It is a universal property of matter, that by the application of heat,
so as to raise its temperature, it suffers an increase in its
magnitude. Also in different substances, when certain temperatures are
attained by the application of fire, or other methods of heating, they
undergo a change of form. Solids, at certain temperatures, are
converted into liquids; and liquids, in like manner, when heated to
certain degrees, become aeriform fluids or gases. These changes are
familiar to every one in the ordinary phenomena attending water. Below
the temperature of 32° of the common thermometer, that substance
exists in the solid form, and is called _ice_. Above that temperature
it passes into the liquid state, and is called _water_; and when
raised to the temperature of 212°, under ordinary circumstances, it
passes into the aeriform state, and is called _steam_. It is to this
last change that we wish at present principally to call the attention
of the reader. In the transition of water from the liquid state to the
state of vapour or steam, an immense change of bulk takes place. In
this change, a solid inch of water enlarges its size about 1,700
times, and forms 1,700 solid inches of steam. This expansion takes
place accompanied with a certain force or pressure, by which the
vapour has a tendency to burst the bounds of any vessel which contains
it. The steam which fills 1,700 solid inches at the temperature of
212°, will, if cooled below that temperature, return to the liquid
form, and occupy only one solid inch, leaving 1,699 solid inches
vacant; and, if it be included in a close vessel, leaving the surfaces
of that vessel free from the internal pressure to which they were
subject before the return of the water to the liquid form. If it be
possible, therefore, alternately to convert water into vapour by heat,
and to reconvert the vapour into water by cold, we shall be enabled
alternately to submit any surface to a pressure equal to the elastic
force of the steam, and to relieve it from that pressure, so as to
permit it to move in obedience to any other force which may act upon
it. Or again, suppose that we are enabled to expose one side of a
movable body to the action of water converted into steam, at the
moment that we relieve the other side from the like pressure by
reconverting the steam which acts upon it into water, the movable body
will be impelled by the unresisted pressure of the steam on one side.
When it has moved a certain distance in obedience to this force, let
us suppose that the effects are reversed. Let the steam which pressed
it forwards be now reconverted into water, so as to have its action
suspended; and at the same moment, let steam raised from water by heat
be caused to act on the other side of the movable body; the
consequence will obviously be, that it will now change the direction
of its motion, and return in obedience to the pressure excited on the
opposite side. Such is, in fact, the operation of an ordinary
low-pressure steam-engine. The piston or plug which plays in the
cylinder is the movable to which we have referred. The vapour of water
is introduced upon one side of that piston at the moment that a
similar vapour is converted into water on the other side, and the
piston moves by the unresisted action of the steam. When it has
arrived at the extremity of the cylinder, the steam which just urged
it forwards is reconverted into water, and the piston is relieved from
its action. At the same moment, a fresh supply of steam is introduced
upon the other side of the piston, and its pressure causes the piston
to be moved in a direction contrary to its former motion. Thus the
piston is moved in the cylinder alternately in the one direction and
in the other, with a force equivalent to the pressure of the steam
which acts upon it. A strong metal rod proceeds from this piston, and
communicates with proper machinery, by which the alternate motion of
the piston backwards and forwards, or upwards and downwards, in the
cylinder, may be communicated to whatever body is intended to be
moved.
The power of such a machine will obviously depend partly on the
magnitude of the piston or the movable surface which is exposed to the
action of the steam, and partly on the pressure of the steam itself.
The object of converting the steam into water by cold, upon that side
of the piston towards which the motion takes place, is to relieve the
piston from all resistance to the moving power. This renders it
unnecessary to use steam of a very high pressure, inasmuch as it will
have no resistance to overcome, except the friction of the piston with
the cylinder, and the ordinary resistance of the load which it may
have to move. Engines constructed upon this principle, not requiring,
therefore, steam of a great pressure, have been generally called
"low-pressure engines." The re-conversion of the steam into water
requires a constant and abundant supply of cold water, and a fit
apparatus for carrying away the water which becomes heated, by cooling
the steam, and for supplying its place by a fresh quantity of cold
water. It is obvious that such an apparatus is incompatible with great
simplicity and lightness, nor can it be applied to cases where the
engine is worked under circumstances in which a fresh supply of water
cannot be had.
The re-conversion of steam into water, or, as it is technically
called, the _condensation_ of steam, is however by no means necessary
to the effective operation of a steam-engine. From what has been above
said, it will be understood that this effect relieves the piston of a
part of the resistance which is opposed to its motion. If that part of
the resistance were not removed, the pressure of steam acting upon the
other side would be affected in no other way than by having a greater
load or resistance to overcome; and if that pressure were
proportionately increased, the effective power of the machine would
remain the same. It follows, therefore, that if the steam upon that
side of the piston towards which the motion is made were not
condensed, the steam urging the piston forwards on the other side
would require to have a degree of intensity greater than the steam in
a low-pressure engine, by the amount of the pressure of the
uncondensed steam on the other side of the piston. An engine working
on this principle has therefore been called a _high-pressure engine_.
Such an engine is relieved from the incumbrance of all the condensing
apparatus and of the large supply of cold water necessary for the
reduction of steam to the liquid form; for instead of being so
reduced, the steam is in this case simply allowed to escape into the
atmosphere. The operation, therefore, of high-pressure engines will be
readily understood. The boiler producing steam of a very powerful
pressure, is placed in communication with a cylinder furnished in the
usual manner with a piston; the steam is allowed to act upon one side
of the piston so as to impel it from the one end of the cylinder to
the other. When it has arrived there, the communication with the
boiler is reversed, and the steam is introduced on the other side of
the piston, while the steam which has just urged the piston forwards
is permitted to escape into the atmosphere. It is evident that the
only resistance to the motion of the piston here is the pressure of
that portion of steam which does not escape into the air; which
pressure will be equal to that of the air itself, inasmuch as the
steam will continue to escape from the cylinder as long as its elastic
force exceeds that of the atmosphere. In this manner the alternate
motion of the piston in the cylinder will be continued; the efficient
force which urges it being estimated by the excess of the actual
pressure of the steam from the boiler above the atmospheric pressure.
The superior simplicity and lightness of the high-pressure engine must
now be apparent, and these qualities recommend it strongly for all
purposes in which the engine itself must be moved from place to place.
The steam-engine therefore consists of two distinct parts,--the
boiler, which is at once the generator and magazine of steam, and the
cylinder with its piston, which is the instrument by which this power
is brought into operation and rendered effective. The amount of the
load or resistance which such a machine is capable of moving, depends
upon the intensity or pressure of the steam produced by the boiler,
and on the magnitude of the surface of the piston in the cylinder,
upon which that steam acts. The rate or velocity of the motion
depends, not on the power or pressure of the steam, but on the rate at
which the boiler is capable of generating it. Every stroke of the
piston consumes a cylinder full of steam; and of course the rate of
the motion depends upon the number of cylinders of steam which the
boiler is capable of generating in a given time. These are two points
which it is essential should be distinctly understood, in order to
comprehend the relative merits of the boilers used in travelling
steam-engines.
The motion which is primarily produced in a steam engine is a
reciprocating or alternate motion of the piston from end to end of the
cylinder; but the motion which is necessary to be produced for the
purposes to which the engine is applied, is rarely or never of this
nature. This primary motion, therefore, is almost always modified by
some machinery interposed between the piston and the object to be
moved. The motion most generally required is one of rotation, and this
is accomplished by connecting the extremity of the piston-rod with a
contrivance constructed on the revolving axle, called a _crank_. This
contrivance does not differ in principle from the common winch, or
from the key which winds a clock. The motion of the piston-rod
backwards and forwards turns such a winch. At each termination of the
stroke, the piston, from the peculiar position of the crank, loses all
power over it. To remedy this two cylinders and pistons are generally
used, which act upon two cranks placed on the axle at right angles to
each other; so that at the moment when one of the pistons is at the
extremity of its stroke, and loses its power upon one crank, the other
piston is at the middle of its stroke, and in full operation on the
other crank. By these means an unremitting force is kept in action.
_Edinburgh Review._
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