9 o

MODERN STEAM PRACTICE.

have given serious trouble through being thus stayed. The shellshould be made quite circular, and the longitudinal seams, whichshould break joint, be so arranged that when the boiler is set allthose below the water-line may be accessible for examination in theflues, and be clear of the brick seatings. Many makers now double-rivet these seams, thus materially increasing their strength, and, whenthe work is well performed, reducing liability to leakage. Flue-tubes are now constructed in various ways, some makers preferringto use thick plates not strengthened in any way, whilst others prefercomparatively thin plates, but flanging them at the ring seams, orby welding each ring of plates and connecting them by solid T-ironhoops, form a much stronger and more reliable flue-tube. Theliability to leakage, fracture, and excessive expansion is thus muchreduced, as the heat is more freely transmitted through the thinplates. The cross -tubes and water-pockets introduced by somemakers in that part of the flue-tubes beyond the furnace bridge areof great value, chiefly from the manner in which they improve theefficiency of the heating surface by the diversion and breaking upof the current of gases, whilst they much increase the strength ofthe tubes to resist collapse. All large tubes exposed to high pres-sure should be strengthened by some of the means described. Wherethe tubes are formed with the ordinary lap-joints the longitudinalseams should break joint, as a tube thus made is much stronger thanwhere those seams are in a line; and at the furnace end all longitu-dinal seams should be below the fire-grate level. The plan of form-ing tubes with the plates longitudinally in narrow Strips is veryobjectionable, as the tubes cannot be made so circular, and theseams above the bars are injured by the action of the fire, whilstsuch tubes are much weaker than those made in the ordinary man-ner Multitubular boilers should, as far as practicable, be so con-structed that every part of the interior may be accessible for cleaningand examination; and it would be a great improvement if those ofportable and locomotive engines were so constructed that the tubescould be drawn out without difficulty, so as to allow occasionalinspection of the internal surface of the plates. External flues arenecessary to stationary cylindrical boilers of this dass, otherwisethe lower seams are strained, and become leaky through excessiveunequal expansion of the boiler. Plain cylindrical externally firedboilers, with egg or saucer shaped ends, are preferred by someowners, chiefly on account of their simple form. Such boilers can