COMPOUND ENGINES.

The Compound Engine dates from the year 1781, when Hornblower, a contemporary of Watt, conceived the idea of utilizing the force in the exhaust steam of the simple engine in a second cylinder.

From his crude design, the constant progress of experiment has developed the marvelous engines now used in ocean steamers, and in both large and small power plants, also on locomotives. Some of the compound engines built in the early part of the century show results, according to the records, not far behind the best attainable in modern times.

The era of the Compound locomotive engine began in Europe in 1876, but in this country half a dozen years would almost cover its history.

However, in this short time, its advantages in putting to profitable use the entire force of the steam supplied, has been so clearly shown, that it has evidently come to stay. Its availability as an efficient, economical, powerful high speed locomotive, demonstrates the value of the Compound as a farm traction engine, and makes it plain that it will be extremely serviceable on this class of engines.

The Compound Traction Engines belonging to the class known as the “Woolf,” or continuous expansion type, are so constructed that the steam passes directly from the high pressure to the low pressure cylinder without the intervention of any receiving chamber or steam chest. This arrangement is considered much better adapted to traction engine work, and to produce superior results under the varying conditions of this class of work than the “cross” compound, or what is generally styled the “receiver” type of compound engines, in which the high pressure cylinder exhausts into a receiver connected with the steam chest of the low pressure cylinder.

In the Woolf Compound as constructed, the cylinders are either cast in one piece, end to end, or cast separately and bolted together in substantially the same way, in order that perfect alignment can be secured by boring both cylinders at the same operation. This makes it not only easy to get them in line at the start, but it prevents any possibility of their getting out of line, which is a very important feature.

The pistons of both cylinders are upon the one rod, thus requiring only one cross-head, connecting rod and crank. There is but one steam chest, in which a valve is placed, with such relation to the valve seat which contain the ports leading to both cylinders, that it performs the double function of first admitting the steam to the high pressure cylinder, cutting off the admission at the proper time to allow expansion to take place there, and after high pressure piston has reached the end of its stroke, passing on the steam to the low pressure cylinder, where it is further expanded and exhausted in the usual manner after its work is done.

The valve is specially designed with cavities so arranged as to co-operate in increasing the area of opening to double the amount obtainable with an ordinary valve having the same travel.

Without some special provision, the full power of the compound engine cannot be exerted in starting, as the steam operates primarily on the high pressure piston only, which has led to the condemnation of the compound as a traction engine. This objection has been thoroughly overcome in the Woolf by means of a “converting valve,” rendering it possible to admit steam directly to the large or low pressure cylinder, thus largely increasing the power obtainable from the engine, even when exerting its maximum power as a compound. This arrangement can be used not only in starting, but also in cases of emergency, such as climbing steep hills, getting out of bad places on the road, or disposing of an especially tough cut in sawing, etc.

Without increasing the boiler pressure beyond that ordinarily used, the compound engine gets fully one-third more force out of the steam used than is at present obtainable with the simple engine as commonly worked.

In other words, the compound will show results compared favorably with a good condensing simple stationary engine doing the same work.

Ball Tandem Compound Engine.

BALL TANDEM COMPOUND ENGINE.

The heavy case iron base is cast in two sections, the rear part being securely bolted to the front section. To the front section is also bolted the main engine frame. This frame contains the bar guides for the cross-head, and pillow block bearing for the double disc crank shaft, and also forms the front head for the low pressure cylinder, which is securely bolted to it.

The high pressure cylinder is attached to the low pressure cylinder by two brackets securely bolted, and is supported by a pedestal, bolted to the rear part of base. By this arrangement, both the high and low pressure pistons are upon the same piston rod, which necessitates of but one cross-head, connecting rod and crank.

The valve of the high pressure cylinder is operated and completely under the control of the automatic shaft governor attached to one of the band wheels, while the valve of the low pressure cylinder receives its motion from a single eccentric on crank shaft at opposite side of engine.

When engine is running, the steam enters the high pressure cylinder first, and after performing its work there, exhausts through the receiver pipe into the low pressure cylinder, and there exerts its minimum force by expansion, and passes out to the condenser, if used, or exhausts into the open air.

EXAMINATION OF ENGINEERS
APPLYING FOR A LICENSE.

QUESTIONS WITH ANSWERS.

Q. How long have you run an engine?

Q. Have you done your own firing?

Q. What kinds of engines have you run?

Q. What would be your first duty if called upon to take charge of an engine?

A. To ascertain the exact condition of the boiler and all its attachments, such as safety valve, steam gauge, water gauge and cocks, pump, injector, blow-off valve, etc.; also the engine.

Q. How often would you blow off your boiler?

A. Once a week or month, according to the condition of feed water used.

Q. How many feet of heating surface is allowed per horse-power by builders of boilers?

A. From 12 to 15 square feet for flue and tubular boilers.

Q. How much steam pressure will be allowed on a boiler 40 inch diameter, ⅜ thick, 60,000 pounds T. S., ⅛ T. S. factor of safety?

A. One-sixth of tensile strength of plate multiplied by thickness of plate, divided by one-half of the diameter of boiler gives safe working pressure.

Q. How do you estimate the strength of a boiler?

A. By its diameter and thickness of material, single or double riveted.

Q. Which is the stronger, single or double riveted?

A. Double riveted is from 14 to 18 per cent. stronger than single.

Q. What is the use of a mud drum on a boiler?

A. To collect all the sediment from the water used in the boiler.

Q. What causes sediment to accumulate in boilers?

A. The use of impure or muddy water.

Q. How often should it be blown out?

A. Three or four times a day.

A. How much grate surface do boiler makers allow per horse-power?

A. About two-thirds of a square foot.

Q. What is the steam dome of a boiler used for?

A. For dry steam to collect in.

Q. Of what use is a safety valve on a boiler?

A. To prevent overpressure.

Q. What is your duty with reference to it?

A. Open it once or twice a day to see that it is in good order.

Q. Of what use is a check valve?

A. To prevent the water in boiler from returning into the pump or injector.

Q. What effect has cold water on hot boiler plates?

A. It will fracture them.

Q. How should the gauge cocks be located on a boiler?

A. So that the lowest gauge cock is about 1½ inches above the top row of flues.

Q. Where should the blow-off valve be located?

A. At the bottom of the fire box in locomotive style of boiler, or in the mud drum when used.

Q. How would you have check valve arranged?

A. With a stop cock between the boiler and check valve.

Q. Does a man-hole in the top shell of boiler weaken it?

A. Yes, to a certain extent.

Q. How many valves in a common plunger pump?

A. Two, a receiving and a discharge valve.

Q. How are they situated?

A. One at suction, the other at discharge end.

Q. How do you find the proper size of safety valve for boiler?

A. Two square feet of grate surface is allowed for one inch area of common lever valve, or three square feet of surface to one inch area of spring valve.

Q. Why do pumps fail to work at times?

A. Leak in the suction, leak around the plunger, leaky check valve, or valve out of order.

Q. Why do injectors fail to work at times?

A. Leak in suction, grit or dirt under seat of valve, or valve not seated properly.

Q. How often should a boiler be examined and tested?

A. Twice a year at least.

Q. How would you test a boiler?

A. By tapping it with a light hammer, and hydrostatic test, using warm water.

Q. Where does the feed water enter the boiler?

A. Below the water level, where the feed water will not strike the heated plates.

Q. What causes priming of boilers?

A. Too high water, not steam space enough, dirty feed water, misconstruction of boiler, or engine being too large for its capacity.

Q. How can you keep boilers clean or remove scale from them?

A. By regularly cleaning them thoroughly, and by the use of compounds.

Q. If you found a thin plate in your boiler what would you do?

A. Patch it on the inside, first cutting out the damaged part.

Q. Why cut out the damaged part of sheet, when putting on a patch?

A. To allow the water to rest against the patch to protect it from the intense heat.

Q. What would be the result if the damaged part of sheet was not cut out?

A. The water not coming in contact with the patch, it would soon bulge from the heat and crack.

Q. Why patch it on the inside?

A. Because the action that has weakened the plate before will act upon the patch, when this is worn it can be replaced.

Q. If you found you had to put on several patches what would you do?

A. Reduce the steam pressure.

Q. If you found a blister what would you do?

A. Cut it out and put a patch on the fire side.

Q. If you found a plate buckled or sagged what would you do?

A. Put a stay bolt through the center of the sag.

Q. What would you do with a cracked plate?

A. Cut out the damaged part and put a patch over it.

Q. How would you change the water in a boiler when steam is up?

A. By supplying more feed water and opening the surface blow-off at short intervals.

Q. When blowing off a boiler, would you leave the blow-off cock to attend to other work?

A. Never.

Q. What would you do to relieve the pressure on the boiler if the safety valve was stuck and steam constantly rising?

A. Cover the fire with coal or ashes, close draught door and open damper in smoke box; work off the steam with the engine and when boiler has cooled down put the safety valve in working order.

Q. What may be the result if you allow the water in the boiler to get low?

A. Burning of the crown sheet and flues and perhaps cause an explosion.

Q. Would you turn feed water into a boiler in which the water was very low?

A. Never, without first pulling the fire or covering it with dry ashes and allowing the steam to go down.

Q. If you allow water in the boiler to get too high what would be the result?

A. It would cause priming or foaming.

Q. Is priming or foaming dangerous to an engine?

A. Yes. It may cause breaking of cylinder head and wrecking of the engine.

Q. What are other causes for foaming or priming of a boiler?

A. Dirty and impure water.

A. W. STEVENS TRACTION ENGINE.

The position of the side crank engine upon the boiler allows of having the Rear Gear traction attachment.

The Engine frame, guides for cross-head, cylinder, steam chest, saddles, brackets and both pillow block bearings for crank shaft are cast in one piece and bolted to the boiler.

The frame is cast oval, and cross-head guides are of the locomotive style.

The Engine is furnished with a [Friction Clutch], a specially designed Reversing Gear, [Governor], [Marsh steam pump], [Injector]; and is mounted upon an open bottom fire box locomotive boiler, with ash pan under fire box and dome at rear end.

The boiler is mounted upon the traction wheels by brackets bolted to the rear end, which contain the boxes for the main axle and cross shaft.

The traction wheels are of the cast iron rim type, with spokes cast in both rim and hub.

The steering wheel and band wheel are on opposite sides of boiler, and both engine and boiler are supplied with all necessary fittings.

A. W. Stevens Traction Engine.

Q. How would you stop foaming?

A. Close the throttle long enough to show the true level of water. If the level of the water is sufficiently high, feeding and blowing off will usually correct the difficulty.

Q. What would you do if you discovered the water gone from sight in the water glass?

A. Pull the fire or cover it over with dry ashes, and allow the boiler to cool off as quickly as possible; and would not open or close any of the steam outlets.

Q, What is a traction engine?

A. A traction engine is an engine the power of which is transmitted to the driving or ground wheels by a combination of gearing.

Q. What is an exhaust pipe?

A. The pipe through which the exhaust steam escapes from cylinder to smoke stack.

Q. What is the feed pipe?

A. The pipe through which the feed water passes from pump or injector to the boiler.

Q. What is the steam pipe?

A. The pipe through which steam is taken from the dome to the steam chest.

Q. What is a pet cock?

A. A small cock used in check valves, pipes and places where draining off water is necessary to prevent freezing.

Q. What is clearance in a steam cylinder?

A. It is the space between the cylinder head and piston head when the latter is at end of the stroke.

Q. What is “cushion” in a steam cylinder?

A. Cushion is the compression of steam let in through the lead of the valve in the clearance of the cylinder, and is for the purpose of catching the weight of the piston and rod, cross-head and connecting rod when the engine reaches the end of each stroke. It also keeps the engine from pounding.

Q. How much water would you blow off at any one time while running?

A. Never blow off more than one gauge.

Q. What are your general views regarding boiler explosions?

A. The greatest causes are from ignorance, carelessness and neglect.

Q. What precaution should you take if necessary to stop with a heavy fire in the furnace?

A. Close the draught door, and put the injector or pump at work.

Q. What is the proper height to carry water in the boiler?

A. About 2½ inches above top row of flues.

Q. At what pressure should you blow off a boiler?

A. At a pressure not to exceed ten pounds.

Q. If you wished to increase the power of an engine what would you do?

A. Increase its speed or get higher steam pressure.

Q. How do you find the horse-power of an engine?

A. Multiply the speed of piston travel in feet per minute by the total effective pressure upon the piston in pounds, and divide the product by 33,000.

Q. What is meant by “brass bound”?

A. Brass bound means that the half brasses touch each other and cannot be driven up any closer by the key.

Q. How would you remedy a brass bound box on crank-pin or wrist-pin?

A. Take off the boxes and file off the top and bottom edges, being careful not to take off too much.

Q. Does a perfect fitting or an imperfect fitting valve have the most friction?

A. An imperfect fitting one.

Q. How would you refit an imperfect fitting or leaky valve?

A. It should be re-faced on a planer or filed and scraped until it fits seat perfectly tight.

Q. How is a steam engine rated?

A. By amount of horse-power developed.

Q. What is a foot-pound?

A. One pound of force exerted through one foot of space.

Q. How many foot-pounds are required to lift 100 pounds one foot?

A. One hundred.

Q. How many foot-pounds required to lift one pound 100 feet?

A. One hundred.

Q. To lift 110 pounds through 300 feet how many foot-pounds required?

A. 300 × 110 = 33,000 foot-pounds.

Q. Would that equal one horse-power?

A. Yes, if done in one minute.

Q. Suppose it took two minutes?

A. Then there would be only half a horse-power, or 33,000 ÷ 2 = 16,500 foot-pounds per minute.

Q. Is it correct to say “horse-power per minute” or “horse-power per hour”?

A. No. When an engine is working at the rate of 10 horse-power, it is doing 10 horse-power all the time. It is an error to assume that such an engine is doing 10 horse-power per minute, and 10 × 60 equals 600 horse-power per hour. When it is said that an engine uses 20 pounds of steam per horse-power per hour, it is meant that this amount of steam is used per hour for each horse-power developed.

Q. How is the foot-pounds of work done by a steam engine, found?

A. Multiply the average pressure per square inch during the stroke by the number of square inches in the piston, and by the number of feet through which the piston has moved.

Q. What do you understand by the “mean effective pressure”?

A. The mean pressure is the average pressure pushing the piston through the stroke, which is about one-third the pressure in the boiler. There is generally some back pressure working against it, therefore the “effective” pressure is only the difference between the two. It can only be determined accurately by measurements from an indicator diagram.

Q. What is a single acting engine?

A. An engine in which the steam acts on one side of the piston only.

Q. How do you find the “piston’s speed”?

A. On double acting engines, multiply the stroke in inches by two and by the number of revolutions per minute and divide by 12.

Q. Why multiply the stroke in inches by 2?

A. Because in double acting engines there are two working strokes to each revolution.

Q. Why do you divide by 12?

A. To reduce the inches to feet.

Q. How is the “piston’s speed” of a single acting engine found?

A. Multiply the stroke in inches by the revolutions per minute and divide by 12.

Q. What is the horse-power developed by an engine, say 12 × 24 inch, running 125 revolutions per minute, with 40 pounds mean effective pressure?

A. Area = 12 × 12 × .7854 = 113.0976 sq. ins.

Q. What is a single valve engine?

A. It is an engine in which a single valve controls the admission and distribution of steam to both ends of the cylinder, or a common slide valve engine.

Q. What is a four valve engine?

A. An engine which has separate steam and exhaust valves for each end both top and bottom of cylinder, such as a Corliss engine.

Q. Into what three classes are engines divided with reference to the manner in which they are governed?

A. Throttling engines, Automatic cut-off and Governor engines.

Q. What is an Automatic cut-off engine?

A. An engine in which the amount of steam supplied is automatically cut off at various points in the stroke, in accordance with the load and pressure. In Throttling engines the volume admitted is constant and the pressure varied. In Automatic cut-off engines, steam is admitted at the highest available pressure, and the volume is varied to meet the requirements of the load. In Governor engines, the steam is admitted and cut off by the governor.

Q. What is a throttle governed engine?

A. An engine in which the amount of steam supplied is regulated by changing the pressure at which it enters the cylinder in accordance with the variation of the load.

Q. What is a Governor engine?

A. An engine in which the supply of steam is regulated by the governor.

Q. Into what classes may the Automatic cut-off engine be divided?

A. Into two classes: The four valve engine, in which the cut-off is usually effected by a detaching mechanism or trip under the control of the governor; the single valve engine, in which the point of cut-off is varied by changing the amount of travel of the valve.

Q. Give examples of the single valve type.

A. High speed, self-contained engines which have shaft governors.

Q. What are their advantages?

A. High rotative speed, compactness, portability, light weight and simplicity.

Q. Are they more economical than the four valve engine?

A. No; the four valve engines are the more economical.

Q. Give a prominent example of the four valve engine.

A. The Reynold’s Corliss.

Q. What is meant by an engine running “over”?

A. The top of the drive wheel running away from the cylinder.

Q. What is meant by an engine running “under”?

A. The top of the drive wheel running towards the cylinder.

Q. Which way are engines generally run?

A. “Over.”

Q. What advantages do engines have in running “over”?

A. The pressure of the cross-head on engines running over, is always downward upon the guides; for when the pressure is on the head end of the piston, the pressure against the connecting rod which is pointing upward, reacts by pressing the cross-head down upon the lower guide, and when the pressure is on the crank end of the cylinder, the cross-head will be dragging the crank, and as the crank is below the center line, it will pull the cross-head down upon the lower guide, while if the engine is running under, the pressure of the cross-head will be upon the top guide, both on the outward and inward strokes, and unless the cross-head is nicely adjusted to its guides and the guides are perfectly parallel, the cross-head will be lifted when subjected to thrust, and will fall on the center by its own weight, causing the engine to pound.

Q. At what point in the stroke is the pressure on the cross-head greatest with a uniform pressure in the cylinder?

A. When the crank is at right angles to the guide.

Q. How does the relative length of the connecting rod affect this pressure?

A. The longer the connecting rod as compared with the crank, the less will be the pressure on the guides.

Q. What is the usual ratio of connecting rod to crank?

A. The connecting rod is from four to six times the length of the crank.

Q. Are there any objections to a long connecting rod?

A. A long connecting rod makes a long engine, and makes extra cost in the bed or frame and the room occupied. The longer rod is heavier, and brings extra weight upon the cross-head, guides and crank-pin. The long rod also lacks stiffness unless excessively heavy.

Q. What determines the length of the crank?

A. The stroke.

Q. What limits the stroke?

A. The piston’s speed limits the length of stroke allowable with a given rotative speed, or the number of revolutions per minute with a given stroke.

Q. What is the practical limit of piston’s speed?

A. Engines of from four to six foot stroke can run at from seven to eight hundred feet piston’s speed per minute. Those of shorter stroke should not run over six hundred feet.

Q. Why do high speed engines have a short stroke in comparison with the diameter of their cylinders?

A. So that they can run at a high rate of speed without exceeding the limit of piston’s travel.

Q. What is the office of the fly-wheel?

A. It maintains a uniformity of motion of the crank, notwithstanding the unequal moving force upon the crank-pin.

Q. Is the force upon the crank-pin unequal, even when the pressure from the cylinder is uniform throughout the stroke?

A. Yes. No matter what the pressure on the piston is, it has no effect in turning the engine when the crank is in line with the guides, which is termed “on the center.” As the crank gets away from the centers, the effect of a given pressure becomes greater, and reaches its maximum when the crank is nearly at right angles with the guides.

Q. How does the fly-wheel counteract the jerky motion of the crank which would result from this?

A. By its tendency to resist an excessive moving force, and by its momentum keeps the engine in motion when the moving force is deficient.

Q. What would you do if the cylinder gets worn or cut from too tight rings or lack of oil?

A. Rebore the cylinder.

Q. What would you do if the crank-pin heats, gets worn or cut?

A. If bent it should be turned true again; if not bent it can be filed and polished perfectly true by hand.

Q. What would you do if the main bearings heat?

A. Loosen the caps and apply plenty of good oil. If this does not stop it take off the caps, examine the oil holes to ascertain why the oil does not reach the bearing. If the bearings have become rough and cut, the shaft will have to be smoothed again.

Q. Would any harm result from starting an engine with the drip cocks closed?

A. Yes, the condensed steam filling the space would smash the cylinder or piston head.

Q. What do you mean by atmospheric pressure?

A. The weight of the atmosphere, which is 14.7 lbs. per square inch at sea level.

Q. How hot can you get water with exhaust steam under atmospheric pressure?

A. 212° Fahr.

Q. Does atmospheric pressure have any influence on the boiling point?

A. It does.

Q. Would you run an engine with throttle wide open, or partly open?

A. Wide open on governor engines, as it is more economical.

Q. How many pounds of water required per horse-power for the best engines?

A. From 25 to 30 pounds.

Q. At what temperature has iron the greatest tensile strength?

A. About 600 degrees.

Q. How much water is consumed (in pounds) per hour per indicated horse power?

A. From 25 to 60 pounds.

Q. How much steam will be evaporated from one cubic inch of water under atmospheric pressure?

A. About one cubic foot, approximately.

Q. How much coal is consumed per hour per indicated horse-power?

A. From two to seven pounds.

Q. How much does one cubic foot of fresh water weigh?

A. 62½ pounds.

Q. How much does one cubic foot of iron weigh?

A. 486⁶/₁₀ pounds.

Q. What does one square foot of half inch boiler iron weigh?

A. Twenty pounds.

Q. For steam purposes, how much wood is required to equal one ton of coal?

A. About 4000 pounds of wood.

Q. Of what does coal consist?

A. Carbon, nitrogen, sulphur, hydrogen, oxygen and ash.

Q. What are their relative proportions?

A. There are different proportions in different specimens of coal. The average per cent is carbon eighty, nitrogen one, sulphur two, hydrogen five, oxygen seven, ash five.

Q. Of what is air composed?

A. It is composed of nitrogen and oxygen in the proportion of seventy-seven of nitrogen and 23 of oxygen.

Q. Of what does water consist?

A. Hydrogen and oxygen in the proportion of one of hydrogen to eight of oxygen by weight.

Q. What are the different kinds of heat?

A. Latent heat, sensible heat, and sometimes total heat.

Q. What is meant by latent heat?

A. Heat that does not affect the thermometer and which expends itself in changing the nature of a body, such as turning ice into water or water into steam.

Q. Under what circumstances do bodies get latent heat?

A. When they are passing from a solid to a liquid state, or from a liquid to a gaseous state.

Q. How can latent heat be recovered?

A. By bringing the body back from a state of gas to a liquid, or from a liquid to a solid.

Q. If the power is in coal, why should we use steam?

A. Because steam has some properties which make it an invaluable agent for applying the energy of the heat to the engine.

Q. What is steam?

A. It is an invisible elastic fluid generated from water by the application of heat.

Q. What are its properties which make it so valuable to us?

A. First. The ease with which we can condense it.

Second. The small space which it occupies when condensed.

Third. Its great expansive power.

Q. What do you understand by the term “horse-power”?

A. A horse-power is equivalent to raising 33,000 pounds one foot per minute.

Q. What do you understand by “lead” on an engine valve?

A. Lead on a valve is the admission of steam into the cylinder before the piston completes its stroke.

Q. What are considered the greatest improvements on the stationary engine in the past forty years?

A. The Corliss valve gear, the governor, the compound and triple expansion.

Q. What is meant by triple expansion engine?

A. A triple expansion engine has three cylinders using the same steam expansively in each one.

Q. What is the clearance of an engine as the term is applied at the present time?

A. Clearance is the space between the cylinder head and the piston head with the ports included.

Q. What is the principal which distinguishes a non-condensing from a condensing engine?

A. Where no condenser is used, and the exhaust steam is open to the atmosphere, it is a non-condensing engine.

Q. Why do you condense steam?

A. To form a vacuum and thus remove the atmospheric and back pressure that would otherwise be on the piston, thereby getting more useful work out of the steam.

Q. What is meant by vacuum?

A. A space void of all pressure.

Q. How can you maintain a vacuum?

A. By the steam used being constantly condensed by the cold water or cold tubes, and the air pump constantly clearing the condenser.

Q. Why does condensing the used steam form a vacuum?

A. Because a cubic foot of steam at atmospheric pressure shrinks into about one cubic inch of water.

Q. What is a condenser as applied to an engine?

A. The condenser is that part of an engine into which the exhaust steam enters and is condensed.

Q. About how much gain is there by using the condenser?

A. Seventeen to twenty-five per cent. where cost of water is not figured.

Q. What do you understand by the use of steam expansively?

A. Where steam admitted at a certain pressure is cut off and allowed to expand to a lower pressure.

Q. How many inches of vacuum gives the best result in a condensing engine?

A. About 25 inches.

Q. What is meant by a horizontal compound tandem engine?

A. One cylinder being back of the other with two pistons on the same rod.

Q. What do you understand by lap?

A. Outside lap is that portion of the valve which extends beyond the ports when valve is placed on the center of its travel; inside lap is that portion of valve which projects over the ports on inside or toward the middle of the valve.

Q. Of what use is lap?

A. It gives expansion to the steam in the cylinder.

Q. What is the dead center of an engine?

A. The point where the center of shaft, center of wrist-pin and center of piston rod are in the same straight line.

Q. From what cause do belts have power to drive shafting?

A. By friction and cohesion.

Q. When would you oil an engine?

A. Before starting it and as often while running as is necessary.

Q. What is the tensile strength of American boiler iron?

A. 40,000 to 60,000 pounds per square inch.

Q. What are the principal defects found in boiler iron?

A. Imperfect welding, brittleness, low ductility.

Q. What is the advantage of steel as a material for boiler plate?

A. Tensile strength, ductility, homogeneity, malleability and freedom from laminations and blisters.

Q. What are the disadvantages of steel as a material for boiler plate?

A. It requires greater care in working than iron and is subject to flaws induced by the pressure of gas bubbles in the ingots from which the plates are made.

Q. How far apart should stay bolts be put in a boiler?

A. They vary from 4 to 6 inches apart, depending on thickness of plates, size of stay bolts and amount of steam pressure to be carried.

J. I. Case Portable Skid Engine.

J. I. CASE PORTABLE SKID ENGINE.

The engine frame is of the girder pattern, cast in one piece, and contains the guides for cross-head and pillow block bearing. It also forms front head of cylinder to which it is firmly bolted. The cylinder is supported and firmly attached to the large feed water heater, and both cylinder and frame are entirely disconnected from the boiler. The heater forms a support for front end of boiler. The engine is placed diagonally with the boiler, allowing the crank shaft to pass over the fire box end. This permits the use of a very large band wheel. The outer end of shaft is supported by a pillow block attached to a large bracket bolted to the boiler.

The locomotive style of boiler has a large steam dome in center and an ash pan under fire box. It is supported upon the long wooden skids by brackets bolted to the sides of boiler, the heater being also bolted to the skids.

The independent pump is connected to the heater, and bolted to skid.

The engine has a Plain Slide Valve, which receives its motion from the rocker arm, operated by the eccentric and rod. It has [Governor], [Steam Gauge], [Pop Valve] and all necessary fittings. These engines range in size from 20 to 30 horse-power, and are extensively used for driving portable saw mills.