282 EXPLOSION OF STEAM BOILERS. three vessels were destroyed, probably from defective machinery, and one hundred and fourteen lives sacrificed. Mr. Reid, writing in the year 1839, states, that the explosions since the commencement of 1837, have been five times that of the average of a similar period in the preceding twenty years; whereas the number of steam vessels since 1837, is less than three times that of the average of three years, from 1817 to 1836 inclusive. The three years 1837-9, being not yet complete, renders the increase in explosions still larger in proportion. This proportion-of explosions to the number of steam vessels-may be said to have been, since 1837, nearly double what it was previously. These accounts are startling,—and the more so, when we are informed, that a few simple contrivances would render explosions impossible, and that it is disgraceful that such an occurrence as the explosion of a steam boiler should ever take place. Explosions of steam boilers are distinguished by Mr. Reid into two kinds explosion outwards, or explosion properly so called, when, from the elastic force of gaseous matter within the boiler being greater than it can support, it is burst, and its sides forced outwards; and collapse, when the sides of the boiler are forced inwards by the atmospheric pressure, from want of support, arising from diminution of the resistance within. When the flues pass through the boiler, explosion of the latter is at the same time collapse of the flue, which must be distinguished from the true collapse first mentioned. It is a collapse as regards the flue, an explosion as regards the boiler." Explosion, properly so termed, may arise from inefficiency of the safety valve, or from weakness of the boiler. 66 190. Explosion from Inefficiency of the Safety Valve.1. The safety valve is sometimes overloaded by the engineman, or kept closed by pressure on the lever connected with it. There is a temptation to resort to this practice in cases of competition among steam vessels plying on the same line, or when a railway engine has to ascend an inclined plane, or when it is desired to accumulate steam of high pressure for INEFFICIENCY OF SAFETY VALVES. 283 the purpose of a good start. Mr. David Napier stated, in his evidence before the jury at the inquest on the explosion of the Victoria, that "the engineer ought to have removed the extra weight on the lever at Blackwall, going and coming, and the accident could not have happened." Every boiler should be provided with two sufficient safety valves, one of which should be inaccessible to the engine-man, and the other accessible both to him and to the persons on board the packet. In cases in which there is only one safety valve, the weights should be so adapted as to be capable of being removed all at once by means of a lever, and not singly, as is the case in some of the smaller boats. Mr. Reid properly observes, that “ no one can foresee those sudden emergencies at which the action of the safety valve is essential to give vent to an accumulation of steam; and that it cannot be looked upon as security at all, unless it be always in such a state as to rise instantaneously whenever the steam tends to acquire undue force." 2. The aperture of the valve may be of insufficient size. On this subject, Messrs. Maudsley and Field observe:-"The safety valves should be large enough to admit of the escape of the whole of the steam, when the engine is suddenly stopped, without its rising more than half a pound on the inch beyond the usual pressure. Two valves, having an area of one square inch for every horse power, are sufficient for this purpose. Those valves should be so constructed that no increase of weight can possibly be put on, even by the engineer; and there should be provided an apparatus by which they may be conveniently lifted from the engine-room, when it is requisite to ease off the steam."-Report of Messrs. Parkes and Pringle. 66 191. Explosion from Weakness of the Boiler.-In the Report" above alluded to, the following statement was made by the Messrs. Seaward:-" Of the various causes which have been suggested by different persons to account for the explosions of steam boat boilers, such as the sticking of the safety valves, the igniting of explosive gases, the loss 284 EXPLOSION FROM WEAKNESS OF THE BOILER. of feed and heating of the metal plates, the sudden immission of a large quantity of feed and consequent generating of an unusual volume of steam, and other pretended causes of similar character, not one, in our opinion, is deserving the smallest attention; they should be all scouted as merely calculated to mislead the inquirer from the only true cause of these accidents, which is simply, that the materials of the boiler are not sufficiently strong to withstand the force of the steam. It is, however, true, that a boiler may lose its water so far as to allow some of the internal parts to become red hot, and thereby assist in producing a collapse when high-pressure steam, or steam of a dangerous pressure, is used; but the circumstance of some internal part of a boiler becoming red hot, ought not to be considered as the true immediate cause of the accident, because the losing of feed in a boiler, and the consequent heating of a flue red hot, is a mishap of very frequent occurrence in low-pressure boilers; but no accident has ever occurred on such occasions, calculated to occasion loss of life or personal injury. The fact is, that the parts of a boiler liable to become red hot, should even in that state be sufficiently strong to resist the force of the steam, so that no dangerous collapse shall take place; and all good low-pressure boilers are so made.” The Messrs. Seaward state their belief, that of the many accidents occurring in steam vessels through imperfection of the boilers, it will invariably be found, that they have happened in vessels in which steam of high pressure has been used, and in no instance with steam of low pressure, that is, of a pressure not exceeding 5 lbs. on the square inch. They further state, that of the numerous accidents occurring to steam boat boilers from the use of high-pressure steam, or steam of dangerous pressure, it will be found, that a large proportion, probably half, have occurred through the collapsing of large internal cylindrical chambers or flues employed in such boilers; the remaining accidents being occasioned through the bursting or rending of the external casings of boilers. ELECTRO-MAGNETIC TRAIN CONTROLLER. 285 III. ELECTRO-MAGNETIC RAILWAY TRAIN CONTROLLER. 192. This apparatus is the invention of Messrs. Wright and Bain, the latter of whom is already well known as the inventor of the electro-magnetic printing telegraph, the electrical clock, &c. From the models and diagrams exhibited at the Polytechnic Institution, it appears that the invention consists in the conveyance of the electric fluid from a battery to be placed in the locomotive engine by which the train is drawn, to a pilot locomotive engine by which the former is to be preceded on the railway at the distance of about a mile and a half. If, therefore, any obstacle occur on the railroad, by which the pilot locomotive is stopped or disturbed in its course, the communication through the electric fluid will be broken, and ocular notice by means of an index attached to the locomotive engine of the train will be immediately given to the engine driver to put on the breaks and stop the train. Should no attention be paid to the index, the apparatus on the locomotive of the train in connexion with the wires extending to the pilot, will cause a gong to be struck, and thus convey further warning; and, should this be neglected, the apparatus will cut off the steam of the locomotive of the train, apply the breaks, and thus, without the interference of the engine driver, prevent the train from proceeding. The electric fluid is conveyed from one locomotive to the other by means of wires laid between the rails of the railroad, which wires are constantly in contact with the locomotive engines by means of feelers, which descend from the locomotive engines and pass along the surface of the wires. According to the miniature exhibition of the models of the machines, the invention is conclusive as to safety and efficacy; but whether it would be equally successful in actual operation, it is impossible at present to say. The wires are not affected, as conductors of the electric fluid, by wet or damp. 286 OBJECT OF ROTARY ENGINES. This fact has been ascertained at the above Institution, by passing them through the tank, and then communicating the fluid. IV. OF ROTARY ENGINES. 193. Object of Rotary Engines.-It occurred to Watt that, in cases in which motions round an axis are required, as in workshops and manufactories, steam vessels might be constructed in the form of hollow rings or circular channels, with proper inlets and outlets for the steam, mounted on horizontal axles, like the wheels of a water mill. By such an apparatus it was proposed to dispense with all the machinery which, in the common steam engine, is interposed between the steam and the axis of motion, for the purpose of converting the alternate rectilinear into the circular motion, as the cylinder and piston, the beam, and the crank. It was thought that a loss of power is sustained by the use of the crank, and that a circular motion, accompanied with the development of the greatest power, might be at once produced, by causing the steam to act immediately upon projections on the circumference of the wheel, and to follow it throughout its entire revolution. 194. Objections to Rotary Engines.—Mr. Scott Russell has pointed out the fallacy of this project. In a paper which appeared in the Transactions of the Scottish Society of Arts, he states that, in theory, the ordinary crank has not the defects usually attributed to it, and which it is the sole object of the rotary engine to remedy-"because, 1, the velocity of the crank is in the inverse ratio of the pressure upon it; because, 2, the mean pressure on the crank during the whole revolution is less than the pressure on the piston, only in the proportion in which the whole space moved over by the latter is less than the space described by the former, so that the whole effect is equal to the whole power; because, 3, the steam is not at all expended at the neutral points, and because its expenditure is at every point exactly |