A. The throttle valve should be kept slightly open while drifting, so as to admit a small quantity of steam in valve chamber and cylinder above atmospheric pressure, to prevent the inrush of hot air and gases which destroy lubrication, also to prevent excessive wear to valve, cylinder and piston rod packing.

13.  Q. How should the water be carried in boiler of superheater locomotives?

A. As low as the conditions will permit, because this practice reduces the tendency to work water over into the dry pipe and units, as the superheater locomotive will use one-third less water than the saturated locomotive.

14.  Q. What care should be exercised in lubricating a superheater locomotive?

A. The supply of oil to steam chest should be watched very closely by the engineer, he to know that lubricator is feeding constantly and evenly over entire division, and according to work performed.

15.  Q. Describe the general form of a locomotive boiler.

A. A locomotive boiler is cylindrical in form, it usually has a rectangular shaped fire-box at one end and a smoke-box at the other, and flues extend through the cylindrical part, and, like the fire-box, are surrounded by water.

16.  Q. How does the wide fire-box type of boiler differ from the ordinary boiler, and what are its advantages?

A. The wide fire-box type of boiler is built so the fire-box is above the frame and extends out over the driving wheels.  The advantages of this are to obtain a larger grate area in the same length of fire-box and to give a slower rate of combustion per square foot of grate surface.  The deep fire-box is limited in width to the distance between the frames, while the shallow fire-box sets on top of the frames and between the driving wheels.

17.  Q. Why have two fire-box doors been placed in the large type of locomotive boilers?

A. For convenience of the fireman on account of the greater width of the fire-box, so that coal can easily be distributed to all parts of the fire-box.

18.  Q. Describe a locomotive fire-box.

A. The modern form is a rectangular shaped structure located at the back end of the boiler.  It has a door and is composed of two side sheets, a crown sheet, a back sheet and a flue sheet from which the flues extend to the smoke-box at the other end of the boiler.

19.  Q. To what strains is a fire-box subjected?

A. To crushing strains and to those of unequal contraction and expansion.

20.  Q. How are the sheets of a fire-box supported?

A. They are supported by staybolts screwed through the inside and outside sheets with their ends riveted over.

21.  Q. In what manner is a crown sheet supported?

A. By crown bars or radial staybolts.

22.  Q. What are the bad features about crown bars?

A. They are hard to keep clean and frequently cause crown sheets to become mud burned.