Fig. 51. Typical Induction Coil.
The Secondary Coil.—The fine wire wrapping represents the secondary coil, which is raised to a high voltage, and this actuates the sparking mechanism.
In the art it is customary to illustrate the various contrivances by certain conventional forms.Fig. 51shows the manner of designating an induction coil in a diagram, in which the heavy zig-zag line indicates the primary, and the lighter zig-zag lines the secondary coil.
Fig. 52. Contact Maker.
Contact Maker.—A simple little device used in the primary circuit of an induction coil, is known as acontact maker. This, as shown inFig. 52, is merely a case A, through which is a shaft B that carries within the shell a cam C. A spring finger D has its free end normally bearing against the cam, and when the nose on the cam moves out the spring finger, the latter is moved outwardly so it contacts with a plug E in the side wall of the case, although it is insulated therefrom. This contact establishes a current through the plug, spring finger and case.
The diagram,Fig. 53, illustrates the principles of construction and arrangement of a high tension jump spark ignition, in which the electrical source is a battery actuating an induction coil.
High Tension With Battery and Coil.—The battery A has one side connected up by wire B with one terminal of the primary C in the induction coil, and the other side of the battery has awire D leading to the contact maker. A switch E is placed in the line of this wire.
Fig. 53. Typical Circuiting, Jump Spark Ignition.
The other terminal of the primary has a wire F leading to the insulated contact plug G of the contact maker. This completes the generating circuit. The cam H is on a shaft I, which travels one half the speed of the engine shaft.
One side of the secondary coil J has a wire K leading to the spark plug, while the other terminal of the secondary has a wire L which is grounded on the engine M.
When the nose of the cam pushes over thespring finger and closes the cam, the circuit through the finger flows through the primary coil and excites the secondary. When the cam again immediately breaks the circuit a high tension current is momentarily induced in the secondary, so that the current leaps the gap in the spark plug and makes the spark.
Fig. 54. Metallic Core, Induction Coil.
Metallic Core for Induction Coil.—In the previous description of the induction coil it was stated that the spool might be made of wood. These coils are also provided with metal cores, which can be used to make what is called a vibratory coil.
The Condenser.—A necessary addition to the circuiting provided by an induction coil, is acondenser. This is used in the primary circuit to absorb the self-induced current of the primary and thus cause it to oppose the rapid fall of the primary current.
The condenser is constructed of a number of tinfoil sheets, of suitable size, each sheet havinga wing at one end, and these sheets are laid on top of each other, with the wings of the alternate sheets at opposite ends. Very thin sheets of waxed paper are placed between the tin foil sheets so that they are thus insulated from each other.
The wings at the ends are used to make connections for the conducting wires. The device is not designed to conduct electricity, but to act as a sort of absorbent, if it might so be termed. The large surface affords a means where more or less of the current moves from the conductor at one end to the conductor at the other end, and as it is designed to absorb a portion of the current in the line it is merely bridged across from one side of the circuit to the other.
Fig. 55. Condenser.
The diagram,Fig. 55, represents the conventional form of illustrating it in sketching electrical devices.
Operation of a Vibrator Coil.—The illustration,Fig. 56, shows the manner in which a vibrator coil is constructed and operated. The coil comprises a metal core A, the primary windingB being connected at one terminal, by a wire C, with a post D, and the other terminal by a wire E with one side of a battery F. A switch G is in the line of this conductor.
Fig. 56. Vibrator Coil and Connections.
The post D holds the end of a vibrating spring H, which has a hammer H´ on its free end, which is adapted to contact with the end of the metal core A, but is normally held out of contact, so that it rests against the end of an adjusting screw I which passes through a post J.
The post J is connected up with the battery by a wire K, and a wire L also runs from the wire K to the conductor C, through a condenser M.
The secondary coil N, has the outlet wires O, P, which run to the spark plug Q on the engine.
The operation is as follows: When the switch G closes the circuit, the battery thus thrown in the primary coil magnetizes the core A, and the hammer H´ is attracted to the end of the core, thus breaking the circuit at the contact screw I. The result is that the core is immediately demagnetized, and the spring H draws the hammer back to be again attracted by the core which is again magnetized, so that the hammer on the vibrator arm H goes back and forth with great rapidity.
From the foregoing explanations it will be understood how the primary induces a high tension current in the secondary, and in order that the spark may occur at the right time, atimerfor closing and opening the primary circuit must be provided. By this means an induced high tension current is caused to flow at the time the spark is needed in the cycle of the engine operation.
The Distributer.—The distributer is a timing device which controls both the primary and the secondary currents, and it also has reference to the revolving switch on the shaft of a magneto whereby the current is distributed to the various cylinders in regular order.
Fig. 57shows a form of distributer whichwill illustrate the construction. A is the shaft which is driven at one half the engine speed. It is usually run by suitable gearing direct from the shaft of the magneto.
Fig. 57. The Distributer.
Its outer end rests in a bearing plate B, of insulating material, which plate serves as the disk to hold the contact plates, 1, 2, 3, 4, to correspond with the four cylinders to which the current is to be distributed.
Wires 5, 6, 7, and 8, run to the respective spark plugs C from these contact plates. The projecting end of the shaft A carries thereon a contact finger D, which is designed to contact with the respective plates, and an insulating ring E is interposedbetween the shaft and finger so as to prevent short circuiting of the high tension current.
On the side of the finger is a hub F, integral therewith, and a wiper attached to a post bears against the hub so as to form continuous contact. A wire leads from the post to one terminal of the secondary coil.
Fig. 58. Circuiting with Distributer.
Circuiting With Distributer.—The diagramFig. 58shows the complete connections of a systemwhich comprises a magneto, induction coil, condenser, and a distributer. The magneto A has on its armature shaft B two revolving disks C, D, one of which must be insulated from the shaft, and one end of the coil E of the armature is connected with one of these disks, and the other end of the coil is attached to the other disk.
Alongside of these disks is another disk F which has projecting points G to engage with and make temporary contact with a spring finger which actuates the interrupter I, this being a contact breaker which breaks the primary current at the time a spark is required.
One terminal of this interrupter is connected by a wire J with one end of the primary winding K, of the induction coil, and the other end of the primary has a wire L which runs to the disk C.
The other terminal of the interrupter has a wire M leading to a condenser N, and from the other side of the condenser is a wire O leading to the wire J before described. The wiper of the other disk D has a wire connection with the wire M.
The distributer shaft P is so mounted that it may receive its motion from the shaft of the magneto, and for this purpose the latter shaft has a gear Q one half the diameter of the gear R on the distributer shaft.
The distributer S has been described with sufficientclearness in a preceding diagram, to show how the wires T lead therefrom and connect up with the spark plugs U. One terminal of the secondary coil V is connected by a wire W with the wiper X which contacts with the hub of the distributer finger X´, and the other terminal of the primary is grounded at Y, which represents the metal of the engine.
CHAPTER IX
MECHANICAL DEVICES UTILIZED IN POWER
One of the most important things in enginery is the capacity to determine the power developed. Although the method of ascertaining this appears to be somewhat complicated, it is really simple, and will be comprehended the more readily if it is constantly borne in mind that a certain weight must be lifted a definite distance within a particular time.
The Unit of Time.—The unit of time is either the second, or the minute, usually the latter, because it would be exceedingly difficult to make the calculations, or rather to note the periods as short as a second, and a very simple piece of mechanism to ascertain this, is to mount a horizontal shaft A,Fig. 59, in bearings B, B, and affix a crank C at one end.
It will be assumed that the shaft is in anti-friction bearings so that for the present we shall not take into account any loss by way of friction.
A cord, with one end attached to the shaft and the other fixed to a weight D, the latter weighing,say 550 pounds, is adapted to be wound on the shaft as it is turned by the crank.
Knowing the length of the cord and the time required to wind it up, it will be an easy matter to figure out the power exerted to lift the weight, which means, the power developed in doing it.
Fig. 59. Illustrating the Unit of Time.
Suppose the cord is 100 feet long, and it requires one and a half minutes to raise the weight the full limit of the cord. It is thus raising 550 pounds 100 feet in 45 seconds.
One horse power means that we must raise 550 pounds one foot in one second of time, hence we have developed only 1/45th of one horse power.
Instead of using the crank, this shaft may be attached to the engine shaft so it will turn slowly. Then add sufficient weight so that the engine will just lift it, and wind the cord on the shaft.
You can then note the time, for, say, one minute, and when the weight is lifted, make the following calculation: Weight lifted one hundred feet in one minute of time was 825 pounds. Multiply 100 by 825, which equals 82,500. This representsfoot pounds.
Fig. 60. The Proney Brake.
As there are 33,000 foot pounds in a horse power, 82,500 divided by this figure will show that 21/2horse power were developed.
The Proney Brake.—Such a device is difficult to handle, but it is illustrated merely to show the simplicity of the calculation. As a substitute for this mechanism, a device, called theProney brakehas been devised, which can be used without rewinding of a cord. This is accomplished by frictional means to indicate the power, and by the use of weights to determine the lift.
The following is a brief description of its construction: The engine shaft A,Fig. 60, which is giving out its power, and which we want to test,has thereon a pulley B, which turns in the direction of the arrow. Resting on the upper side of the pulley is a block C, which is attached to a horizontal lever D by means of bolts E, these bolts passing through the block C and lever D, and having their lower ends attached to the terminals of a short sprocket chain F.
Block segments G are placed between the chain and pulley B, and when the bolts E are tightened the pulley is held by frictional contact between the block C and the segments G.
The free end of the lever has a limited vertical movement between the stops H, and a swinging receptacle I, on this end of the lever, is designed to receive weights J.
The first thing to do is to get the dimensions of the pulley, its speed, and length of the lever. By measurement, the diameter of the pulley is six inches. To get the circumference multiply this by 3.1416. The distance around, therefore, is a little over 18.84 inches. The speed of the pulley being 225 times per minute, this figure, multiplied by 18.84, gives the perimeter of the pulley 4239 inches.
As we must have the figures in feet, dividing 4239 by 12, we have 353.25 feet.
The length of the lever from the center of the pulley to the suspension point of the receptacle,is 4 feet, and this divided by the radius of the pulley (which is 6 inches), gives the leverage. One half of six inches, is three inches, or 1/4 of one foot, and 4 divided by this number, is 1' 4", or 11/3feet, which is theleverage.
Now, let us suppose the weight J is 1200 pounds. This must be multiplied by the leverage, 11/3feet, which equals 1800, and this must be multiplied by the feet of travel in the pulley, namely, 353.25, which is equal to 635,850. This representsfoot pounds.
Now, following out the rule, as there are 33,000 foot pounds in a horse power, the foregoing figure, 635,850, divided by 33,000, equals 19 horse power within a fraction.
Reversing Mechanism.—A thorough knowledge of the principles underlying the various mechanical devices, and their construction, is a part of the education belonging to motors. One of the important structures, although it is very simple, when understood, requires some study to fully master.
This has reference to reversing mechanism, which is, in substance a controllable valve motion, whereby the direction of the valve is regulated at will.
All motions of this character throw the valve to a neutral point which is intermediate the twoextremes, and the approach to the neutral means a gradual decrease in the travel of the valve until the reciprocating motion ceases entirely at the neutral position.
Fig. 61. Double Eccentric Reversing Gear. Fig. 62. Reversing Gear, Neutral.
Double Eccentric Reversing Gear.—A well known form of gear is shown inFig. 61, in which the engine shaft A has two eccentrics B, C, the upper eccentric B being connected with the upper end of a slotted segment D by means of a stem E, and the other eccentric C is connected with the lower end of the segment by the stem F. The eccentricsB, C, are mounted on the shaft so they project in opposite directions.
The slotted segment carries therewith the pin G of a valve rod H, and the upper end of the segment has an eye I, to which eye is a rod J operated by a lever.
Fig. 63. Reversing Gear, Reversed. Fig. 64. Single Eccentric Reversing Gear.
By this arrangement the link may be raised or lowered, and as the valve rod pin has no vertical movement, either the connecting link E or F may be brought into direct line with the valve rod H.
Fig. 61shows the first position, in which the valve rod H is in direct line with the upperconnecting rod E, actuated by the cam B.
Fig. 62shows the neutral position. Here the pin G serves as a fulcrum for the rocking movement of the segment; whereas inFig. 63the valve rod H is in line with the lower connecting rod F, so that the valve is pushed to and fro by the eccentric C.
Fig. 65. Balanced Slide Valve.
It is more desirable, in many cases, to use a single eccentric on the engine shaft, which can be done by pivoting the segment L,Fig. 64, to a stationary support M, and connecting one end of the segment by a link N with the single eccentric O.
In this construction the valve rod P is shifted vertically by a rod Q, operated from the reversing lever, thus providing a changeable motion through one eccentric.
Balanced Slide Valves.—In the chapter pertainingto the steam engine, a simple form of slide valve was shown, and it was stated therein that the pressure of the steam bearing on the valve would quickly grind it down. To prevent this various types of balanced valves have been made, a sample of which is shown inFig. 64.
The valve chest A has in its bottom two ports C, D, leading to the opposite ends of the cylinder, and within is the sliding valve E, which moves beneath an adjustable plate F connected with the top or cover G of the valve chest.
Fig. 66. Valve Chest. Double Port Exhaust.
This is also modified, as shown inFig. 66, in which case the slide valve H bears against the cover I at two points, so that as there is steam on the upper surface to a slightly greater area than on the lower side, there is sufficient downward pressure to hold it firmly on its seat, and at the same time not cause any undue grinding. This valve also has double exhaust ports J, J.
Balanced Throttle Valve.—Fig. 67will give a fair idea of the construction of throttle valves, the illustration showing its connection with a simple type of governor.
Fig. 67. Balanced Throttle-Valve.
Engine Governors.—Probably the oldest and best known governor for regulating the inlet of steam to an engine, is what is known as the Watt design. This is shown inFig. 68.
The pedestal A which supports the mechanism, has an upwardly-projecting stem B, to the upper end of which is a collar C, to which the oppositely-projectingpendent arms D are hinged. These arms carry balls E at their free ends.
Fig. 68. Watt's Governor.
The lower part of the stem has thereon a sliding collar F, and links G, with their lower ends hinged to the collar, have their upper ends attached to the swinging arms D. The collar has an annular groove at its lower end, to receive therein the forked end of one limb of a bell-crank lever H, the other limb of this lever being connected up with the engine throttle, by means of a link L.
Centrifugal motion serves to throw out the balls, as indicated by the dotted lines J, and this action raises the bell-crank lever, and opens the throttle valve.
Numerous types of governors have been constructed, some of which operate by gravity, in connection with centrifugal action. Some are made with the balls adapted to swing downwardly, and thrown back by the action of springs. Others have the balls sliding on horizontally-disposed arms, and thrown back by the action of springs; and gyroscopic governors are also made which are very effective.
Fig. 69. The Original Injector.
Fly wheel governors are not uncommon, which are placed directly on the engine shaft, or placed within the fly wheel itself, the latter being a well known form for engines which move slowly.
Injectors.—The Injector is one of the anomalies in mechanism. It actually forces water into a boiler by the action of the steam itself, against itsown pressure. It is through the agency of condensation that it is enabled to do this.
The illustration,Fig. 69, which represents the original type of the device, comprises a shell A, within which is a pair of conically formed tubes, B, C, in line with each other, the small ends of the tubes being pointed towards each other, and slightly separated. The large end of the conical tube C, which points toward the pipe D, which leads to the water space of the boiler, has therein a check valve E.
The steam inlet pipe F, has a contracted nozzle G, to eject steam into the large end of the conical tube B, and surrounding the nozzle F is a chamber which has a pipe H leading out at one side, through which cold water is drawn into the injector.
Surrounding the conical pipes B, C, is a chamber I, which has a discharge pipe J. The action of the device is very simple. When steam is permitted to flow into the conical tube B, from the nozzle G, it passes out through the drain port J, and this produces a partial vacuum to form in the space surrounding the nozzle G.
As a result water is drawn up through the pipe H, and meeting with the steam condenses the latter, thereby causing a still greater vacuum, and this vacuum finally becomes so great that, withthe inrushing steam, and the rapid movement through the conical tubes, past their separated ends, a full discharge through the drain J is prevented.
Fig. 70. Injector with Movable Combining Tube.
As it now has no other place to go the check valve E is unseated, and the cold water is forced into the boiler through the pipe D, and this action will continue as long as condensation takes place at the nozzle G.
Many improvements have been made on the original form, mostly in the direction of adjusting the steam nozzle, and to provide the proper proportion of flow between the steam and water, as this must be adjusted to a nicety to be most effective.
An example of a movable tube which closes theoutlet to the overflow, is shown inFig. 70. The steam inlet tube A is at one end of the shell, and the outlet tube B to the boiler, at the other end, and intermediate the two is a tube C, with its open flaring end adapted to receive the steam from the tube A. This tube is longitudinally-movable, so that the controlling lever D may move it to and fro.
A chamber E surrounds the nozzle A, and has a water inlet pipe F, while the space G between the ends of the pipes B, C, has an outlet H, a single check valve I being interposed. In operation the tube C may be adjusted the proper distance from the end of the pipe B, and when the current is once established through the injector, the pipe C may be brought into contact with B, and thus entirely cut out the movement of the water to the overflow.
Feed Water Heater.—An apparatus of this kind is designed to take the exhaust steam from the engine and condense it, and from the condenser it is again returned to the boiler. The water thus used over again goes into the boiler at a temperature of over 180 degrees, and thus utilizes the heat that would otherwise be required to raise the temperature of the water from the natural heat, say 70, up to that point.
InFig. 71the illustration shows a typicalheater, which comprises an outer shell A, each end having a double head, the inner head B being designed to receive the ends of a plurality of horizontally disposed pipes, and the outer heads C, separated from the inner head so as to provide chambers, one end having one, and the other head being provided with two horizontal partitions D, so the water may be diverted back and forth through the three sets of pipes within the shell.
Fig. 71. Feed Water Heater.
The three sets of pipes, E, F, and G, are so arranged that they carry the water back and forth from one head to the other, and for this purpose the water for cooling the steam enters the port H at one end, passes through the upper set of pipes E to the other end, then back through the same set of pipes on the other side of a partition, not shown, and back and forth through the two lower sets of pipes F, G.
The steam enters at the port I at the top of the shell, and passes down, as it is condensed, being discharged at the outlet J.
CHAPTER X
VALVES AND VALVE FITTINGS
In the use of steam, compressed gas, or any medium which must have a controllable flow, valves are a necessary element; and the important point is to know what is best adapted for the use which is required in each case.
For this reason one of the best guides is to fully understand the construction of each. The following illustrations and descriptions will give a good idea of the various types in use.
Fig. 72. Check Valve.
Check Valve.—Fig. 72shows a longitudinal section of a check valve, which is designed to preventthe water from returning or backing up from the pressure side. The cylindrical body A is threaded at each end, and has an inclined partition B therein which has a circular aperture.
Fig. 73. Gate Valve.
The upper side of the shell has an opening, adapted to be closed by a cap C, large enough to insert the valve D, which is hinged to the upper side of the partition. Water or gas is forced in through the valve in the direction of the arrow, and the hinged valve is always in position to close the opening in the partition.
In case the valve should leak it may be readily ground by taking the small plug E from the opening, and with a screw driver, turning the valve, and thereby fit it snugly on its seat.
Fig. 74. Globe Valve.
Gate Valve.—The cylindrical shell A has its ends internally threaded, and is provided, midway between its ends, with a partition wall B, having a central aperture. The upper side of the shell has an opening to receive the bonnet C, through which the valve stem D passes. This stem carries at its lower end a gate E which rests against the partition B.
The stem D is threaded to screw into the threaded bore of the gate. A packing gland F surrounds the stem D. It will thus be seen that the turning of the stem D draws the gate up or down, and thus effects an opening, which provides a direct passage for the water through the valve body.
Globe Valve.—A globe valve has the advantage that the valve is forced against its seat by the pressure of the wheel, differing from the gate valve, that depends on the pressure of the fluid to keep it tight.
The valve body A has therein a Z-shaped partition B, the intermediate, horizontally-disposed limb of the partition being directly below the opening through the body, which is designed to receive the bonnet C.
The bonnet has a central vertical bore, the lower end of which is threaded to receive the wheel spindle. The lower end of the spindle carries the circular valve, which is seated in the opening of the Z-shaped partition.
The Corliss Valve.—The valve itself is of the rotary type, as shown inFig. 75, in which the port A goes to the cylinder, and B is the passage for the steam from the boiler. The cylindrical valve body C has within the aperture B a gate D, one edge of which rests against the abutment throughwhich the port A is formed, and this gate has within it the bar E which is connected with the crank outside of the casing.
The Corliss Valve-Operating Mechanism.—As the operation of the valves in the Corliss type of engine is so radically different from the ordinary reciprocation engine, a side view of the valve grouping and its connecting mechanism are shown inFig. 76.
Fig. 75. Corliss Valve.
The cylinder has an inlet valve A at each end, and an outlet valve B at each end for the discharge of the steam. C is a valve rod from the eccentric which operates the valves, and D a wrist plate, having an oscillatory or rocking motion around its center E. The attachments F F, of the steam rods, open the inlet ports A A, and G G, are the attachments of exhaust rods which open and close the exhaust valves B B. H H are catches which can be unhooked from the stems of the valves A by the governor rods J J.
The vertical links K, K are connected at their lower ends with the pistons of dash pots, and have their upper ends attached to the valve spindles, and act to close the valves A A when the catches H are released by the governor rods J by means of the weights of the pistons in the dash pots.
Fig. 76. Corliss Valve-operating Mechanism.
The dash pots L L act in such a manner as to cushion the descent of the links K and thus prevent undue shock. M is a wrist plate pin by which the valve rod C can be released from the wrist plate.
The whole purpose of the mechanism is to provide a means for closing the valves which are atthe steam inlet ports, by a sudden action. The exhaust valves, on the other hand, are not so tripped but are connected directly with the wrist plate which drives all four of the valves.
The wrist plate or spider has a rocking motion, being driven by an eccentric rod from the engine-shaft. The mechanism thus described gives a variable admission as the load varies, but a constant release of the exhaust and a constant compression to act as a cushion.
Fig. 77. Angle Valve.
It gives a high initial pressure in the cylinder, and a sharp cut off, hence it is found to be very efficient.
Angle Valve.—One of the most useful is the angle valve, which is designed to take the place of an angle bend or knee in the line of the piping. The mechanism is the same as in the well knownglobe valve construction, the bonnet A being on a line with one of the right-angled limbs of the body.
The pressure of the fluid should always be on the lower side of the valve C, coming from the direction of the arrow B, for the reason that should the steam pressure be constant on the other side, it would be difficult to repack the gland D without cutting off the steam from the pipe line.
Fig. 78. Rotary Valve. Fig. 79. Two-way Rotary.
Referring back to the illustration of the globe valve, it will be noticed that the same thing, so far as it pertains to the direction of the steam, applies in that construction, and a common mistake is to permit the pressure of the steam to be exerted so that it is constantly acting against the packing of the spindle.
Rotary Valves.—Two forms of rotary valves are shown, one as illustrated inFig. 78, where therotating part, or plug, A has one straight-way opening B, which coincides with two oppositely-projecting ports C, D.
The other form,Fig. 79, has an L-shaped opening E through the rotating plug F, and the casing, in which the plug is mounted has three ports, one, G, being the inlet, and the other two H, I, at right angles for the discharge of the fluid.
Fig. 80. Rotary Type. Fig. 81. Two-way Rotary Type.
Rotable Engine Valves.—So many different forms of the rotable valve have been made, that it is impossible to give more than a type of each. For engine purposes the plugs are usually rotated in unison with the engine shaft, and a single delivery valve of this kind is shown inFig. 80.
This has three ports in the casing, namely the inlet port A, and two outlet ports C, D. The plug has a curved cut out channel E, and this extends around the plug a distance equal to nearly one-halfof the circumference, so that the steam will be diverted into, say, B, for a period equal to one-quarter turn of the plug, and then into port C, for the same length of time.
Fig. 81shows a valve which has a double action. The plug G has two oppositely-disposed curved channels, H, I, and the casing has a single inlet port J, and two oppositely-disposed outlet ports K, L.
Fig. 82. Butterfly Throttle. Fig. 83. Angle Throttle.
When the plug turns the port L serves to convey the live steam to the engine, while the other port K at the same time acts as the exhaust, and this condition is alternately reversed so that L acts as the discharge port.
Throttle Valves.—The throttle valves here illustrated are those used in connection with gasoline engines. The best known is theButterflyvalve, shown inFig. 82, and this is also used as adamper, for regulating the draft in furnaces and stoves.
This type is made in two forms, one in which the two wings of the valve are made to swing up or down in unison, and the other, as illustrated, where the disk A is in one piece, and turns with the spindle B to which it is fixed.
Fig. 84. Slide Throttle. Fig. 85. Two-slide Throttle.
InFig. 83the wing C is curved, so that by swinging it around the circle, the opening of the discharge pipe D is opened or closed.
Another design of throttle is represented inFig. 84. One side of the pipe A has a lateral extension B, which is double, so as to receive therein a sliding plate C, which is easily controllable from the outside.
Fig. 85shows a form of double sliding plate, where the double lateral extensions project out in opposite directions, as at D, D, and within these extensions are sliding plates which are secured together in such a way that as one is pushed inthe other also moves in, and thus acts in unison to close or to open the space between them. It is the most perfect form of throttle valve, as it causes the gases to open directly into the center of the outgoing pipe.
Blow-off Valves.—The illustration shows a type of valve which is used on steamboats and very largely on farm boilers throughout the country. The pipe A from the boiler has cast therewith, or otherwise attached, a collar B, which has a standard C projecting upwardly at one side, to the upper end of which is hinged a horizontal lever D, which has a weight at its other end.