The development of higher pressures, and the difficulty of overcoming trouble by the breaking of the side stay-bolts near the top of the furnace, have led to the Zoption by many of a construction in which the fire-box crown is arched, and sup ported by radial stay-bolts tapped through the crown-sheet and roof of boiler and riveted over. The arched form of crown-sheet allows the sediment to drain off without obstruction. By entering the boiler through the dome the entire crown is easily accessible for re moving scale. It is therefore especially suitable for locations where impure water must be used. The removal of the weight of the crown-bars permits the heating-surfaces to be increased without ex ceeding a fixed limit. The gradual lengthening of the stays from the short ones supporting the side-sheets to the long ones sup porting the crown, prevents distortion by concentration of strain at a particular point, and therefore overcomes the breakage of bolts, which is frequent in boilers of the crown-bar or Belpaire patterns, to carry high pressures, unless constant vigilance is exercised. (For the above description of American types of loco motives we are indebted to D. K. Clark's work on the Siertni-Evine edition of 1802.) The Woolle'n Locomotive Boiler (shown in Fig. 1) is the inven thin of Mr John E. Wootten, and is the subject of six United States letters-patent, granted from 1877 to 1887. It has been largely adopted in the Philadelphia and Heading and other railroads using anthracite coal. The distinguishing features of the Wootten locomotive as cotnpared with others. is a much greater breadth of furnace and larger area of grate with less depth of fool thereon, it change in the location of the cab from the rear of the engine and at the sides of the fire-box to a position above the furnace in some instances, and in others on each side of the waist of the boiler immediately in front of the fire-box. the steam-dome being located in the eab. The construction of frames, driving-wheels, cylinders. and steam chests is not strikingly different from other well-known and usual types of engines. The constantly increasing weight of train-loads has necessitated more powerful engines: and while it was not difficult to increase the cylinder capacity or piston displacement of the en gines, the limit of the boiler to supply adequate steam to such engines was soon reached. The gauge of the railroad appeared to limit the width of the boilers mlinissible, the frames could not he spread any farther apart, and, under the practice of placing the furnace of the boiler between the frames, the only increase of grate-surface practicable was in the direction of length. This rendered firing more difficult, and a deep bed of fuel was required to main tain steam-pressure; the draft of air to maintain combustion demanded greater pressure on the exhaust, which could only be enforced by contracting the nozzle of the exhaust-pipe, and imposing as pressure upon the steam-pistons during the return strokes. This, in view of the large piston-surface recently coming into vogue, especially in compound locomotives, means a serious waste of force. The solution of this difficulty was found in an increased breadth of furnace-grate and fire-box to accommodate it. Space to contain such boilers without interfering with the driving-wheels was procured by placing the boiler above the driving-wheels and frames, which were protected from ashes by a hopper-shaped ash-pit, A report of series of tests made by Dr. Charles M. Cresson of the Standard locomotive boiler of the Baldwin Locomotive Works, and of a Wootton boiler burning several kinds of fuel, which shows the claims for the capacity of the Wootten boiler as an efficient steam generator with different varieties of fuel. including some incapable of use in ordinary locomotives, to be fully sustained, is quoted as follows by a committee of the Franklin Institute (see Jour. Frank. Inst., September, 1891): For 18 X 24 to 20X 24 road locomotives with the Wootten boiler, a grate-surface of 70 sq. ft. is obtained, the length of the grate being Di ft. and its width 8 ft. Between the grates and the tube-plate, and separated from the first by a fire-brick bridge wall, is a combustion-chamber about 3 ft. long, which is set into the cylindrical part of the boiler, and correspondingly shortens the tnbes. By adopting so large a grate-area is obtained a low velocity of air pass ing through the fuel, and a slowness of combustion, which are of the utmost value in horning fuel too light to remain on the grates of ordinary locomotives, or impure fuel requiring the combustion of a large volume to produce sufficient heat. This type of boiler has been adopted by many of the railways in the anthracite coal regions. which are not only carriers but pro ducers of anthracite coal. and mast therefore utilize the cheap grades in order to market the more valnable grades, a fixed proportion of both attending the production. Separate cabs are provided for the engineer Itral fireman, as the former is preferably located in front of the fire bux, while the latter must stand on the tender.
Comeousn LocomoTivEs.—Doring the past. three years much attention has been given to developing and perfecting compound locomotives. They have been the subject of numerous patents. which may be divided into four classes, viz.: 1. Those with concentric cylinders. the high-pressure cylinder inclosed in the low-pressure cylinder, of which the most important example is the design of Mr. F. W. Johnstone, Super intendent of Notive-Power of the Mexican rentrn1 Railway, of which a number of engines have been constructed by the Rhode Island Locomotive Works, of Providence. R. I.
2. Those with cylinders placed tandem, the high-pressure cylinder being usually in front of the low-pressure cylinder. Engines of this type at this time (December, 1891) appear not to have passed the experimental stage. An important objection is the necessary length of the steam-ports connecting the two cylinders.
3. Those halving two unequal cylinders, 114.'1101 one on each side of the engine, and ex hausting from the smaller ol- low-pressure cylinder into a receiver exposed to the heated products of combustion in the smoke-box. The original] patent covering this system was grano.41 hi 1873 to Mr. W. S. Hudson, late Superintendent of the Rogers Locomotive Works, of Paterson, N. .1. This system has been further developed by Worsdell, Von Borries, La page, Lindner, and :Mallet, in Europe, and by Pitkin, Dean, Lytligue, and others in the United States.
4. Those having cylinders, of which one high-pressure and one low-pressure cylinder are placed on each side of the engine, the steam passing from one to the other by continuous expansion, without passing through a receiver. This system, which is the invention of Samuel M. Vanclain, Superintendent of the Baldwin Locomotive Works, has thus far been more exten sively adopted than any other in the United States, about 150 locomotives having been con structed in the two and a half years following the date of the patent, June 25, 18,•t9.
The general appearance of a recent Ten-wheel freight compound locomotive is shown in the full-page illustration.
The valve is of the type known as piston-valve, con sisting of a hollow plug with cylindric rings at proper intervals, fitting into a east-iron bushing.
with apertures registering with the rim of the plugs, and leading to and from the ends of the cylinders, from the steam-pipe and to the exhaust - pipe. The movement of the steam from the steam - pipes through the steam - chest, high-pressure cylinder, pis ton-valve, low-pressure cyl inder, and out at the final exhaust-port, is shown by the diagram Fig. 2. Fig. 3 shows an external view of the cylinders and chest. The arrangement of the cylinders is imma terial; in locomotives with small driving- wheels, the large or low-pressure cyl inder may be placed over the small or high-pressure cylinder, in order to obtain more clearance from the track. The following ad vantages were discovered in this type of compound lo comotives by the Committee of Sciences and Arts of the Franklin Institute, which caused it to be awarded a gold medal : "It can be applied to locomotives having outside cylinders, without increasing the entire breadth of the engines at the cylinders beyond the restrictions made necessary by bridges, tunnels, and trains upon parallel tracks. The transfer of steam from the low to the high pressure cylinder is effected by the shortest possible conduit. The valve construction is simple, and, being balanced, requires a minimum of force to work it, irrespective of the steam pressure upon it. The distribution of force upon each side of the engine is equal. Each side of the engine is capable of working when the other is disconnected, and when so operated can produce a draft sufficient to maintain effective steam generation for running purposes—a feature of decided importance in cases of accident disabling the engine on one side. The engine always starts promptly and steams readily with the diminished exhaust-pressure, the volumes of the exhaust being greater than with the Standard or non-compound engine, and occurring twice as often in the revolution of the shaft as in either the Webb or I ludson type of engine. It is not pretended that this compound engine imparts any new properties to the steam that is used in it, so as to surpass other well-proportioned compound engines in degree of expansion, and consequent economy of steam, but that it does diminish the clearance spice between the high and low pressure pistons, and promptly proceeds with the expansion in the low-pressure cylinder, while in other types of engines the exhaust from the high-pressure cylinder must lie retained in a receiver to await the opening of the valve admitting it to the low-pressure cylinder." A number of tests have been made, with much care and accuracy. The results justify the conclusions reached by the committee, and show a gratifying economy of fuel.