BOILERS.] STEAM-ENGINE 497 (A, A, fig. 40), and finally pass to the back again by an underneath flue B. The arrangement in the Lancashire boiler is the same, ex- cept that there are two internal flues, each with its own furnace. The shell is made up of rings of riveted plates, larger and smaller in diameter alternately to allow the circumferen- tial seams to be made without bending the edges. The flue is made up of a series of welded rings, joined to each other by a flanged joint with a stiffening ring. This form of joint was in- troduced by Mr Adam- son to stiffen the flue against collapse un- der external pressure. Other joints, designed with the same object, are shown in figs. 42 and 43. The grate is made up of firebars, FIG. 41. Lancashire Boiler: Transverse Section. sloping down towards the back, where they terminate at the " bridge" of fire-brick (C, fig. 39). Beyond the bridge the flue is crossed by a number of tapered "Galloway" tubes D, D, which increase the heating surface, promote circulation of the water, and stiffen the flue. The end plates are strengthened by gusset slays FIGS. 42 and 43. Joints for Furnace Tubes. E, E, riveted to them and to the circumference of the shell by means of angle-irons. The gusset-stays do not extend so far in as to the circumference of the flue (fig. 40), in order that the end plates may retain enough flexibility to allow the flue to expand and contract under change of temperature. To provide for unequal expansion is one of the most important points in the design of boilers ; when it is neglected the boikr is subjected to a racking action which induces leakage at joints and tends to rupture the plates. For this reason the flue is attached to the boiler shell at the ends only, so that it may be free to take an upward camber in consequence of the greater heating of the upper side. Mild steel is now very generally used for boiler plates, being superior even to the best Yorkshire iron in the qualities of ductility and tensile strength. The following particulars refer to the Lanca- shire boiler of fig. 41, which may be taken as representative of a large number of stationary boilers. 128. The shell is 28 feet long and 7 feet in diameter, and is made up of 9 rings, each of two semi- cylindrical plates. The shell plates are f inch thick ; their edges are planed and fullered, and the rivet holes are drilled. The longitudinal seams, which break joint from ring to ring, are lap-joints double-riveted ; the circular seams are single-riveted. Each end plate is a solid piece of steel inch thick ; the front plate is attached to the shell by riveting to an angle ring ; the back plate is flanged. The flues are each 2 feet inches in diameter, made up of rings of steel | inch thick ; the longitudinal joints welded and the are circular joints are flanged and strength- ened with stiffening rings. The flues are tapered somewhat at the back end to facil- itate expansion, and are attached to the end plates by welded angle -rings. Each flue contains 5 Gallo- way tubes, tapering from 10^ inches diameter at top to 5^ inches at bot- tom. On the top of the boiler is the manhole, covered with a cast-iron plate ; also a nozzle for the steam-pipe and two others for safety-valves. One of the safety-valves is connected with a float so that it opens if the water-level becomes too low. At the bottom, in front, is another nozzle for the blow-out tap ; and FIG. 44. Anti-Priming Pipe and Stop- Valve. in the front plate below the flues is another manhole. Feed-watei is supplied by a pipe which enters through the front plate on one side, near the top of the water, and extends for a considerable dis- ance along the boiler, distributing the water by holes throughout he length. A pipe at the same level on the other side serves to collect scum. The fire doors are pro- vided with sliding shutters by means of which the amount of air admitted above FIG. 45. Lever Safety-Valve. the fire may be regulated. On the top of each fur- nace is fitted a fusible plug which melts if the furnace crown becomes overheated. No separate steam dome is used ; the steam is collected by an "anti-priming" pipe shown in fig. 44, which also illustrates the stop- valve by which the delivery of steam from the boiler is started or stopped at will. On the front plate are a pair of glass gauge- tubes for showing the water-level, and a Bourdon pres- sure-gauge. This last important tit- ting consists of a bent tube of oval section, one end of which is closed and free to move while the other is open to the steam and is / fixed. The pres- 1 sure within the tube tends to straighten F , G 40. GaUoway Boiler : Section beyond the Bridge, it, and the extent to which this takes place is shown by a pointer which travels over a circular dial. A common lever safety-valve is shown in fig. 45. In other forms the valve is kept down by a weight directly applied to it, or by means of springs. Spring safety-valves are liable to the objection that when the valve opens the load on it increases ; to remedy this, forms have been proposed in which the spring acts through a bent lever in such a way that when the strain on it increases the leverage at which it acts is re- duced. If the spring is of reasonable length, however, the objection is not seri- ous. 129. A modifica- tion of the Lanca- shire type the ' ' Galloway " boiler is shown in sec- tional elevation in fig. 46. In it the two flues are joined beyond the bridge into a single flue, of the form shown in the figure, which is traversed by nu- merous Galloway tubes and is also fitted with water- nonkpts at its sides F IG. 47.- Vertical Boiler FIG. 48. Vertical Boiler pockets at il sides. wifh Hol . izontal Wa u-r 130. In other Tubes> Tubes . types of boiler an tubular extensive heating surface is obtained by the use of a large number hoilers. of small tubes through which the hot gases pass. This construc- tion is universal in locomotive and marine boilers. It is appiu in some instances to boilers of the ordinary cylindrical form by making small tubes take the place of that part of the flue or flues which lies behind the bridge, or by using small tubes as channels through which the gases return from back to front after passing through the main Hue. Another form of tubular boiler is an exter- nally fired horizontal cylinder fitted with tubes which carry the hot gases from the back to the front. XXII. 63 Water L,ne ( ^E2^~ ?v^ f "( I AJ pi / IJ Galloway boiler.
