40.  Q. How should you build up the fire when at stations in order to avoid black smoke?

A. Put in small quantities of coal at a time, have the door slightly open and have the blower on lightly; good judgment must be exercised by the fireman.

41.  Q. Why is it that if there is a thin fire with a hole in it the steam pressure will fall at once?

A. Because too much cold air is drawn into the fire-box and through the tubes, retarding combustion and cooling the fire-box and tubes.

42.  Q. If the injector is to be used after throttle is shut off, how should the fire be maintained?

A. A sufficient quantity of coal should be placed on the grates to maintain the maximum steam pressure and the blower used to keep the fire burning brightly.

43.  Q. What would be the result of starting a heavy train or allowing drivers to slip with the fire too thin on the grates?

A. The fire would be pulled off the grates and into the tubes, leaving the fire bed full of holes and some of the fire remaining on the grates turned over.  Large quantities of cold air would be drawn in, resulting in a rapid decrease of temperature and pressure.  The tubes would possibly start leaking and the fire would be in such condition that it could not be built up properly in a long distance.  Possibly the grates would become clogged up with green coal—­an excellent opportunity for forming clinkers.  In this condition, the engine would fail to make steam for the entire trip.

44.  Q. Where should the coal, as a rule, be placed in the fire-box?

A. As a rule, more coal is burned along the sides and in the corners than in the middle of the grates; the fire should consequently be kept somewhat heavier along the sides and corners than in the middle.

45.  Q. How is the fire affected by and what causes clinkers?

A. A clinker shuts off area of grate surface according to its size, and thereby shutting off that much of the air supply and interfering with proper combustion.  Clinkers are caused by firing too heavy in spots, which prevents sufficient air passing up through these spots and allows the coal to run together, melting the ash, and sand; running a hoe or slash bar through the fire will bring the points of melted sand together, thereby causing a clinker.

46.  Q. How can you best avoid their formation and dispose of them?

A. Light firing and occasionally moving the grates lightly is the best preventive.  When once formed, they should be removed if possible by firing around and burning them out.

47.  Q. How can you explain the slower burning of the coke and how understand the proper manner of supplying fresh coal?

A. The gases of coal are lighter than air and will pass away whether consumed or not.  The slow burning of the coke is due to the fact that it burns from the outside only.  When a fire reaches a white or incandescent heat it indicates that the gases are burned and a fresh supply of coal should be added; this is to be done as light as the service performed by the engine will permit.