411. Q.—­How is the proper level of the water in the boiler of a steam vessel maintained when, the engine is stopped for some time, and the boiler is blowing off steam?

A.—­By means of a separate pump worked sometimes by hand, but usually by a small separate engine called the Donkey engine.  This pump, by the aid of suitable cocks, will pump from the sea into the boiler; from the sea upon deck either to wash decks or to extinguish fire; and from the bilge overboard, through a suitable orifice in the side of the ship.

LOCOMOTIVE BOILERS.

412. Q.—­Will you recapitulate the general features of locomotive boilers?

A.—­Locomotive boilers consist of three portions (see fig. 29):  the barrel E, E, containing the tubes, the fire box B, and the smoke box F; of which the barrel smoke box, and external fire box are always of iron, but the internal fire box is generally made of copper, though sometimes also it is made of iron.  The tubes are sometimes of iron, but generally of brass fixed in by ferules.  The whole of the iron plates of a locomotive boiler Which are subjected to the pressure of steam, should be Lowmoor or Bowling plates of the best quality; and the copper should be coarse grained, rather than rich or soft, and be perfectly free from irregularities of structure and lamination.

413. Q.—­What are the usual dimensions of the barrel?

A.—­The thickness of the plates composing the barrel of the boiler varies generally from 5/16ths to 3/8ths of an inch, and the plates should run in the direction of the circumference, so that the fibres of the iron may be in the direction of the strain.  The diameter of the barrel commonly varies from 3 ft. to 3 ft. 6 inches; the diameter of the rivets should be from 11/16ths to 3/4ths of an inch, and the pitch of the rivets or distance between their centres should be from 17/8th to 2 inches.

414. Q.—­How are the fire boxes of a locomotive constructed?

A.—­The space between the external and internal fire boxes forms a water space, which must be stayed every 4-1/2 or 5 inches by means of copper or iron stay bolts, screwed through the outer fire box into the metal of the inner fire box, and securely riveted within it:  iron stay bolts are as durable as copper, and their superior tenacity gives them an advantage.  Sometimes tubes are employed as stays.  The internal and external fire boxes are joined together at the bottom by a N-shaped iron, and round the fire door they are connected by means of a copper ring 1-1/4 in. thick, and 2 in. broad,—­the inner fire box being dished sufficiently outward at that point, and the outer fire box sufficiently inward, to enable a circle of rivets 3/4 of an inch in diameter passing through the copper ring and the two thicknesses of iron, to make a water-tight joint.  The thickness of the plates composing the external