IMPROVEMENTS IN GAS MOTOR ENGINES
Year: 1879

page_text: PROVISIONAL SPECIFICATION left by Joseph John Butcher at the Office of
the Commissioners of Patents on the 28th June 1879.
JOSEPH JOHN BUTCHER, of Gateshead, in the County of Durham, Engineer.
"IMPROVEMENTS IN GAS MOTOR ENGINES."
My Invention consists of improvements in gas motor engines, that is to say
engines in which the impulse given to the piston or pistons is chiefly due to the
expansive force developed in the gases that result from a process of combustion;
and their chief objects are to render possible the construction of powerful reversing
engines, such as those used for locomotive purposes, as well as the simpler types
of stationary engines; and also to render such engines more compact in form and
more economical in effect.
My improvements mainly relate to the method of carrying on the combustion of
an inflammable mixture of air and gas in such a manner that while the continuity
of combustion is maintained at a pressure greater than that of the atmosphere, and
which is the maximum attained in the working of the whole arrangement, the
incandescent constituents or the products of combustion may be allowed to enter
the working cylinder at various parts of the stroke of the piston, so as to effect the
starting and continue the motion of the same as required. Thus a type of valve
may be used for effecting the distribution of the gas within the cylinder similar in
most respects to an ordinary slide valve, a constant pressure of gas being maintained
outside of it, which is approximately the greatest pressure used in the operation of
the entire arrangement; such gas, whether in process of combustion or already
entirely consumed, rushing freely into the cylinder and doing work upon the piston
immediately upon the uncovering of one of the cylinder ports. In these respects
my engines are similar to steam engines, and differ essentially from those gas
engines in which an explosive mixture is ignited in an enclosed space behind the
piston, and an increased pressure thus obtained; combustion in my engine resulting
in an increase of volumo only.
I may produce these results in either of two separate ways.
According to the first of these I connect with some of the moving parts of my
engine a compressing pump or pumps, which deliver the combustible mixture of
air and gas through suitable valves into a separate reservoir or reservoirs at the
pressure which is intended to be the maximum used in the entire arrangement and
the initial pressure in the working cylinders. This reserve of stored and compressed
gas and air may be subsequently prevented from escaping by stop valves or other-
wise during temporary stoppages of the engine, and so form a source of power, by
means of which by suitable arrangements of valves, such for instance as slide valves
and link motion, the engine may be restarted when desired.

page_text: 1 connect the reservoir and pumps with the valve casing of the power cylinder
by means of a pipe, which discharges its mixture of combustible gas in the vicinity
of or immediately over the cylinder port in such a manner that when the valve
is withdrawn and uncovers this port the gas has free access to the cylinders.
The orifice through which the gas enters the valve casing is provided with a
suitable flame check of wire gauze in layers, or other suitable appliance for a
similar purpose; and it is upon the surface of this flame check or in its immediate
vicinity, within the valve casing and at the back of the induction valve, that the
continuity of ignition of the gas is maintained. During the period when the
cylinder port is open and the inrush of gas is taking place the base of the flame
only is contiguous to the flame check in the valve casing, but during the time when
the valve has closed the port the whole flame is in such contiguity, this flame being
then reduced to much smaller dimensions. To maintain the slight flow of gas
through the flame check during the period in which the valve is closed, which is
necessary to secure the permanence of the flame, the valve is so constructed that as
it closes the port opening into the cylinder a slight escape valve or port is opened
leading to the exhaust pipe or elsewhere. This escape may be fitted with an
adjustable cock or valve, so as to enable the flame retained within the valve casing
to be turned down at will and otherwise adjusted as desired, so as to keep it as low
as possible during the working of the engine consistently with steadiness of com-
bustion. As when the engine is standing for a few moments or reversing, the
inrush of gas through the ports to the cylinders may be checked; while the vent
hole is closed a means is provided by which it can be kept permanently slightly.
open, or a second vent may be provided in each valve casing for this purpose. I may
connect the valve controlling this permanent vent either with a hand lever beside
or upon the starting lever of the engine, so as to enable the main instantly to open
it when requisite, or with an antomatic governor, which will always open it when
the engine is at rest.
To ignite the gas within the valve casing a suitable aperture may be provided
for the admission of a light by hand from without the cock or adjustable valve
between the gas reservoir and this casing, having been first partially closed to
prevent the too rapid escape of the gas during the time that the casing is open.
The aperture above mentioned having been closed by a suitable pressure resisting
cover or valve, and the cock between the reservoir and the slide valve casing
turned full on, the engine may be started by bringing the main valve to its open
position either by means of a suitable reversing gear or by turning the engine round
by hand. When the valve is open the pressure within the cylinders being less
than that within the valve casing and reservoir an immediate inrush of gas towards
the piston takes place, and the flame formerly contained within the valve casing is
prolonged into the cylinder. The result of combustion is not therefore in any way
an increase in the pressure of the gas, but is simply an enlargement in volume due
to the heat resulting, the pressure being during the process rather decreased than
otherwise.
To prevent the inrush of flaine injuriously affecting the machined valve facing
a loose slide forming an extension of the lip or lap of the slide valve, if such is
employed, may be attached to it by means of spring bolts or other suitable
appliance or pressed against it when the port is covered in such a manner that
when the valve is moving towards the open position it may draw this loose sliding
cover over the planed face until it reaches the edge of the port opening in the
cylinder casing, at which point by means of a suitable stop its further progress may
be arrested, the valve being left free to move forward by the spring attachment
before mentioned. Thus the machined portion of the valve seating may be always
kept protected from contact with the flame.
To prevent certain parts, such as the cylinder covers and certain portions of the
valve casing, from becoming overheated I may provide water channels through
them, the water being kept in circulation by means of the pumps supplying the
piston water lining, to be subsequently described.

page_text: To prevent the internal skin of the cylinder from becoming overheated, and at
the same time to avoid the necessity of employing a cumbrous water jacket, I may
introduce water through a channel bored out in the piston rod to a water space in
the piston, terminating in an annular channel formed round the periphery of the
Latter between the packing rings. These rings, if of metal, may be so formed that
when water is forced round the edge of the piston it may be left in the track of the
latter in a thin film over the skin of the cylinder, at the same time lubricating and
cooling it, and the amount of water thus introduced may be so regulated by means
of a small force pump, or otherwise, that it is completely evaporated by the heated
gas within the cylinder before the end of the stroke of the piston and thus utilized
as an expansive force, assisting the gas in driving forward the piston. To induce a
tendency to form a water tight joint on the part of the ring or rings on that side
of the annular water channel towards which the piston is advancing each ring may
have its edge which comes into contact with the side of the grooved recess within
which it is sunk in the casting of the piston so bevelled away, the corresponding
side of the grooved recess being similarly bevelled, that when by the motion of the
piston the annular water space is advancing towards the side on which the ring is
situated the bevelled edge may have a alight tendency to wedge the ring against
the cylinder skin and so form a perfectly tight joint and resist on this side of the
piston the escape of the water. If on the other band the outer corresponding edge
of the ring on that side next the cylinder skin and water space respectively be
similarly bevelled away so as to admit a wedge of water between the bevelled edge
of the ring and the cylinder skin the water will constantly have the tendency to
force the ring and the skin apart, and when the piston is moving in such a direction
as to leave the ring only in contact with the external edge of the recessed groove
in the piston, which is formed as usual perpendicularly to the cylinder axis, a film
of water or steam of any desired thickness may be left by the advancing piston
behind it, covering the skin of the cylinder. The operation of forcing the water
into the channel within the piston rod may be effected by allowing a portion of the
latter to pass through and reciprocate within a closed reservoir of water, an opening
being formed connecting the internal channel in the rod with the water space
eurrounding it. If now water is forced into the reservoir by means of a pump it
will find its way through the tubular piston rod and hollow piston to the lining of
the cylinder or cylinders, as previously described. Where the compressing pumps
are situated immediately behind the motor cylinders, as in most instances will be
convenient, and the piston rods connect both pistons through stuffing boxes in the
motor cylinder and pump barrel, covers respectively one of the most compact
positions, for the reservoir will be between these two stuffing boxes. To effect this
purpose the glands used may each have cast upon them a small hollow cylinder or
pipe, so made that one of these pipes will telescope into and form a joint with the
other in the manner usual with water pipes; the ends of these short pipes forming
the glands entering the stuffing boxes may have the usual flanges formed upon
thein and be drawn up tight by means of studs screwed, as is usual, into the cylinder
covers. If the joint between the two portions of the reservoir is made tight by
means of an india rubber ring this will freely admit of the parts moving over one
another, as required when tightening up the stuffing boxes.
In the above description, one end of the cylinder only has been thoroughly
described, but it will be obviously convenient in most cases to make the engines
double acting, while when they are wanted for locomotive, tramway, or ther similar
purposes, two cylinders may be employed, acting upon cranks at right angles to one
another, and fitted with link motion or other reversing gear.
According to my second plan of producing the before named results, I construct
the engines with a gas furnace composed of stationary fire balls or other similar
arrangement us now used for household gas fires, and so piled together as to allow
free passage for the gases among them through the interstices of the heap, and
adinit of complete gaseous combustion, while they at the same time form a solid-
pucleus of heat whereby the temperature of ignition may be sustained during

page_text: temporary stoppages in the current of combustible gases, the said gas furnace being
entirely separated from all working parts, and both supplied with unconsumed and
supplying consumed gas at the maximum pressure attained in the operation of the
entire arrangement, so that under normal conditions, the immediate result of com-
bustion is an increase in gaseous volume, apart from any increase in pressure.
Secondly. I reduce the temperature of the products of combustion in the gas
furnaces worked under pressure as above referred to, and having a solid nucleus of
heat, the said reduction of temperature being effected by means of a water jacket or
common boiler, the steam generated being employed in driving the engines either
mixed with the products of combustion, or separately; or the reduction of tempe-
rature may be effected by jacketing and dilution with cold air.
Thirdly. I construct gas motor engines with a single combustion chamber wherein
a solid sustained red hot nucleus of heat for ignition is employed for a double acting
cylinder, both ends of which draw their gas from the said chamber, or for two or
more cylinders under similar circumstances.
Fourthly. I use within a separate combustion chamber having a permanent solid
nucleus of heat, and in its immediate vicinity a sufficiently large reserve of high
pressure products expanded by heat to ensure the maintenance of a moderately
steady pressure somewhat similar to that in a small boiler, the radiant heat given
off by the permanent heated solids in the furnace, assisting during temporary
stoppages to maintain such pressure which can therefore be utilized in restarting
the engine after an interval of rest.
Fifthly. I provide two governors for the regulation respectively of the supply
and discharge of gases to and from a high pressure combustion chamber, having a
solid nucleus of heat as above referred to, the governor which controls the supply
being acted upon by the pressure in the receiver, and that which controls the
discharge into the engines being acted upon as is usual with the engine governors
by the speed of the engine.
Sixthly. I combine with a separate combustion chamber, having a solid nucleus
of heat, compressing pumps so arranged as only to discharge the compressed gas at
the highest pressure in use at the time for the engines, the pressure of their dis-
charge being at all times the maximum available.
Seventhly. I fit a wire gauze screen or other suitable device at the entrance to
the separate combustion chamber in cases where the gas and air are mixed before
their entry into the pump barrels, the said device being to prevent the flame
travelling backward along the delivery pipe.
The improvements above mentioned and other minor improvements will be
understood by the following description of a gas motor engine constructed in
accordance with my Invention.
I construct the engine proper (that is, the motor cylinder or cylinders, and its or
their connection with the revolving or reciprocating gear required to be actuated)
in any usual or desirable manner as far as convenient in accordance with the best
methods in practice in steam engines. Connected with the moving parts of this
engine and preferably attached in most cases to an extension of the piston rod or
rods, the force pump piston or pistons work, their function being to compress the
gaseous elements required for combustion, and discharge them through any con-
venient pipes or channels into the chamber within which this combustion is required
to be effected, the ultimate pressure in the pump barrels being therefore equal to or
somewhat in excess of the pressure within the combustion chamber.
To prevent any waste of gas in the process of its being drawn into the pumps I
sometimes provide a check or clack valve, which is made common to both the air
and gas supplies, and is so arranged that no gas can escape from the reservoirs
except during the period of the suction of the pumps, so that therefore the entire
volume of the gas released shall be drawn inwards towards the pump barrels with
the influx of air. The amount of supply may either be regulated by the lift of the
valve or by an independent cock.
As it may in some cases be desirable to entirely separate the air and gas supplies

page_text: until the moment of their entry into the combustion chamber, I may for this
purpose either use separate pumps or separate ends of the same pump (should the
latter be double acting) for delivering the air and gas. In cases where the gas and
air are mixed, as first described, prior to their entry into the pump barrels a
provision (such as a wire gauze screen) may be made at the entry to the combustion
chamber to prevent the flame travelling backwards along the delivery pipe to the
pump barrels.
Within the combustion chamber it is necessary that a reserve or steady nucleus
of heat should be kept up to effect with certainty the ignition and complete
combustion of the gases, and for this purpose I employ stationary fire balls or other
similar arrangement, as now used for household gas fires, and so piled together as
to allow free passage for the gases among them.
As the temperature resulting from this combustion would be too high to permit
of the gaseous products being brought into direct contact with the working parts
of the engine without injuring them, I preferably surround the combustion chamber
with a water jacket, thus at the same time cooling the gases, and heating and
evaporating the water. The gases, if requiring still further cooling, may be passed
through tubes also surrounded by water on the plan common in multitubular
boilers; or the whole or part of them may be brought into direct contact with the
water by being discharged into it at the bottom of the reservoir or boiler, so that
they may rise to the surface in separate bubbles, and thus the more effectually part
with their excessive heat. If this latter plan is adopted and the proportional part
of the gaseous products of combustion coming into direct contact with the water is
made either adjustable or self adjusting, the whole of the gaseous results, both of
combustion and evaporation, being subsequently allowed to mix in one common
stream, it will be found that the temperature of these common products is
completely under control, and that they may be even reduced to nearly that of the
boiling point of water at the pressure in use at the time in the combustion
chamber.
A similar adjustment of temperature may also be effected where the gases are
not brought into direct contact with the water, but cooled in tubes surrounded by
it or otherwise beyond the combustion chamber by allowing part of the products
of combustion to flow off direct from the furnace and part to flow through the
cooling apparatus, these parts being made by means of proper valves adjustable in
quantity and subsequently mixed. An adjustment of temperature may also be
effected by altering the length of cooling surface surrounded by water, and this
may be done by increasing the immersion of the tube or tubes.
I thus obtain within the pipes leading from this combustion chamber and steam
generator a much larger volume of heated gas than was originally supplied to them
by the force pumps, and at such a temperature as to be non-injurious to the working
parts of an engine. The pressure, always above that of the atmosphere, may be
controlled in a manner to be subsequently described.
As therefore the supply of gas coming from this combustion chamber and steam
generator is larger than that requiring to be discharged into it, in consequence of
the expansion due to the heat of combustion it is capable of giving out a power
greatly in excess of that required to drive the force pumps compressing the air and
gas prior to their combustion, and as I by this means obtain a mixed gas at a nearly
uniform pressure above the atmosphere, and of which there may be always a
quantity kept in reserve as in a steam space of an ordinary boiler, such gas being
considered mechanically similar in all respects to steam, I may utilize it in driving
any engine like those now generally in use, whether compounded or otherwise, with
simple slide valve or valves of some other description.
Comparing the immediate effects due to combustion as carried on in the gas
engines now commonly in use with those resulting from the method described above,
it will be seen that whereas in the case of the former combustion results in the
immediate increase of pressure within the working parts of the engine, in the latter
it results in a gradual increase of volume within a combustion chamber entirely

page_text: separated from all moving parts, the pressure within which may be kept moderately
constant. Comparing the mechanical effects due to the two methods of working, it
will be seen that in the former case, after the explosion of gas has taken place, the
continuance of the movement of the engine during the interval occurring from the
end of the stroke to the next subsequent explosion depends solely upon the mo-
mentum acquired by the moving parts, whereas as I apply it the thrust of gas
against the piston may be made as nearly continuous as in a steam engine, or worked
as expansively as desired, while it will be possible by employing a pair of cylinders
and link motion or other device for a similar purpose so to turn on the pressure
by opening the valves when the engine is temporarily standing, as to start it either
backward or forward without any other impulse than that of the gas.
Where the intervals during which it is required to run the engine are sufficiently
long to materially reduce the water level in the steam generator I provide a pump
injector or other appliance to supply the deficiency, for which purpose a water tank
is also provided. Such tank may, where desired, be formed round the cylinders or
pump barrels. A safety valve may be also provided as in ordinary boilers in case
of an excessive accumulation of pressure. I sometimes employ separate safety
valves, one for the steam space and another for the combustion chamber and
gas space so as to avoid risk of accident in getting up pressure in those spaces
separately.
As it may probably be found convenient (in both my arrangements) to exercise
a more complete and steady control over the pressure than can be done by a safety
valve, which only fixes a maximum limit beyond which it cannot rise, I make a
provision where required for throttling the supply of combustible gas, whether coal
gas only, or both that and air, which is drawn into the pump barrels and subse-
quently forced into the combustion chamber. This may be done either by hand
under the direct control of the man in charge, or automatically. In the latter case
prefer to choose some limit of greatest pressure desirable (which limit may by
suitable means be made, itself, adjustable) and then to effect the throttling described
by means of the pressure within the steam generator or combustion chamber when
that pressure rises above such limit. This may be done in several ways, one of
which would be to pass a rod or piston through a stuffing box in such a manner that
the pressure of the internal gases tends to move it through the same.
If a spring
be fixed so as to resist this tendency to movement and so arranged that no such
movement will take place until the maximum pressure described has been reached,
and that when the pressure increased beyond this limit a gradually increasing move-
ment takes place, and if the rod or piston be connected with the suction throttle
valve or valves in such a manner that the movement occasioned by this excessive
pressure tends to close this valve, the control desired will be effected. Another
means of effecting the same end is to employ a bent spring tube (such as is used
in the Bourdon steam gauges) closed at both ends and subjected internally to the
pressure of the gas, such pressure tending to extend the tube into a straight line. If
the free ends of this tube be now attached to the throttle valve which it is desired
to affect, the limits before mentioned being fixed by suitable means, the excess
of pressure will again effect the purpose desired.
Thus as a steady pressure is obtained in the steam generator mechanically similar
to ordinary steam pressure in a boiler; I may use any convenient form of governor
to regulate the supply of gas and steam entering the engine, such supply being
made to depend, as is usual, upon the speed attained by the latter. Upon this
arrangement an increasing load upon the engines affecting their speed occasions
them by means of the governor to make an increasing demand upon the gas and
steam supply, thus tending to decrease the pressure in the steam generator and
combustion chamber. This decreasing pressure acting through the appropriate
valve previously described allows the force pumps to deliver a greater charge of gas
to the combustion chamber with each stroke, and thus is the pressure therein
sustained at the limit desired, and the series of compensations in the engine com-
plete. Where a pair of cylinders and link motion are used the engines may be

page_text: slowed, stopped, or reversed by hand, quite independently of the governor, which
may be used or not, as preferred.
As previously stated, it will probably in some cases be found convenient to have
the temperature of the gases, together with the proportional admixture of steam
passing forward with them to the engine also under control. One convenient way
of effecting this would be to make it depend upon the water level within the steam
generator, so that a slight increase in such level beyond a certain chosen height
should increase the proportional amount of heat discharged by the gas into the
water, and so lower the temperature of the output of gas and increase the output of
steam. One means of readily effecting this object is to allow a part of the gases
heated in the combustion chamber to pass off at once towards the engine, the
remainder being discharged in a divided form through the water with which they
come into direct contact, and subsequently rising through it mixes with the steam,
such steam and its accompanying gas afterwards merging into one common stream
with the first part of the gases of combustion which were allowed to pass off direct
from the furnace. As in the passage of the gas through the water, it will encounter
more resistance than it would in passing directly from the combustion chamber
towards the engine, the tendency of the entire product of combustion will be to
take this route. Taking advantage of this tendency a throttle valve or other mode
of interference may be arranged in the passage lending direct from the combustion
chamber towards the engine, and connected with a float upon the surface of the
water (an ordinary hollow ball and lever will effect the purpose), so that the higher
the water rises the greater will be the interference with the direct escape of the
gases, and thus the greater will be the proportion of gas which is forced through
the more tortuous route and comes into direct contact with the water. All then
that will be needed to lower the temperature of the gases and increase the output
of steam will be simply to increase the delivery of water from the small force pump,
for which purpose a suitable hand valve may be provided, thus slightly raising the
water level in the steam generator, the increased feed of water being soon compen-
sated by an increased evaporation consequent upon the increased volume of gas
coming into direct contact with it.
In cases where lightness is of great importance, and it may be found convenient
to partly or completely do away with the use of water around the combustion
chamber for lowering the gaseous temperatures, this purpose may be effected by the
admixture with the products of combustion of a large excess of atmospheric air, to
which the excess of heat will be communicated, proportionally expanding it in the
process. Such excess of air may either be supplied together with the air and gas
necessary for the completion of combustion, or through a separate aperture, so as to
mix with the heated gases after that combustion is complete.
It may be desirable (with either of my arrangements) where more than one
cylinder is required, together with simple valve gear and high degrees of expansion,
to adopt the usual compounded engine. This may be done in the same way as
with steam, by expanding the gas first in a small cylinder, and then passing it on
to a larger one, or one having a more rapid motion of piston. In cases where very
high pressures are used, it may be convenient to compress the gas required for
combustion by more than one stage, the first pump compressing it partially, and the
second increasing the compression. Where any inconvenient break in the continuity
of flow of gas into the combustion chamber is experienced, this may be obviated by
placing an intermediate gas receiver of suitable dimensions between the pumps and
the inlet to the combustion chamber, which latter inlet, if formed into a neck suffi-
ciently contracted to slightly throttle the stream of gases, will set up some degree
of back pressure in the intermediate receiver, which will occasion the flow of gas
into the combustion chamber, to continue for a short time after the pumps have
ceased their discharge.
When it is requisite to condense the small quantity of steam developed, this may
be done by permanently separating the outputs of steam and gas coming from the
steam generator and combustion chamber, and supplying them either to separate

page_text: cylinders or to separate ends of the same cylinder. The latter plan has the advan-
tage of exposing the skin of the cylinder alternately to the actions of steam and
gas, while on either plans the steam may be led through a separate exhaust port to
the condenser.
Several methods may be employed of starting the engine. If a means be em-
ployed of passing a light from the outside into the closed combustion chamber, so
as to effect in the first instance the ignition of the gases; the engine may be
simply turned round by hand, until the pressure in the receiver is sufficient to
overcome the friction of the moving parts and starting is effected. As however it
is not desirable, except in small engines, to adopt this method; I sometimes provide
a separate burner, preferably of the Bunsen type, capable of injecting gas and air to
the combustion chamber at the atmospheric pressure, and in such a manner that
they may be ignited from without. Two separato methods may be employed to
obtain pressure by this means. In the first place the injection of the explosive
mixture may be allowed to continue until the combustion chamber is partially filled
with it, and the whole may be then exploded from without; a stop valve or other
means being provided at the neck through which the gases enter to prevent their
ejection when an internal pressure is set up by their explosion. As this method
although very ready and rapid, somewhat liable to the objection that it is
attended by sudden strains, another plan may be adopted which, though_scarcely
so speedy, is both very effectual, and entirely unattended with danger to the parts.
Upon this latter plan a means is employed (by stop valve or otherwise), of shutting
all connection between the steam space and the combustion chamber, and also (by
three way cock, stop valves, or otherwise) of completely severing the connection
between the combustion chamber and the engines, and opening a connection with
the waste pipe or chimney. If now the gases be turned on from the Bunsen burner
and ignited, so that the flame and heated products pass up through the combustion
chamber to the waste pipe or chimney, and thence to the atmosphere, a gradual
heating of the water and accumulation of steam will ensue, and when a pressure
sufficient has been obtained, the engine may be started on steam alone. The
Bunsen burner and waste pipe having previously been shut off, and the stop valve
making connection with the combustion chamber having been kept closed to prevent
the inrush of steam, until by means of the force pumps, a sufficient pressure has
been developed, tho said valve may now be opened so as to allow the products of
combustion to unite with those of evaporation in driving the engine, and the supply
of gas and the consequent combustion and evaporation will be continued by the
force pumps in the normal way.
Sometimes where great compactness of stowage is an object, or coal gas is not
available, I work my engines with gas obtained from volatile dydro carbon oils.
In cases where the temperature in the pump barrels or working cylinders is
found inconveniently high, so as to render it desirable to jacket them with water,
this water may be completely enclosed and so connected by pipes with the steam
generator, that any steam arising from the jackets may flow into it, and thus be
usefully employed.
Where,
With the object of obtaining a noiseless exhaust, I prefer as far as practicable to
expand the gases in the cylinders down to the pressure of the atmosphere.
however, this cannot be effectually done, I may place impediments in the exhaust
passages which will prevent their too sudden expansion effecting this pmrpose
gradually, and then allowing them to discharge themselves through an unusually
large pipe into the atmosphere.