STEAM BOILER STEAM CARRIAGE 335 and with the pressure rising from 2| to 4 at- mospheres. In a locomotive boiler, he esti- mates the time required to raise the pressure from 90 Ibs., the working point, to 175 Ibs., the assumed exploding point, at 3| minutes. A safety valve, of sufficient size and of good design, is the safeguard against such accidents, being so weighted that it will never allow the steam to rise above a pressure at which a proper "factor of safety" is given. ' Steam boilers should be designed with a factor of safe- ty of at least 6 or 8, but they are frequently, even when new, capable of sustaining with- out rupture no more than four times the reg- ular working pressure. As they deteriorate with age, the factor of safety is too generally allowed to decrease, until it becomes as small as legal requirements permit. This has usu- ally been far too low in the United States, and frequently boilers are legally passed by the inspectors when their factors of safety are less than 1-J. The accumulation of steam to an excessively high pressure is found to be most imonly due to defective pressure gauges, to itire deficiency of pressure indicators, and to the absence of or defects in safety valves. Boilers fail in consequence of deficiency of water, which, causing exposure of heating sur- faces to the heated gases without protection, permits them to become overheated and weak- ened. Braces and straps are weakened by cor- sion, and even entirely severed. Plates are racked by changes of temperature and irregu- expansion and contraction, or even burned as just described, and are blistered in conse- quence of defective manufacture. Deposits cover the heating surfaces, and, interposing a non-conducting coating between the metal and the water, permit overheating to take place even when the boiler is amply supplied with water. Corrosion produces extended and uni- form weakening of sheets, or, forming grooves along the lines of junction of the plates, creates long lines of weakness. The sudden evolution of steam, in such volumes that the pressure is increased too rapidly to be effectively relieved by the safety valve or by the supply pipe lead- ing to the engine, is considered by many au- thorities to be an occasional cause of explo- sion. This may be caused by the overheating of a portion of heating surface not in contact with the water (as when a deficiency of water occurs, or when the surface is heavily coated with scale), and a subsequent sudden return of the water into contact with the metal. The occurrence of the "spheroidal state" may, it is supposed, sometimes produce this effect when the liquid is restored to contact with the plate. (See BOILING POINT.) A committee of the Franklin institute in 183 3-' 6 experimented upon the first of the above named conditions, and found that very considerable accessions of pressure might be caused by the sudden return of the water upon overheated surfaces. The superheating of the water, as in experi- ments of Donny, Dufour, and others, is also 762 VOL. xv. 22 supposed to be a possible cause of explosions. The United States government has appointed a commission to investigate this subject. They have produced many explosions by over pres- sure, by injecting feed water upon overheated iron in boilers, but have not yet (1876) made their report. It is generally supposed by en- gineers that good design, good materials and workmanship, and skilful and intelligent man- agement, will almost invariably insure perfect immunity from danger of explosion ; but the phenomena of sudden evolution of steam in steam boilers have not yet been fully inves- tigated by any thoroughly scientific series of experimental researches. Steam boilers are usually tested at regular intervals. Careful and skilful inspection will almost invariably detect all serious defects. Every sheet should be exam- ined to discover blisters, lamination, fracture, or corrosion. The use of a light hammer, tapping its surface and following the seams, will gen- erally in practised hands reveal such defects and indicate their extent. All stays and braces should be carefully examined, and the boiler fittings, valves, and gauges should be inspected, and the last should be tested. After such an inspection and the repair of injured parts, it is considered by many engineers to be advisa- ble to subject the boiler to a hydrostatic test. This consists in filling it with water, and rais- ing the pressure to a point exceeding by one half or more the regular working pressure.. This form of test is prescribed by the laws of the United States regulating steam-boiler man- agement on steam vessels. (See BOILING POINT, EVAPORATION, HEAT, and VAPORIZATION.) See N. Burgh, "Steam Boilers" (London, 1871); E. B. Martin, "Steam Boiler Explosions" (London, 1871); L. Delvordre, Traite pratique mr les chaudieres d vapeur (Paris, 1872); Trowbridge, "Heat as a Source of Power" (New York, 1874); R. Wilson, "Treatise on Steam Boilers" (London, 1874); and J. Lau- rent, Chaudieres d vapeur (Paris, 1875). STEAM CARRIAGE. Road locomotives and traction engines have been frequently con- structed for the transportation of both freight and passengers, and for hauling wagons carry- ing heavy loads. The latter application only has been permanently successful, although re- peated attempts have been made to perfect steam carriages of high speed. As early as 1759 Dr. Robinson called the attention of Watt to the possibility of constructing a car- riage to be driven by a steam engine. The first actual experiment was made, as is sup- posed, by a French army officer, Nicolas Jo- seph Cugnot, in 1769. Encouraged by the partial success of the first locomotive, he con- structed a second in 1770, which is still pre- served in the conservatoire des arts et metiers, Paris. Watt patented a road engine in 1784. About the same time his assistant, Murdoch,, completed and made a trial of a model locomo- tive, driven by a " grasshopper engine," hav- ing a steam cylinder f in. in diameter and 2 in.
