383. Q.—­Can any constructive precautions be taken to prevent the furnaces and tube plates of the boiler from being burned by the intensity of the heat?

A.—­The sides of the internal furnaces or flues in all boilers should be so constructed that the steam may readily escape from their surfaces, with which view it is expedient to make the bottom of the flue somewhat wider than the top, or slightly conical in the cross section; and the upper plates should always be overlapped by the plates beneath, so that the steam cannot be retained in the overlap, but will escape as soon as it is generated.  If the sides of the furnace be made high and perfectly vertical, they will speedily be buckled and cracked by the heat, as a film of steam in such a case will remain in contact with the iron which will prevent the access of the water, and the iron of the boiler will be injured by the high temperature it must in that case acquire.  To moderate the intensity of the heat acting upon the furnace sides, it is expedient to bring the outside fire bars into close contact with the sides of the furnace, so as to prevent the entrance of air through the fire in that situation, by which the intensity of the heat would be increased.  The tube plate nearest the furnace in tubular boilers should also be so inclined as to facilitate the escape of the steam; and the short bent plate or flange of the tube plate, connecting the tube plate with the top of the furnace, should be made with a gradual bend, as, if the bend be sudden, the iron will be apt to crack or burn away from the concretion of salt.  Where the furnace mouths are contracted by bending in the sides and top of the furnace, as is the general practice, the bends should be gradual, as salt is apt to accumulate in the pockets made by a sudden bend, and the plates will then burn into holes.

384. Q.—­In what manner is the tubing of boilers performed?

A.—­The tubes of marine boilers are generally iron tubes, three inches in diameter, and between six and seven feet long; but sometimes brass tubes of similar dimensions are employed.  When brass tubes are employed, the use of ferules driven into the ends of the tubes is sometimes employed to keep them tight; but when the tubes are of malleable iron, of the thickness of Russell’s boiler tubes, they may be made tight merely by firmly driving them into the tube plates, and the same may be done with thick brass tubes.  The holes in the tube plate next the front of the boiler are just sensibly larger in diameter than the holes in the other tube plate, and the holes upon the outer surfaces of both tube plates are very slightly countersunk.  The whole of the tubes are driven through both tube plates from the front of the boiler,—­the precaution, however, being taken to drive them in gently at first with a light hand hammer, until the whole of the tubes have been inserted to an equal depth, and then they may be driven up by degrees with a heavy hammer, whereby