801 



STEAM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



the further end of the boiler, where they open into the chimney. These 



tubes, which are entirely immersed in the water, constitute the flue, 



Fig. 13. 



aii'l thus increase considerably the heated surface in contact with the 

 water, and therefore promote the rapid generation of steam. The 

 introduction of these tubes forms an epoch in the construction of 

 boilers, and was the suggestion of Mr. R. Stepheuson, and simultaneously 

 of M. Seguin, in 1829. [RAILWAY.] 



The cylinders of the engine, of which one only can be seen in the 



i . are fixed at the bottom of the chimney. The steam passes to 



the slide-valve by the pipe s, s, while the waste steam escapes up the 



funnel at T. The cylindrical vessel with the spherical head v, is called 



the steam-rhfi(, the steam-pipe terminating in it at s. The object of 



this arrangement ia to prevent the water, which is agitated by the 



'i, from passing through the steam-pipe to the cylinder, where it 



would be highly detrimental; the main steam-pipe divides into two 



branches in the chimney, one passing to each cylinder. 



The boiler is supplied with water from the tender by two force- 

 pumps worked by the engine, and has a gauge, try-pipes, safety- 

 valves, Ac., in common with other engines, though all peculiar in their 



"tion. 



This brief account of the boiler, and of its various arrangements, 

 will hardly be considered complete without some notice of the explo- 



to which it is liable. 



Tin- bursting of boilers presents very different phenomena, being 

 simple rent in the metal, allowing the harmless escape of 

 .Hid water ; and at others accompanied by an explosion in its 

 violence equal to that produced by gunpowder; it has hence been 

 ' iiivl that on these occasions some explosive gases are formed in 

 tin: I Boiler ; but this does not appear probable, nor is it reconcileable with 

 any known physical laws, while the elastic force of steam is capable of 

 indefinite increase, and is quite adequate to produce any mechanical 

 effect whatever. It is always difficult to get any satisfactory evidence 

 as to the facts of an explosion of a boiler ; the terror of the moment 

 j >! events the survivors from accurately recalling the phenomena imuie- 

 iliiti'ly antecedent, while those who from their proximity would be 

 best capable of affording this evidence, are either killed, or are too 

 interested in exculpating themselves to be impartial witnesses. With 

 regard to the formation of explosive gaseous compounds in the boiler, 

 it is generally admitted that hydrogen gas is the only ingredient of 

 such that can be formed ; and that is obtained by the decomposition 

 of the steam when in contact with the red-hot iron ; but pure hydrogen 

 is not explosive ; and to render it so it must be mixed with oxygen or 

 atmospheric air. It has been suggested that the latter may be intro- 

 duced along with the water by a defect in the feeding-pump ; but the 

 pn ^portions of the air and of the hydrogen must be definite to produce 

 an explosion ; and it is difficult to suppose that in such a situation 

 I'itln r xhonld continue to accumulate till the quautity is exactly that 

 necessary to produce an explosion. In short this explanation of the 

 subject is beset with difficulties which have not yet been removed, 

 though the attention of scientific men both in Europe and America 

 has been frequently directed to it. 



Whenever the steam-engine is employed to execute any work which 

 a variable in its quantity or intensity, there must be some means of 

 adjusting the force of the engine to this varying resistance it has to 

 overcome : if the engine were suddenly relieved of half that resistance, 

 I example, would be the case if half the machinery it was driving 

 were suddenly stopped or thrown out of gear, the engine, if impelled 

 by the nine quantity of steam, would immediately move with a rapidly 

 accelerating velocity, till that velocity were doubled, which would be 

 prejudicial to the engine itself and to the rest of the machinery it was 

 working, n well as be a useless expenditure of force or fuel. The 

 object of that beautiful piece of mechanism termed the governor, is to 

 enable the engine to regulate the supply of steam admitted to the 

 ABia ASD stl. Civ. vol.. vil. 



:ylinder, or in other words to adapt the force it has to transmit to the 



resistance it has to overcome. 



Fig. 11. 



The governor, the form and principle of which will be better under- 

 stood from the figure (jiy. It) than from any description, is made to 

 revolve on its vertical axis rt, by a baud passing round a pulley and round 

 one on the axle of the fly-wheel, or else by bevilled wheels, as shown in the 

 figure; so that its velocity of rotation varies with that of the fly-wheel. 

 If this velocity increase, the heavy balls 6 b diverge by the increased 

 centrifugal force, and cause the collar c to slide up the axis ; this by 

 means of the intermediate cranks partially closes the valve at v, in the 

 steam-pipe , termed the t/truttle-valve, and consequently diminishes 

 the quantity of steam passing to the cylinder : if, on the contrary, the 

 velocity of the fly-wheel is diminished by an increase in the resist- 

 ance, the balls of the governor collapse, aud the throttle- valve is opened 

 so as to admit more steam to the cylinder to augment the force in 

 proportion to the increased resistance. 



The effect on the governor is equivalent to varying the diameter of 

 a second fly-wheel, the circumference of which is represented by the 

 balls b b, consequently the governor itself, independently of its action 

 on the throttle-valve, has a tendency to effect the adjustment required ; 

 since a portion of the surplus force of the engine, on the first supposi- 

 tion, is absorbed in overcoming the increased inertia of the governor ; 

 while, on the second supposition, that inertia, being diminished, relieves 

 the engine of a portion of resistance. This accounts for the prompt 

 action of the governor in effecting the adjustment, which it does with- 

 out those fluctuations in the velocity of the piston which would be 

 prejudicial to the effect of the machinery actuated by this prime- 

 mover. 



In the marine engine no governor is requisite ; the resistance being 

 so great compared to the force, that the velocity can never be exces- 

 sive, and all the power the engine is capable of exerting is required to 

 turn the paddle-wheels ; added to which the resistance is nearly uni- 

 form, only varying with the draught of the vessel or the state of the 

 weather, but never diminishing below that originally calculated on aud 

 provided for by the power of the engines ; an increase of the resistance 

 accordingly produces a diminution in the velocity of the vessel, but 

 the engine can never work so fast as to be injured by its own velocity. 



The governor is equally unnecessary to the locomotive engine, since 

 the attendant must constantly have the engine under his control, aud 

 be watching its action ; he can therefore regulate the throttle-valve by 

 hand gear placed within his reach ; and in this case also, as with the 

 marine engine, the resistance is nearly constant, consisting chiefly in 

 the weight of the engine and the train of carriages to be moved, a 

 weight not liable to any sudden change during the transit, while the 

 momentum of the whole mass acts as a fly-wheel or regulator to 

 equalise the motion of the pistons. 



We have not entered into any investigations of the formula) for 

 determining the dimensions of au engine and of its boiler, the pressure 

 at which it must be worked, the velocity of the piston, &c., in order to 

 produce any proposed available or net force ; or into the account of 

 the experiments by which the amount of friction, the effects of the 

 uncondensed steam, the quantity of caloric obtained from different 

 kinds of fuel, &c., all of which must be taken into account in these 

 investigations, aud have been approsimatively determined. 



3 s 



