IMPROVEMENTS IN MOTIVE-POWER ENGINES
Year: 1866

page_text: LETTERS PATENT to Caleb Pratt and Josiah Pratt, of Wilderness Row,
in the County. of Middlesex, for the Invention of "IMPROVEMENTS IN
MOTIVE-POWER ENGINES."
Sealed the 16th November 1866, and dated the 4th August 1866.
PROVISIONAL SPECIFICATION left by the said Caleb Pratt and Josiah
Pratt at the Office of the Commissioners of Patents, with their Petition,
on the 4th August 1866.
We, CALEB PRATT and JOSIAH PRATT, of Wilderness Row, in the County
of Middlesex, do hereby declare the nature of the said Invention for "Ims
PROVEMENTS IN MOTIVE-POWER ENGINES," to be as follows:-
Our Invention consists in a combination of machinery for obtaining motive
power by the alternate expansion and contraction of compressed air combined
with the vapour of water. Our machinery combines and embodies the
following actions :-
The use of the expansion and contraction of air by heat and cold to produce
motive power.
The use of air in a state of great compression that large power may be
derived from a small engine.

page_text: The use of air in a moist condition to facilitate its conduction, trans-
mission, and cession of heat.
The use of an alternate state of plenum and vacuum existing in parts of
engine to induce or compel the outer air to enter the engine to intensify the
pressure within it; to superceed the use of air pumps.
The use of a peculiarly sudden transference of pressure by heat from one
vessel to vacuum by cold in another vessel at the end of each stroke of
the working pistou by means of a balanced rotary slide called an equation
valve, such equilibrium baving the effect of releiving the working piston of
all pressure at its dead point and correspondingly intensifying pressure at &
useful point of its next stroke, while at the same time diminishing resistance
on the exhaust side during its stroke.
An elevation and section of the machinery is shown in the three Drawings
annexed.
At the base of the engine are seen two furnace doors, and over these
furnaces are two cylindrical reservoirs of compressed air; the bottoms only
of these reservoirs being exposed to the fire become hot, their top ends
remaining cold.
Looking at the longitudinal section it will be seen that two hollow pistons
occupy two-thirds of the capacity of their respective reservoirs, which are in all
respects counterparts of each other, and fulfil the same duties and offices only
at opposite moments. These occupying pistons are formed as buckets, and
contain cold water; they are suspended from either end of a beam taking
its motion in either direction for lead from a pair of heart eccentrics put in or
out of action by a link motion as shown in the front elevation. The buckets
have an outer casing, the space between which and the inner casing is filled
with a non-conducting substance to prevent the water it contains from
becoming hot when by the descent of its piston rod it occupies the hot bottom
part of the reservoir; these buckets being slightly smaller than the reservoirs
move in them without friction, but a slight packing seen at their upper ends
(section) prevents any of the water from falling to the heated bottom of their
reservoirs. The top of each reservoir is always open to its respective end of
the working cylinder, and the top and bottom of each reservoir is in constant
communication by means of a wide pipe passing right up through each bucket,
so that when the bucket is at the top the air displaced by it is heated at the
bottom and expanded ready to pass through the slanting pipes seen at back
and front of the elevation and give motion to the working piston. When
either bucket is at the bottom of its reservoir the hot air displaced by it is
at once reduced in bulk by the coldness of the top end of the reservoir

page_text: assisted by thin copper plates (not shown in any of the Drawings) which
have been left cold by their former immersion in the cold water of the
decended bucket; thus is the air alternately expanded and contracted, and
employed to move the piston in the working cylinder. The working cylinder
is shown as an inverted oscillating cylinder, this being the best form for giving
compactness to the engine; its piston gives motion to the crank in the centre
of the Drawing. The cylinder is not bored out, as it does not touch the
piston; it is cast in two parts; at the junction is inserted the packing, and
the piston is as thick as its length of stroke plus the thickness of the packing
rings in the cylinder, and being tight only at the packing is driven as a bolt
to and fro by the pressure from the reservoirs. One of the best forms of this
cylinder is shown in the part sectional Drawing. The equation valves are
situated on the tops of the reservoirs and connected by a pipe as seen in the
front elevation. With respect to their action.-When the working piston is
near the end of its stroke the reservoirs are in an opposite state of pressure
by temperature, although each containing the same bulk of air minus the
contents of the working cylinder. If a communication be now suddenly
opened between these reservoirs by these valves the excess of pressure in the
reservoir that has just ceased its effect on the working piston will be trans-
ferred to the reservoir that is then just about to begin its duty on the opposite
side of the working piston, and so establish an equilibrium of pressure, but
not temperature, between the reservoirs. Now when the bucket ascends this
increased pressure receives a farther increment by the contact of the air
with the heated bottom of the reservoir at the up stroke of the bucket, and is
thus made proportionately more effective on the working piston than it could
have been without the reinforcement derived from the equation transfer.
These equation valves are kept in rotation by a pair of excentrics on the main
shafts, and are connected by a coupling bar, this triangular leverage giving
easier motion than geering. The buckets are supplied with water by a pair
of pumps (not shown) which deliver at the moment of least pressure in the
reservoir; any excess of water will run off into the annular channel seen at
the top of the reservoirs; these channels open to two stand pipes. In
one of these stand pipes is seen a float, which opening a valve allows surplus
water to run out. The other stand pipe is called an inspirator (on the right
of the Drawing); it is connected with the water in the bucket by a pipe
furnished with a cock, the plug of which is seen in the sectional Drawing,
connected to two other cocks near the base of the inspirator, so that when the
upper one is open both the lower ones shall be shut. When these lower air
and water ways are open the upper cock is shut, therefore any water in the

page_text: stand pipe will escape and be replaced with air, but when the lower passages
are shut and the upper one open the pressure in the reservoir forces water
from the bucket into the stand pipe and equalizes the pressure in both, the
bucket being at top; when however the bucket decends the pressure in its
reservoir is reduced by the cooling of its air, the consequence being that the
air is "inspirated" into the reservoir. The cocks can be reversed, stand pipe
emptied of water, air taking its place, to be exchanged for waste hot water at
pleasure by a simple motion of the handle; this answers its purpose as a
condenser, and is not so eisily disordered as an air pump. When the engine
is stopped to prevent any injurious overheating of the reservoirs there are two
horizontal dampers arranged so as to run over the fires by a rack and pinion
motion; they thus become a heat screen and deflect the products of the fire
into other flues, as shown in the long section. The square ends of the
pinion shafts are seen equidistant from the centre of the base of the engine,
and are eisily moved by a wrench or spanner. The dampers themselves are
made of reticulated ironwork filled in with fire-brick.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Caleb Pratt and Josiah Pratt in the Great Seal Patent
Office on the 29th January 1867.
TO ALL TO WHOM THESE PRESENTS SHALL COME, we, JOSIAH
PRATT and CALEB PRATT, of Wilderness Row, in the County of Middlesex,
send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Fourth day of August, in the year of our Lord One
thousand eight hundred and sixty-six, in the thirtieth year of Her reign,
did, for Herself, Her heirs and successors, give and grant unto us, the said
Caleb Pratt and Josiah Pratt, Her special licence that we, the said Caleb
Pratt and Josiah Pratt, our executors, administrators, and assigns, or such
others as we, the said Caleb Pratt and Josiah Pratt, our executors, adminis-
trators, and assigns, should at any time agree with, and no others, from
time to time and at all times thereafter during the term therein expressed,
should and lawfully might make, use, exercise, and vend, within the United
Kingdom of Great Britain and Ireland, the Channel Islands, and Isle of
Man, an Invention for "IMPROVEMENTS IN MOTIVE-POWER ENGINES," upon the
condition (amongst others) that we, the said Caleb Pratt and Josiah Pratt,.
our executors or administrators, by an instrument in writing under our, or

page_text: their, or one of their hands and seals, should particularly describe and ascer-
tain the nature of the said Invention, and in what manner the same was
to be performed, and cause the same to be filed in the Great Seal Patent
Office within six calendar months next and immediately after the date of
the said Letters Patent.
NOW KNOW YE, that we, the said Caleb Pratt and Josiah Pratt, do,
hereby declare the nature of our said Invention, and in what manner the same,
is to be performed, to be particularly described and ascertained in and
by the following statement:-
.
Our improvements have relation to air motive engines, and provide for the
use of the following actions, which will be shown by the Drawings annexed.
First, the use of the expansion and contraction of air by heat and cold to
produce motion.
Second, the use of such air in a state of great condensation that large
power may be derived from a small engine..
Third, the use of such air in a moist condition to facillitate its conduction,
transmission, and cession of heat.
Fourth, the use of an alternate state of plenum and vacuum existing in
parts of the engine to induce or compel the outer air to enter the engine
to intensify the pressure within it, to supersede the necessity of an air pump.
Fifth, the use of a peculiarly sudden transference of air under pressure
by heat from one vessel into air constringed by cold in another vessel
at the end of each stroke of the working piston by means of a balanced
rotary slide valve, called an equation valve; the equilibrium of pressure
independent of temperature gained by this transfer having the effect of
relieving the working piston of all pressure at its dead point, and corres-
pondingly augmenting pressure at a useful point of its next stroke, while
at the same time diminishing resistance on the exhaust side during its stroke.
At the base of the engine are seen two furnace doors, and over the furnaces
are two cylindrical reservoirs of compressed air, the bottoms of these
reservoirs only being exposed to the fire become hot, their top ends remaining:
cold.
Two hollow pistons occupying two-thirds of the capacity of their respected
reservoirs, which are in all respects counterparts of each other, and fulfil the
same duties and offices only at opposite moment. These occupying pistons
are formed as buckets and contain cold water; they are suspended from a
beam taking its motion for lead in either direction from a pair of heart.
excentrics put in or out of action by a link motion, as shown. The buckets
have an outer casing, the space between which and the inner casing is filled

page_text: with a non-conducting substance, such as carbon or gypsum, to prevent the
water it contains from becoming hot when by the decent of its piston rod it
occupies the bottom of the reservoir; these buckets being slightly smaller
than the reservoirs move in them without friction, but a slight packing at
their upper ends prevents any of the water from falling to the heated bottom
of their reservoirs. The top of each reservoir is always open to its respective
end of the working cylinder, and the top and bottom of each reservoir is in
constant communication, by means of a wide pipe passing right up through
each bucket, so that when the bucket is at the top the air displaced by it is
heated at the bottom and expanded ready to pass through the slant pipes seen
at back and front, and give motion to the working piston. When either
bucket is a bottom of its reservoir the hot air displaced by it is at once
reduced in bulk by the coldness of the top of the reservoir, assisted by contact
with the cold water in the bucket; this condensatión is still further assisted
by thin plates of copper (not shown in any of the Drawings) which have been
left cold by their former immersion in the water of the desended bucket;
thus is the air alternately expanded and contracted and employed to move the
piston in the working cylinder. The working cylinder is shown as an inverted
oscillating cylinder, this being the best form for giving compactness to the
engine its piston gives motion to the main shaft in the centre of the
Drawing.
;
Now as the working pressure in this engine is to be very great it was
necessary to devise some kind of piston which, while it was prevented from
all leakage packing could not have the pressure of its packing increased
beyond that given to it by its packing spring by any amount of extra pressure
it might be called on to act under, and become jammed in the cylinder.
To prevent this effect the cylinder is made in two halves and flanged and
bolted together. It is not bored out, as it does not touch the piston. At
the junction is inserted the packing ring, and the piston is as thick as its
length of stroke plus the thickness of the packing rings in the cylinder, and
being tight only at the packing is driven as a bolt to and fro by the pressure
from the reservoirs. Any pressure of air between the rings and piston cannot
cause a jamming action, aud may be perfectly balanced by a due apportion-
ment of the strength of the packing spring to the pressure it has to resist.
The juncture of the cylinder is such that although its two halves are rigidly
fixed together there are spaces left around for the insertion of the gland and
tightening by its screws. One of the best forms of this cylinder, showing the
mode of junction, is shown in the part section at centre, and in the longitudinal
section.

page_text: The equation valves (from which this engine takes its name) are situated
on the covers of the reservoirs and connected by a pipe, as seen in the front
elevation. With respect to their action.-When the working piston is near the
end of its stroke the reservoirs are in an opposite state of pressure by tem-
perature, although each containing the same bulk of air minus the contents of
the working cylinder. If a communication be now suddenly opened between
the reservoirs the excess of pressure in the one that has just ceased, its
effect on the working piston will be transferred to the reservoir which is just
then about to begin its duty on the opposite side of the working piston, and
so establish an equilibrium of pressure, but not of temperature, between the
reservoirs. This transfer always takes place into the reservoir whose bucket
is at the bottom, hence its cold pressure is made as great as the pressure by
heat in the other reservoir. Now when the bucket ascends this increased
pressure receives a further increment by the contact of the air with the heated
bottom of the reservoir at the up stroke of the bucket, and is then pro-
portionately more effective on the working piston than it could have
been without the reinforcement derived from the equation transfer. And,
again the reservoir from which this extra pressure was derived has its pressure
farther diminished by contact with the cold water in its bucket at its down
stroke, which of necessity lessens its reaction on the exhaust side of the
working piston. This equation or transfer of pressure is governed by the
equation valve, which is in duplicate to avoid the loss of the pressure of air in
the pipe of communication between the reservoirs to the extent of its capacity,
which in a large engine would be considerable, and would have the same
disadvantage in loss of useful effect as long passages to and from the ports to
the valve chest would have in a steam engine; but their duplication admits
of their having communicated to them a complete rotary motion, much pre-
ferable to any reciprocating action, by simple leverage; they are kept in
rotation by a pair of excentrics on the main shaft, and are connected by a
coupling bar, thus by triangulation of of leverage having an easy continuous
motion superior to that from gearing. These valves give passage to the air or
equate" twice in each revolution. The body of the valve is not rigidly
fixed to its shaft, and is only in contact with its cylinder at its lower half where
the ports are situated, so that being free to find its own seat, and having no
unbalanced pressure on it, it will not absorb much power to work it, nor will
it be subject to any great or unequal wear, while it admits of easy lubrication.
These excentrics may be placed in any position for starting by a pair of bevil
wheels which when not in use are thrown out of gear.
"
The buckets of the reservoirs are supplied with water by a pair of pumps

page_text: (not shown) which deliver it through pipes in the covers of the reservoirs; it
is then distributed like rain from the perforations of an annular rose and
enters at the moment of least pressure in the reservoirs. The heated and
waste water, always the lightest, is at the surface, and when in excess of the
contents of the bucket on any up stroke it will run off into the annular
channel provided round the reservoirs seen in the Drawings as an enlargement
at their upper ends. These channels open into two stand pipes; in one of
these stand pipes is a float regulated to act at a given elevation; it is attached
to a slide valve at the bottom, and when it rises by flotation it allows the
surplus water to run out. The other stand pipe has a more curious office,
and is called the "inspirator; " it stands on the right hand of the Drawing,
and it communicates with the water in the bucket by a pipe, and is furnished
with a cock moved by a handle without. The plug of this cock is prolonged
to the plugs of two other cocks near the base of the inspirator, so that when
the upper one is open both the lower ones shall be shut. When these lower
air and water ways are open the upper cock is shut, therefore any water in the
stand pipe will fall through the lowest opening and escape, while the atmos-
phere enters at the side passage. Thus the stand pipe is always full of air;
but when both the lower passages are shut, and the upper one in connection
with the reservoir is open, the pressure in the reservoir forces water from the
bucket into the stand pipe, and equalizes the pressure in both, the bucket
being at the top. When, however, the bucket decends the pressure in the
reservoir is reduced by the condensation by cooling of the air in the reservoir,
the consequence being that air is inspirated into the reservoir. The handle
governing the cocks can then be reversed, the water in the stand pipe emptied,
fresh air taking its place, to be again exchanged for waste hot water at
pleasure by a simple motion of the handle; and this apparatus is far more
simple and efficient than an air pump.
When the engine is stopped, to prevent any injurious overheating of the
reservoirs, there are two horizontal dampers arranged so as to run over the
fires by rack and pinion motion; they thus become a heat screen and deflect
the products of the fire into other flues. The square ends of the pinion shafts
are seen equidistant from the centre of the drawing of the base of the engine,
and are easily moved by a portable winch or a spanner. The dampers them-
selves are made of reticulated ironwork filled in with fire-brick.
In order to enable the when working at high speeds to receive heat
readily, we place in each reservoir a superheater, which is an inner casing
through which the air is discharged on any upstroke of the bucket, and during
passage the air is thereby forced into contact with the heated sides of the

page_text: reservoir and escapes in a heated film all round its upper edge; this is shown
in the sectional Drawing.
Having now described the principles we wish to embody in our engine, and
the mechanical details to give them effect to the fullest advantage, we claim
the genera combination of machinery as described for the use and purpose
as described, but we do not limit ourselves to the arrangements of such
parts, or in any way confine ourselves to the particular proportions shown in
our Drawings.
In witness whereof the said Caleb Pratt and Josiah Pratt, have hereunto
set their hands and seals, this Twenty-ninth day of January, in the
vear of our Lord 1867.
CALEB PRATT.
(Ls.)
JOSIAH PRATT. (L.S.)
CALEB PRATT.
(L.B.)
JOSIAH PRATT. (L.S.)