vM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



more >K '" one-third the pressure the boiler U just capable of with- 

 standing. The rapid diminution in the number of accident* from 

 explosions, notwithstanding the increased employment of steam, suffi- 

 ciently proves that they are nine times out of ten caused by gross 

 negligence or culpable recklessness ; but to obviate as much as possible 

 the recurrence of explosions, every boiler should have at least two 

 safety-valves, both secured from access, and yet both capable of being 

 raised by hand from time to time : one should be loaded with a less 

 weight than the other, that by the escape of the steam from it the 

 engineer may be warned to reduce the quantity of steam generated, by 

 " damping " his draught ; and the other safety-valve should be only 

 loaded with a weight equivalent to one- third the pressure which, by 

 computation founded on actual experiments, would bunt the boiler, if 

 made with metal of a given thickness ; for, however carefully the boiler 

 may have been made, it is impossible to ensure equal strength of it in 

 every part. 



A plan was originally suggested by Trevithick for insuring safety 

 from a boiler by inserting in it a plug of metal, which, melting at the 

 temperature attained by the steam when its tension became dangerous, 

 might open an exit for it. Thin plan is adopted in France, but besides 

 that it is repugnant to our ideas of mechanical fitness, it is liable to 

 many objections ; none of the pure metals melt at a temperature suffi- 

 ciently low to be available, and all the fusible alloys soften long before 

 they melt, and vary in these respects with minute differences in the 

 proportion of their ingredients, so that the plug might be driven out 

 before the proper time. 



The boiler, besides the danger of bursting from over-pressure of the 

 steam within it, is also liable to injury by the external pressure of the 

 air, if the steam within be condensed, an it must be on cooling, when 

 the fire is let out, thus causing a comparative vacuum in the boiler. 

 To guard against this, there should be a safety-valve to act in the direc- 

 tion opposite to the usual one, which, yielding to the pressure of the 

 atmosphere, would allow of the entrance of the air, when this pressure 

 exceeded the one exerted by the steam to keep the valve closed. 



Another source of accident, which should be guarded against most 

 sedulously, is the formation in the boiler of a deposit of the earths, &c. 

 chemically united with the water or held suspended in it, and which 

 are deposited from all water when long kept boiling in any vessel, as is 

 illustrated and proved by the furring of old tea-kettles. This deposit 

 is intensely hard, and adheres so closely to the metal, that it requires a 

 chisel and hammer to detach it when accumulated to any thickness. 

 Being a bad conductor of heat, it prevents the rapid generation of 

 steam, and by not allowing the water to be in contact with the metal, 

 so as to carry off the heat imparted to the latter, the metal gets red-hot 

 and is burnt, or, in chemical language, becomes oxidised by long 

 exposure to a high temperature. If, under these circumstances, a 

 fissure should be produced in the earthy crust, the water, suddenly 

 admitted to contact with the red-hot iron, is converted instanta- 

 neously into steam of such high pressure as to risk the bursting of the 

 boiler. 



It U one of the advantages held out as an inducement to their 

 adoption by the inventors of tubular boilers, that, owing to the .in- 

 equality of the temperature of the liquid at different distances from the 

 source of heat, a circulation U continually going on, which mechanically 

 prevents the formation of a deposit, while in large open boilers no such 

 cause can operate to any extent. In these the remedy appears to be 

 the frequent cleaning out of the boiler, to prevent the accumulation of 

 the deposit, and the admission of the water which supplies the waste 

 to the part not over the furnace, which should be separated from the 

 rest by a partition extending upwards nearly to the level of the water ; 

 the fresh water is thus allowed to deposit its sediment in the part 

 where it can be least productive of the evils alluded to, and whence it 

 may be removed collectively from time to time. 



To indicate the actual pressure at any time of the steam within the 

 boiler, this is furnished with a gauge (fig. 12), consisting of a bent tube, 

 open at both ends, one orifice A opening into the boiler. This tube con- 

 tains mercury, which will obviously be at the some level in both branches 

 when the steam in the boiler is of the same pressure as the atmosphere, 

 but will rise in the longer leg as the pressure of the steam increases, 

 and will thus by its altitude indicate that pressure. If the longer leg 

 be of a certain length, the mercury would flow over, or be blown out ; 

 altogether, if the steam were suddenly to increase in its elastic force, 

 the gauge would in this case act as a safety-valve. The loss of the 

 mercury may be guarded against, under these circumstances, by a 

 cinteni placed round the orifice of the gauge to receive it, aa shown in 

 the figure. 



A water-gauge is often used instead of a mercurial one, to save the 

 expense of this metal ; but then the tube must be long enough to 

 allow a sufficient column of water to balance the pressure of the 

 team ; and by making the tube of sufficient diameter, this water-gauge 

 then constitutes an efficient safety-valve. As these gauges cannot, for 

 obvious reasons, be made of glass, to allow of the height of the fluid 

 within them being directly observed, this height is indicated by a light 

 wooden rod projecting beyond the end of the gauge, which floats on, 

 and therefore rises and falls with, the mercury or water. 



In locomotive-engines, where the use of a mercurial and still more 

 of a water-gauge it impracticable, the same end is attained by a ther- 

 mometer, on the well-known principle that the temperature of steam 



U always in a constant relation to its pressure ; or by Bourdon's steam- 

 gauge, in which the steam acts upon a diaphragm, as in the aneroid. 



. 11. 



n 



I 



If there be not water in contact with that part of the boiler exposed 

 externally to the direct action of the flame and hot air in the furnace 

 and flues, the iron would become red-hot, and so suddenly increase the 

 pressure of the steam in contact with it, that an explosion would pro- 

 bably ensue ; and if not, the iron in that part would be more oxidised 

 or burnt. To prevent this it is necessary that there should be always 

 water in the boiler above the level of the highest part of the flues ; and 

 to enable the engineer to ascertain whether this is the case, there an- 

 in all boilers two pipes with cocks, one of which dips down into the 

 water, while the other reaches only to within a few inches of its surface 

 when at the right height in the boiler ; the consequence of this arrange- 

 ment is, that if the cock of the shorter pipe be opened steam will issue 

 from it, and water from the other when that is opened in its turn ; but 

 if steam escapes from both cocks, the engineer is warned that there is 

 not sufficient water in the boiler, and therefore directs his attention to 

 remedy the deficiency. 



The boiler is always supplied with water by the action of the engine ; 

 the hot water, pumped out of the condenser, is raised into a cistern 

 placed at a sufficient height above the boiler, by the force-pump men- 

 tioned in the general description of Watt's engine ; from this cistern :i 

 pipe passes through the top of the boiler, and reaches nearly to the 

 bottom, where it is bent at right angles ; the upper orifice in the cistern 

 is closed by a valve connected by a spindle-rod with one end of a simple 

 lever, from the end of the other arm of which a wire is suspended, sup- 

 porting a stone float in the boiler, the valve being weighted just to 

 counterbalance the specific gravity of the float; M the water subside* 

 in the boiler in consequence of its evaporation, the float falls and raises 

 the valve, allowing sufficient water to descend into the boiler till the 

 float, rising again, causes the valve to close ; the weight of the column 

 of water iu the pipe prevents the steam from escaping in that direei imi 

 during the action of the valves ; hence the necessity for the feeding- 

 cistern being raised sufficiently above the boiler. The lien.l at, tin: 

 bottom of the feeding-pipe is intended to cause the water, when i 

 near to the bottom of the boiler, to wash away the sediment which 

 would otherwise collect there. 



The locomotive-engine, as has been stated, requires a boiler of a form 

 and principle totally different from those of an ordinary one. 



The boiler A (/ff. 13) is a cylinder made of wrought-iron plates, rivet, -,1 

 together in the usual way, but it is covered with a wooden casing, t.i 

 prevent, as far as possible, the great waste of heat which would radiate 

 from a metal surface moving through the air with great velocity. At 

 one end of the boiler is the furnace B, consisting of a double cose, the 

 outer one of iron with a semi-cylindrical head, but quite open at 

 bottom ; within this is an inner square case of sheet copper, riveted all 

 round the bottom edge to the outer one, but leaving on its three sides 

 a space of three inches between them, which is filled with water, and 

 indeed forms a continuation of the boiler. The bottom of this inner 

 case is the grating on which the fuel is laid. r is the feeding door in 

 front, opening of course through both cases, which arc therefore riveied 

 together, so as to be steam-tight all round. A series of upwards of 

 100 brass tubes of small diameter pass from the back of the furnace to 



