77 



STEAM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



798 



hitherto proposed, the principle has been to admit the steam to act 

 on a fan or fans revolving round an axis of the cylinder ; and, by 

 ingenious excentric movements, the surface of these fans is made to 

 increase aa the steam diminishes in elastic force from the enlargement 

 of the space it occupies. Many such engines have been used for a 

 time, but commonly after a few years' trial they have been abandoned, 

 and the reciprocating principle substituted, thus proving that experience 

 confirmed the deductions from theory. 



In all mechanical combinations, the object to be effected necessitates 

 a certain characteristic form of the machine, which it retains, whatever 

 improvements may have been successively introduced either in its 

 principle or in the details of its construction. We can recognise in a 

 modern Sussex plough the general form of that used by the subjects 

 of the Pharaohs to till the banks of the Nile ; and Newcomen would 

 acknowledge a marine engine made by Maudslay and Field as a 

 descendant of his atmospheric one : but he would for some time be at 

 a loss to tell the object of a locomotive engine of Stephenson's, if he 

 could see it at rest only, and the connection between it and its tender 

 would be beyond his comprehension. The reason of this is, that a 

 locomotive engine is a perfectly new application of the power, having 

 no other analogy to an ordinary engine except that steam is the source 

 of power in both ; but all locomotive engines will hereafter possess a 

 family likeness. 



The principal causes of this novelty of form are, that great velocity 

 being the object, the boiler must constitute the greatest part of the 

 bulk, in order to supply a sufficient quantity of steam to meet the 

 demauds of the two pistons making 200 strokes per minute, and even 

 then it requires a tender to accompany it to carry a supply of fuel and 

 water to keep up the quantity. The locomotion is usually produced 

 by a pair of itriving-ickeeli made to revolve by the engine by means of 

 cranks on their axle, receiving motion almost directly from the piston- 

 rods ; the adhesion which takes place between these wheels and the 

 edge-ban or rails, causes the carriage to move on, just as the paddle- 

 wheels of a steam-boat propel the vessel by the resistance of the water 

 to the float-boards. The driving-wheels of a locomotive engine do not, 

 any more than the paddle-wheels of a steam-boat, act as fly-wheels to 

 regulate the velocity of the engine ; this is effected in the former by 

 the inertia of the whole mass ; the great velocity consequently ensures 

 the steady action of the engine. In the steam-boat this great velocity 

 of the paddles is unattainable, and consequently two powerful engines 

 are requisite to maintain even a moderate velocity ; but air being the 

 medium iu which the driving-wheels of a locomotive engine act, almost 

 any velocity can be imparted to them, and for obvious mechanical 

 reasons ia best attained by steam acting with a force almost amounting 

 to impact on small pistons through a short stroke, two alternately 

 acting cylinders being necessary, as in the steam-boat, to equalise the 

 action of the cranks. 



The two internal cylinders are moat conveniently placed under the 

 boiler in nearly a horizontal position ; intermediate cranks preserve the 

 rectilinear motion of the piston-rods, and connect them with the cranks 

 on the axle; while excentric frames on this axle actuate the simple 

 slide-valves required in a non-condensing engine. The steam which is 

 driven out of the cylinder at each stroke, instead of escaping directly 

 into the open air, passes into the funnel of the furnace, and thus 

 increases the draught ; but as the action of the engine cannot be 

 umli rxtood independently of the boiler, it becomes necessary to turn 

 our attention to that subject, without a general description of which, 

 any account of the steam-engine would be incomplete. 



I'pon the South-Western Railway of England, and on the Chemins de 

 For de 1'Ouest of France, the engines employed are made with external 

 cylinders, and the piston-head is connected with the crank fixed on the 

 driving-wheel by a link or connecting-rod. There are great advantages 

 in this system, so far as economy of construction is concerned ; for the 

 crank" are simply keyed on the ends of the axles of the driving-wheels, 

 instead of being forged upon the axles themselves, and the whole of 

 the machinery is of a simpler and more efficient character ; but at the 

 same time the condensation in the cylinders is far more serious than it 

 would be in the case of internal cylinder engines, on account of the more 

 exposed position of the external cylinder engines. The latter engines, it 

 may be added, present the advantage of having a smaller dead weight, per 

 hone-power exercised, than is the case with internal cylinder engines. 



Since some of the principal objects to be kept in view in the con- 

 struction of boilers are incompatible with each other, one or more must 

 be dispensed with in order to secure the rest. The specific purpose 

 for which the engine is constructed must determine the general prin- 

 ciple nl the boiler : thus, rapid generation of steam, security, compact- 

 ness, and lightness must be aimed at in boilers for marine or locomotive 

 engines, even at the cost of a comparative waste of fuel ; while for those 

 intended for pumping or driving machinery, economy of fuel mu.st be 

 the paramount object, the weight, form, and space occupied by the 

 boiler being secondary considerations. And whenever steam of a high 

 elastic force is to be used, that form of boiler should be adopted which 

 will moot nearly equalise the strain on it. The following requisites are 

 therefore to be understood as being those which it would be desirable 

 to combine, though, united, they are unattainable in practice. 



First, the boiler should have the greatest capacity with the least 

 lurface, to save material, diminish the weight, and increase the strength : 

 hence a spherical form would be best in this respect, but it is incom- 



patible with an economical application of the heat to a great extent of 

 surface, which is essential to the rapid generation of steam. 



Secondly, the form should be as simple as possible, both for the 

 sake of reducing the expense of construction and most readily admitting 

 of repair. Boilers are made of iron or copper plates riveted together 

 at their edges ; and if one of these is cracked, or has been burnt, that 

 plate can be taken out and another put in without pulling the whole 

 to pieces, which must be done when the boiler is of a complex form ; 

 added to which, all angles are sources of weakness, owing to the 

 inequality of the strain on the adjoining surfaces, and the injury done 

 to the metal by bending it to form the angle. 



The form of boiler used for Savery's, Blakey's, Newcomen's, and 

 other engines of the 17th and 18th centuries, up to the time of Watt, 

 was that of an inverted frustum of a cone, with a spherical top, and its 

 bottom slightly concave. This boiler was set in brick-work like a 

 common copper, the flame playing round the whole of the lower part. 

 The steam -pipe was connected in the usual way to a flange of a collar 

 in the spherical head. Watt adopted a long rectangular form, with a 

 semi-cylindrical top ; the ends were flat and upright, the sides slightly 

 curved inwards, as was also the bottom. From this form it is termed 

 the waggon-head boiler. It is set in a rectangular mass of masonry, 

 the cylindrical head alone projecting above the level of the brick- work ; 

 the fire-place was underneath one end of the boiler, and extended 

 backwards for one-third of its length ; the flue, after proceeding to the 

 further end, returned along one side, across the end, over the furnace, 

 and along the other side into the chimney-shaft, the boiler itself every- 

 where forming one side of the flue, and consequently having the flame 

 and heated air directly in contact with it at the bottom and sides. In 

 some cases, when the boiler was very large, a cylindrical iron flue was 

 formed through the boiler longitudinally, opening at each end into and 

 forming a continuation of the brick one, thus increasing the surface to 

 be acted on by the heat. 



The next important modification in form was that of making the 

 boiler entirely cylindrical with hemispherical ends, which is probably 

 the form best fitted to combine as many requisites as possible. With 

 this form the furnace is often placed in a second cylinder within the 

 boiler, and forming the first part of the flue : thus the fuel being 

 entirely surrounded by the water, little or no heat is lost by radiation ; 

 but there are serious objections to this practice on the score of acci- 

 dents, as well as the deficiency in draught, owing to the confined space 

 in which the combustion takes place. 



When an engine is of such a size as to require more steam than one 

 boiler of the ordinary magnitude can supply, it has two or more, set 

 side by side, communicating with a common steam-pipe. Since the 

 extent of surface exposed to the pressure of the steam, and therefore 

 the liability to bursting, increases in a greater ratio than the capacity, 

 there is obviously a limit to size, which can never safely be surpassed, 

 while the security is proportionally increased by diminishing that capa- 

 city : hence the necessity for using two or more small boilers instead 

 of one large one ; and the principle, carried to its limit, constitutes 

 that of the lafmlar boiler, in which the steam is generated in a series of 

 independent metal pipes of small diameter, all communicating with a 

 common steam-chamber or reservoir, itself small, and strong enough to 

 resist great pressures. 



These tubular boilers, however, have not come into general use, not 

 only on account of their complexity, and consequent liability to derange- 

 ment, but because, unless in the case of locomotives, or steam-boat 

 engines, there does not exist any demand for steam of such high 

 pressure as they are intended to generate. 



Since, in accordance with the general hydrostatic law, every unit of 

 surface of the boiler has to sustain the same pressure, if a small portion 

 of that surface can be opened by the pressure of the steam, when it 

 has attained that which the boiler was only intended to withstand, by 

 the escape of a quantity of the vapour at this orifice, the elasticity of 

 the remainder is again reduced below the limit. This is the object of 

 the safelij-vaire, which is such an aperture, kept closed by a valve 

 retained in its seat by a weight calculated to yield to, or be raised by, 

 the pressure the moment the steam exceeds its proper elastic force. 



The safety-valve was first contrived by Papin, and used in his digester 

 and boilers, and has ever since constituted an essential appendage to 

 every boiler. In its simplest form it is an obtuse conical valve, kept in 

 its seat (which is at the end of a short collar, standing up from the 

 surface of the boiler) by a weight acting at the end of a lever, resting 

 on the spindle of the valve, and having its fulcrum or hinge at the 

 other extremity. The effective weight by which the valve is kept 

 down may be varied by shifting the position of the weight on the arm 

 of the lever ; and as this alteration might be unintentionally made by 

 carelessness or accident, the valve should be inclosed in a box under 

 lock and key, to prevent its being tampered with. A chain attached to 

 the valve, and accessible to the engineer, should be provided, to enable 

 him to raise the valve, to ascertain that it is in efficient order, and has 

 not become fixed in its socket. But the best mode of applying the 

 weight is directly ou the valve, so that it cannot be increased, as long 

 an it is inclosed, by any accidental alteration in its position. In loco- 

 motive engines the weight would be liable to derangement by the 

 motion, and a spiral spring is employed to keep down the valve. 



The safety-valve is perfectly effective as long as it is free to rise in 

 its seat, and is loaded with a constant weight, which ought never to be 



