In the case of the Post Office

steamers called the DOLPHIN and the THETIS (Liverpool station,) the

power has been successively increased, and the speed proportionably

augmented; but the consumption of fuel per voyage between Liverpool

and Dublin has been diminished. This, at first view, appears

inconsistent with the known theory of the resistance of solids moving

through fluids; since this resistance increases in the same proportion

as the square of the speed. But this physical principle is founded on

the supposition that the immersed part of the floating body remains

the same. Now I have myself proved by experiments on canals, that when

the speed of the boat is increased beyond a certain limit, its draught

of water is rapidly diminished; and in the case of a large steam raft

constructed upon the river Hudson, it was found that when the speed

was raised to 20 miles an hour, the draught of water was diminished by

7 inches. I have therefore no doubt that the increased speed of

steamers is attended with a like effect; that, in fact, they rise out

of the water; so that, although the resistance is increased by reason

of their increased speed, it is diminished in a still greater

proportion by reason of their diminished immersion.

Meanwhile, whatever be the cause, it is quite certain that the

resistance in moving through the water must be diminished, because the

moving power is always in proportion to the quantity of coals

consumed, and at the same time in the proportion to the resistance

overcome. Since, then, the quantity of coals consumed in a given

distance is diminished while the speed is increased, the resistance

encountered throughout the same distance must be proportionally

diminished.

(122.) Increased facility in the extension and application of steam

navigation is expected to arise from the substitution of iron for

wood, in the construction of vessels. Hitherto iron steamers have been

chiefly confined to river navigation; but there appears no sufficient

reason why their use should be thus limited. For sea voyages they

offer many advantages; they are not half the weight of vessels of

equal tonnage constructed of wood; and, consequently, with the same

tonnage they will have less draught of water, and therefore less

resistance to the propelling power; or, with the same draught of water

and the same resistance, they will carry a proportionally heavier

cargo. The nature of their material renders them more stiff and

unyielding than timber; and they do not suffer that effect which is

called _hogging_, which arises from a slight alteration which takes

place in the figure of a timber vessel in rolling, accompanied by an

alternate opening and closing of the seams. Iron vessels have the

further advantage of being more proof against fracture upon rocks. If

a timber vessel strike, a plank is broken, and a chasm opened in her

many times greater than the point of rock which produces the

concussion. If an iron vessel strike she will either merely receive a

dinge, or be pierced by a hole equal in size to the point of rock

which she encounters. Some examples of the strength of iron vessels

was given by Mr. Macgregor Laird, in his evidence before the Committee

of the Commons on Steam Navigation, among which the following may be

mentioned:--An iron vessel, called the ALBURKAH, in one of their

experimental trials got aground, and lay upon her anchor: in a wooden

vessel the anchor would probably have pierced her bottom; in this

case, however, the bottom was only dinged. An iron vessel, built for

the Irish Inland Navigation Company, was being towed across Lough Derg

in a gale of wind, when the towing rope broke, and she was driven upon

rocks, on which she bumped for a considerable time without any injury.

A wooden vessel would in this case have gone to pieces. A further

advantage of iron vessels (which in warm climates is deserving of

consideration) is their greater coolness and perfect freedom from

vermin.

(123.) The greatest speed which has yet been attained upon water by

the application of steam has been accomplished in the case of a river

steamer of peculiar form, which has been constructed upon the river

Hudson. This boat, or rather raft, consisted of two hollow vessels

formed of thin sheet iron, somewhat in the shape of spindles or cigars

(from whence it was called the _cigar boat_.) In the thickest part

these floats were eight feet in diameter, tapering towards the ends,

and about 300 feet long: these floats or buoys, being placed parallel

to each other, having a distance of more than 16 feet between them,

supported a deck or raft 300 feet long, and 32 feet wide. A

paddle-wheel 30 feet in diameter and 16 feet broad revolved between

the spindles, impelled by a steam engine placed upon the deck. This

vessel drew about 30 inches of water, and attained a speed of from 20

to 25 miles an hour: she ran upon a bank in the river Hudson, and was

lost. The projector is now employed in constructing another vessel of

still larger dimensions. It is evident that such a structure is

altogether unfitted for sea navigation. In the case of a wide

navigable river, however, such as the Hudson, it will no doubt be

attended with the advantage of greater expedition.

(124.) Several projects for the extension of steam navigation to

voyages of considerable length have lately been entertained both by

the public and by the legislature, and have imparted to every attempt

to improve steam navigation increased interest. A committee of the

House of Commons collected evidence and made a report in the last

session in favour of an experiment to establish a line of steam

communication between Great Britain and India. Two routes have been

suggested by the committee, each being a continuation of the line of

Admiralty steam packets already established to Malta and the Ionian

Isles. One of the routes proposed is through Egypt, the Red Sea, and

across the Indian Ocean to Bombay, or some of the other Presidencies;

the other across the north part of Syria to the banks of the

Euphrates, by that river to the Persian Gulf, and from thence to

Bombay. Each of these routes will be attended with peculiar

difficulties, and in both a long sea voyage will be encountered.

In the route by the Red Sea, it is proposed to establish steamers

between Malta and Alexandria (860 miles). A steamer of 400 tons

burthen and 100 horse-power would perform this voyage, upon an average

of all weathers incident to the situation, in from 5 to 6 days,

consuming 10 tons of coal per day. But it is probable that it might be

found more advantageous to establish a higher ratio between the power

and the tonnage. From Alexandria, the transit might be effected by

land across the Isthmus to Suez--a journey of from 4 to 5 days--by

caravan and camels; or the transit might be made either by land or

water from Alexandria to Cairo, a distance of 173 miles; and from

Cairo to Suez, 93 miles, across the desert, in about 5 days. At Suez

would be a station for steamers, and the Red Sea would be traversed in

3 runs or more. If necessary, stations for coals might be established

at Cosseir, Judda, Mocha, and finally at Socatra--an island

immediately beyond the mouth of the Red Sea, in the Indian Ocean: the

run from Suez to Cosseir would be 300 miles--somewhat more than twice

the distance from Liverpool to Dublin. From Cosseir to Judda, 450

miles; from Judda to Mocha, 517 miles; and from Mocha to Socatra, 632

miles. It is evident that all this would, without difficulty, in the

most unfavourable weather, fall within the present powers of steam

navigation. If the terminus of the passage be Bombay, the run from

Socatra to Bombay will be 1200 miles, which would be, upon an average

of weather, about 8 days' steaming. The whole passage from Alexandria

to Bombay, allowing 3 days for delay between Suez and Bombay, would be

26 days: the time from Bombay to Malta would therefore be about 33

days; and adding 14 days to this for the transit from Malta to

England, we should have a total of 47 days from London to Bombay, or

about 7 weeks.

If the terminus proposed were Calcutta, the course from Socatra would

be 1250 miles south-east to the Maldives, where a station for coals

would be established. This distance would be equal to that from

Socatra to Bombay. From the Maldives, a run of 400 miles would reach

the southern point of Ceylon, called the Point de Galle, which is the

best harbour (Bombay excepted) in British India: from the Point de

Galle, a run of 600 miles will reach Madras; and from Madras to

Calcutta would be a run of about 600 miles. The voyage from London to

Calcutta would be performed in about 60 days.

At a certain season of the year there exists a powerful physical

opponent to the transit from India to Suez: from the middle of June

until the end of September, the south-west monsoon blows with unabated

force across the Indian Ocean, and more particularly between Socatra

and Bombay. This wind is so violent as to leave it barely possible for

the most powerful steam packet to make head against it, and the voyage

could not be accomplished without serious wear and tear upon the

vessels during these months--if indeed it would be practicable at all

for any continuance in that season. The attention of parliament has

therefore been directed to another line of communication, not liable

to this difficulty: it is proposed to establish a line of steamers

from Bombay through the Persian Gulf to the Euphrates. The run from

Bombay to a place called Muscat, on the southern shore of the Gulf,

would be 840 miles in a north-west direction, and therefore not

opposed to the south-west monsoon. From Muscat to Bassidore, a point

upon the northern coast of the strait at the mouth of the Persian

Gulf, would be a run of 255 miles; from Bassidore to Bushire, another

point on the eastern coast of the Persian Gulf, would be a run of 300

miles; and from Bushire to the mouth of the Euphrates, would be 120

miles. It is evident that the longest of these runs would offer no

more difficulty than the passage from Malta to Alexandria. From

Bussora near the mouth of the Euphrates, to Bir, a town upon its left

bank near Aleppo, would be 1143 miles, throughout which there are no

physical obstacles to the river navigation which may not be overcome.

Some difficulties arise from the wild and savage character of the

tribes who occupy its banks. It is, however, thought that by proper

measures, and securing the co-operation of the Pacha of Egypt, any

serious obstruction from this cause may be removed. From Bir, by

Aleppo, to Scanderoon, a port upon the Mediterranean, opposite Cyprus,

is a land journey, said to be attended with some difficulty but not of

great length; and from Scandaroon to Malta is about the same distance

as between the latter place and Alexandria. It is calculated that the

time from London to Bombay by the Euphrates--supposing the passage to

be successfully established--would be a few days shorter than by Egypt

and the Red Sea.

Whichever of these courses may be adopted, it is clear that the

difficulties, so far as the powers of the steam engine are concerned,

lie in the one case between Socatra and Bombay, or between Socatra and

the Maldives, and in the other case between Bombay and Muscat. Even

the run from Malta to Alexandria or Scandaroon is liable to objection,

from the liability of the boiler to deposite and incrustation, unless

some effectual method be taken to remove this source of injury. If,

however, the contrivance of Mr. Hall, or of Mr. Howard, or any other

expedient for a like object, be successful, the difficulty will then

be limited to the necessary supply of coals for so long a voyage.

This, however, has already been encountered and overcome on four

several voyages by the HUGH LINDSAY steamer from Bombay to Suez: that

vessel encountered a still longer run on these several trips, by

going, not to Socatra but to Aden, a point on the coast of Arabia near

the Straits of Babel Mandeb, being a run of 1641 miles, which she

performed in 10 days and 19 hours. The entire distance from Bombay to

Suez was in one case performed in 16 days and 16 hours; and under the

most unfavourable circumstances, in 23 days. The average was 21 days

for each trip.

(125.) Another projected line of steam communication, which offers

circumstances of equal interest to the people of these countries and

the United States, is that which is proposed to be established between

London and New York. On the completion of the London and Liverpool

railroad, Dublin will be connected with London, by a continuous line

of steam transport. It is proposed to continue this line by a railroad

from Dublin to some point on the western coast of Ireland; among

others, the harbour of VALENTIA has been mentioned. The nearest point

of the western continent is St. John's, Newfoundland, the distance of

which from Valentia is 1900 miles; the distance from St. John's to New

York is about 1200 miles, Halifax (Nova Scotia) being a convenient

intermediate station. The distance from Valentia to St. John's comes

very near the point which we have already assigned as the probable

present limit of steam navigation. The Atlantic Ocean also offers a

formidable opponent in the westerly winds which almost constantly

prevail in it. These winds are, in fact, the reaction of the trades,

which blow near the equator in a contrary direction, and are produced

by those portions of the equatorial atmosphere which, rushing down the

northern latitudes, carry with them the velocity from west to east

proper to the equator. Besides this difficulty, St. John's and Halifax

are both inaccessible, by reason of the climate, during certain months

of the year. Should these causes prevent this project from being

realized, another course may be adopted. We may proceed from the

southern point of Ireland or England to the Azores, a distance of

about 1800 miles: from the Azores to New York would be a distance of

about 2000 miles, or from the Azores to St. John's would be 1600

miles.[46]

[Footnote 46: A treatise on the Steam Engine is not the place to enter

into discussion on the causes of the several constant, periodic, and

prevailing winds, otherwise we should feel it our duty to correct the

opinion adopted by Dr. Lardner from the older authorities, in relation

to the course of the westerly winds. These winds are, in the South

Atlantic, and in both South and North Pacific, constant winds. In the

North Atlantic between the latitudes, of 35° and 45°, and, therefore,

in the track of the vessels which navigate between the United States

and Great Britain, they are the most frequent prevailing winds, except

in the months of April and October. They are certainly not the

reaction of the trade winds, which is in a well known zone, to the

south of the region in which these westerly winds prevail, under the

name of the Horse latitudes of our navigations, and the Grassy sea of

the Spaniards. Those who wish to study the true theory of these winds

will find it in Daniell's learned and ingenious work, "On Atmospheric

Phenomena," or in the analysis of that work in the American Quarterly

Review.]

(_k_) While the inhabitants of Great Britain are discussing the

project of the communication with New York, by means of the stations

described by Dr. Lardner, those of the United States appear to be

seriously occupied in carrying into effect a direct communication from

New York to Liverpool. At the speed which has been given to the

American steam boats, this presents no greater difficulties than the

voyage from the Azores to New York, would, to one having the speed of

no more than 7-1/4 miles per hour. As this attempt is beyond the limit

of individual enterprise, there is, at the present moment, an

application before the Legislature of the State of New York for a

charter to carry this project into effect. It will be difficult to

estimate the results of this enterprise, which will bring the old and

new world within 12 or 15 days voyage of each other.--A. E.[47]

[Footnote 47: Dr. Lardner appears purposely to have omitted any detail

of the history of Steam Navigation. It would be an invidious task on

the part of a mere editor to attempt to supply what he has thought

proper to avoid. We therefore merely refer to this subject for the

purpose of expressing the hope, that this silence is an earnest that

the writers of Europe are about to abandon the claims they have set up

for their countrymen to the merit of introducing the successful

practice of Steam navigation, and that the respective services of

Fitch, Evans, Fulton, and the elder Stevens will soon be universally

acknowledged.--A. E.]

CHAPTER XII.

GENERAL ECONOMY OF STEAM POWER.

Mechanical efficacy of steam -- proportional to the quantity of

water evaporated, and to the fuel consumed -- Independent of the

pressure. -- Its mechanical efficacy by condensation alone. -- By

condensation and expansion combined -- by direct pressure and

expansion -- by direct pressure and condensation -- by direct

pressure, condensation, and expansion. -- The power of engines.

-- The duty of engines. -- Meaning of horse power. -- To compute

the power of an engine. -- Of the power of boilers. -- The

structure of the grate-bars. -- Quantity of water and steam room.

-- Fire surface and flue surface. -- Dimensions of steam pipes.

-- Velocity of piston. -- Economy of fuel. -- Cornish duty

reports.

(130.) Having explained in the preceding chapters the most important

circumstances connected with the principal varieties of steam engines,

it remains now to explain some matters of detail connected with the

power, efficiency, and economy of these machines, which, though

perhaps less striking and attractive than the subjects which have

hitherto engaged us, are still not undeserving of attention.

It has been shown in the first chapter, that water exposed to the

ordinary atmospheric pressure (the amount of which may be expressed by

a column of 30 inches of mercury) will pass from the liquid into the

vaporous state when it arrives at the temperature of 212°; and the

vapour thus produced from it will have an elastic force equal to that

of the atmosphere. If the water, however, to which heat is applied, be

submitted to a greater or less pressure than that of the atmosphere,

it will boil at a greater or less temperature, and will always produce

steam of an elastic force equal to the pressure under which it boils.

Now it is a fact as remarkable as it is important, that to convert a

given weight of water into vapour will require the same quantity of

heat, under whatever pressure, and at whatever temperature the water

may boil. Let us suppose a tube, the base of which is equal to a

square foot, in which a piston fits air-tight and steam-tight.

Immediately under the piston, let a cubic inch of water be placed,

which will be spread in a thin layer over the bottom of the tube. Let

the piston be counterbalanced by a weight (acting over a pulley) which

will be equivalent to the weight of the piston, so that it shall be

free to ascend by the application of any pressure below it. Now let

the flame of a lamp be applied at the bottom of the tube: the water

under the piston being affected by no pressure from above, except that

of the atmosphere acting upon the piston, will boil at the temperature

of 212°, and by the continued application of the lamp it will at

length be converted into steam. The steam into which the cubic inch of

water is converted will expand into the magnitude of a cubic foot,

exerting an elastic force equal to the atmospheric pressure;

consequently the piston will be raised one foot above its first

position in the tube, and the cubic foot beneath it will be completely

filled with steam. Let us suppose, that to produce this effect

required the lamp to be applied to the tube for the space of fifteen

minutes.

The water being again supposed at its original temperature, and the

piston in its first position, let a weight be placed upon the piston

equal to the pressure of the atmosphere, so that the water beneath the

piston will be pressed down by double the atmospheric pressure. If the

lamp be once more applied, the water will, as before, be converted

into vapour; but the piston will now be raised to the height of only

six inches[48] from the bottom, the steam expanding into only half

its former bulk. The temperature at which the water would commence to

be converted into vapour, instead of being 212°, would be 250°; but

the time elapsed between the moment of the first application of the

lamp, and the complete conversion of the water into steam, will still

be fifteen minutes.

[Footnote 48: Strictly speaking, the height to which the piston would

be raised would not diminish in so great a proportion as the pressure

is increased, because the increase of pressure being necessarily

accompanied by an increase of temperature, a corresponding expansion

would be produced. Therefore there will be a slight increase in the

total mechanical effect of the steam. The difference, however, is not

very important in practice, and it is usual to consider the density of

steam as proportional to the pressure.]

Again, if the piston be loaded with a weight equal to double the

atmospheric pressure, the water will be pressed down by the force of

three atmospheres. If the lamp be applied as before, the water would

be converted into steam in the same time; but the piston will now be

raised only four inches above its first position, and the steam will

consequently be three times as dense as when the piston was pressed

down only by the atmosphere.

From these and similar experiments we infer:--

_First_, That the elastic pressure of steam is equal to the mechanical

pressure under which the water producing the steam has been boiled.

_Secondly_, That the bulk which steam fills is diminished in the same

proportion as the pressure of the steam is increased; or, in other

words, that the density of steam is always in the same proportion as

its pressure.

_Thirdly_, That the same quantity of heat is sufficient to convert the

same weight of water into steam, whatever be the pressure under which

the water is boiled, or whatever be the density and pressure of the

steam produced.

_Fourthly_, That the same quantity of water being converted into

steam, produces the same mechanical effect, whatever be the pressure

or the density of the steam. Thus, in the first case, the weight of

one atmosphere was raised a foot high; in the second case, the weight

of two atmospheres was raised through half a foot; and, in the third

case, the weight of three atmospheres was raised through the third of

a foot; the weight raised being in every case increased in the same

proportion as the height through which it is elevated is diminished.

Every increase of the weight is, therefore, compensated by a

proportionate diminution of the height through which it is raised, and

the mechanical effect is consequently the same.

_Fifthly_, That the same quantity of heat or fuel is necessary and

sufficient to produce the same mechanical effect, whatever be the

pressure of the steam which it produces.

If steam be used to raise a piston against the atmospheric pressure

only, although a definite physical force will be exerted by it, and a

mechanical effect produced, yet under such circumstances it will exert

no directly useful efficiency; but after the piston has been raised,

and the tube beneath it filled with steam balancing the atmosphere

above it, a useful effect to the same amount may be obtained by

cooling the tube, and thereby reconverting the steam into water. The

piston will thus be urged downwards by the unresisted force of the

atmosphere, and any chain or rod attached to it will be drawn

downwards with a corresponding force. If the area of the piston be, as

already supposed, equal to the magnitude of one square foot, the

atmospheric pressure upon it, being 15 pounds for each square inch,

will amount to 144 times 15 pounds, or 2160 pounds. By drawing down a

chain or rope acting over a pulley, the piston would in its descent

(omitting the consideration of friction, &c.) raise a weight of 2160

pounds a foot high. Since 2160 pounds are nearly equal to one ton, it

may, for the sake of round numbers be stated thus:--

"_A cubic inch of water, being converted into steam, will, by the

condensation of that steam, raise a ton weight a foot high._" Such is

the way in which the force of steam is rendered practically available

in the atmospheric engine.

(131.) The method by which steam is used in the single-acting steam

engine of Watt is, in all respects, similar, except that the piston,

instead of being urged downwards by the force of the atmosphere, is

pressed by steam of a force equal to the atmospheric pressure. It is

evident, however, that this does not alter the mechanical result.

We have stated that a considerable increase of power, from a given

quantity of steam, was produced by cutting off the steam after the

piston had made a part of its descent, and allowing the remainder of

the descent to be produced by the expansive force of the steam already

admitted. We shall now more fully explain the principle on which this

increase of power depends.

Let A B (_fig. 69._,) as before, represent a tube, the bottom of which

is equal to a square foot, and let P be a piston in it, resting upon a

cubic inch of water spread over the bottom; and let w be an empty

vessel, the weight of which exactly counterpoises the piston. By the

application of the lamp, the water will be converted into steam of the

atmospheric pressure, and the piston will be raised from P to P´,

through the height of one foot, the space in the tube beneath it being

filled with steam, and the vessel w will have descended through one

foot. Let half a ton of water be now poured into the vessel W; its

weight will draw the piston P´ upwards, so that the steam below it

will expand into a larger space. When the piston P´ was only balanced

by the empty vessel W, it was pressed downwards by the whole weight of

the atmosphere above, which amounts to about one ton: now, however,

half of this pressure is balanced by the half ton of water poured into

the vessel W; consequently the effective downward pressure on the

piston P´ will be only half a ton, or half its former amount. The

piston will therefore rise, until the pressure of the steam below it

is diminished to the same extent. By what has been already explained,

this will take place when the steam is allowed to expand into double

its former bulk; consequently, when the piston has risen to P´´, one

foot higher, or two feet from the bottom of the tube, the steam will

then exactly balance the downward pressure on the piston, and the

latter will remain stationary; the vessel W, with the half ton of

water it contains, will have descended one foot lower, or two feet

below its first position. Let the steam now be cooled and reconverted

into water, and at the same time let another half ton of water be

supplied to the vessel W; the pressure below the piston being entirely

removed, the atmospheric pressure will act above it with undiminished

force; and this force, amounting to one ton, will draw up the vessel

W, with its contents. When the piston descends, as it will do, to the

bottom of the tube, the ton of water contained in the vessel W will be

raised through two perpendicular feet.[49]

[Footnote 49: Strictly speaking, the quantity of water supposed in

these cases to be placed in the vessel W would just _balance_ the

atmospheric pressure. A slight preponderance must therefore be given

to the piston, to produce the motion.]

Now, in this process it will be observed that the quantity of steam

consumed is not more than in the former case, viz. the vapour produced

by boiling one cubic inch of water. Let us consider, however, the

mechanical effect which has resulted from it; half a ton of water has

been allowed to descend through one foot, while a ton has been raised

through two feet: deducting the force lost by the descent of half a

ton through one foot from the force obtained by the ascent of one ton

through the two feet, we obtain for the whole mechanical effect one

ton and a half raised through one foot; for it is evident that half a

ton has been raised from the lowest point to which the vessel W

descended one foot above that point, and one ton has been raised

through the other foot, which is equivalent to one ton and a half

through one foot.

Comparing this with the effect produced in the first case, where the

steam was condensed without causing its expansion, it will be evident

that there is an increase of 50 per cent. upon the whole mechanical

effect produced.

But this is not the limit of the increase of power by expansion.

Instead of condensing the steam when the piston had arrived at P´´,

let a further quantity of water amounting to one sixth of a ton be

poured into the vessel W, in addition to the half ton which it

previously contained; the effective pressure on the piston P´´, being

only half a ton, will be overbalanced by the preponderating weight in

the vessel w, and the piston will consequently ascend. It will become

stationary when the steam by expansion loses a quantity of force equal

to the additional weight which the vessel W has received: now, that

vessel, having successively received a half and a sixth of a ton will

contain two thirds of a ton; consequently the effective downward

pressure on the piston will be only a third of a ton, and the steam to

balance this must expand into three times the space it occupied when

equal, to the atmospheric pressure. It must therefore ascend to P´´´,

three feet above the bottom of the tube. If the steam in the tube be

now condensed, and at the same time one third of a ton of water be

supplied to the vessel W, so as to make its total contents amount to

one ton, the piston will descend, being urged downwards by the

unresisted atmospheric pressure, and the ton of water contained in the

vessel W will be raised through three perpendicular feet.

In this case, as in the former, the total quantity of steam consumed

is that of one cubic inch of water; but the mechanical effect it

produces is still further increased. To calculate its amount, we must

consider that half a ton of water has fallen through two feet, which

is equivalent to a ton falling through one foot, besides which the

sixth part of a ton has fallen through one foot. The total loss,

therefore, by the fall of water has been one ton and one sixth through

one foot, while the force gained by the ascent of water has been one

ton raised through three feet, which is equivalent to three tons

through one foot. If, then, from three tons we deduct one and one

sixth, the remainder will be one ton and five sixths raised through

one foot; this effect being above 80 per cent. more than that which is

produced in the first case, where the steam was not allowed to expand.

To carry the inquiry one step further: Let us suppose that, upon the

arrival of the piston at P´´´, a further addition of water to the

amount of one twelfth of a ton be added to it: this, with the water it

already contained, would make the total contents three fourths of a

ton; consequently, the effective pressure upon the piston would now be

reduced to one fourth of the atmospheric pressure. The atmospheric

steam would balance this when expanded into four times its original

volume: consequently, the piston would come to a state of rest at

P´´´´, four feet above the bottom of the tube, and the vessel W would

consequently have descended through four perpendicular feet. If the

steam in the tube be now condensed as in the former cases, and at the

same time a quarter of a ton of water be added to the vessel W, the

piston will descend to the bottom of the tube, and the ton of water in

the vessel W will be raised through four perpendicular feet. To

estimate the mechanical effect thus produced, we have, as before, to

deduct the total force lost by the fall of water from the force gained

by its elevation: the water has fallen in three distinct portions:

first, half a ton has fallen through three perpendicular feet, which

is equivalent to one ton and a half through one foot; secondly, one

sixth of a ton has fallen through two perpendicular feet, which is

equivalent to one third of a ton through one foot; and thirdly, one

twelfth of a ton has fallen through one foot: these added together

will be equivalent to one ton and eleven twelfths through one foot.

One ton has been raised through four feet, which is equivalent to four

tons through one foot: deducting from this the force lost by the

descent, the surplus gained will be two tons and one twelfth through

one foot, being about 108 per cent. more than the force resulting from

the condensation of steam without expansion.

To the increase of mechanical effect to be produced in this way, there

is no theoretical limit. According to the manner in which we have here

explained it, to produce the greatest possible effect by a given

extent of expansion, it would be necessary to supply the water or

other counterpoise to the vessel W, not in separate masses, as we

have here supposed, but continuously, so as to produce a regular

motion of the piston upwards.

Such is the principle on which the advantages of the expansive engine

of Watt and Hornblower depend, explained so far as it can be without

the aid of the language and reasoning of analysis.[50]

[Footnote 50: A strict investigation of this important property, as

well as of the other consequences of the quality of expansion, would

require more abstruse mathematical processes than would be consistent

with the nature of this work.]

(132.) We have here, however, only considered the mechanical effect

produced by the condensation of steam. Let us now examine its direct

action.

Let the piston P be supposed to be connected by a rod with a load or

resistance which it is intended to raise, and let the load placed upon

it be supposed to amount to one ton, the total pressure on the piston

will then be two tons; one due to the atmospheric pressure, and the

other to the amount of the load. Upon applying heat to the water,

steam will be produced; and when the water has been completely

evaporated, the piston will rise to the height of six inches from the

bottom of the tube. The total mechanical effect thus produced will be

one ton weight raised through six perpendicular inches, which is

equivalent to half a ton raised through one foot.

Again, let the load upon the piston be two tons; this will produce a

total pressure upon the water below it amounting to three tons,

including the atmospheric pressure. The water, when converted into

vapour under this pressure, will raise the piston and its load through

four perpendicular inches: the useful mechanical effect will then be

two tons raised through the third of a foot, which is equivalent to

two thirds of a ton raised one foot. In the same manner, if the piston

were loaded with three tons, the mechanical effect would be equivalent

to three fourths of a ton raised through one foot, and so on.

It appears therefore from this reasoning, that when the direct force

of steam of greater pressure than the atmosphere is used without

condensation, the total mechanical effect is always less than that

produced by the condensation of atmospheric steam without expansion;

but that the greater the pressure under which the steam is produced,

the less will be the difference between these effects. In general, the

proportion of the mechanical effect of high-pressure steam to the

effect produced by the condensation of atmospheric steam, will be as

the number of atmospheres expressing the pressure of the steam to the

same number increased by one. Thus, if steam be produced under the

pressure of six atmospheres, the proportion of its effect to that of

the condensation of atmospheric steam will be as six to seven.

(133.) Another method of applying the power of steam mechanically is,

to combine its direct action with condensation but without expansion.