789 



STEAM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



790 



could not be used when the piston had to raise the beam, as it had to 

 do in Watt's engine, instead of being raised by it, as in Newcomen's. 

 The object of these " arched heads " is attained in modern engines 



Fig. 9. 



Fig. 10. 



by a system of simple rods or levers, BO combined that one point may 

 move in a straight line nearly. There are a variety of combinations 

 by which this may be effected, but that termed the " parallel motion," 

 invented by Mr. Watt for the purpose, is the only one which need be 

 here noticed, as being that most commonly used. 



The geometrical principle of this motion is shown in fiy. 9 and 10 as 

 well as the whole arrangement when the piston is near the top of the 

 cylinder : D D are rods fixed by one end to the frame supporting the 

 beam, while the three other pair of levers being jointed together and 

 to the beam, must obviously, in every position, form a parallelopipedon, 

 whence the name is derived ; p is the piston-rod attached to H ; Q that 

 of the hot-water pump connected with the parallel motion at I in the 

 centre of that side. 



When the engine is employed to drive machinery of any kind, a ///- 

 vlietl become* a necessary adjunct to it. A fly-wheel is one in which 

 the principal quantity of the matter is distributed in the periphery ; 

 when such a wheel revolves on an axis perpendicular to its plane, the 

 greatest quantity of matter moving with a maximum velocity, the 

 momentum of the whole is a maximum, while its inertia, if it be large, 

 causes it to control, or equalise, the motion of the machinery through 

 which it receives its own. It is the momentum of such a wheel which 

 constitutes the disposable force available for the multifarious purposes 

 to which machinery can be applied ; so that in the case of the steam- 

 engine, although the elasticity of steam is the original source of power, 

 the immediate one by which the work is executed is the momentum 

 of the fly-wheel 



It is consequently necessary to adapt some contrivance to the end 

 of the beam, which shall convert the alternating circular motion of the 

 latter into a continuous one of the fly-wheel ; this is effected by the 

 rod and crank, a piece of mechanism of such frequent occurrence that 

 it is unnecessary to describe it ; the treadle of a lathe is a familiar 

 instance of its application, and for a similar purpose, that of connecting 

 the alternate motion of the turner's foot with the continuous one of 

 the wheel of the lathe ; the principle of the treadle, or rod and crank, 

 is in fact the only one by which an alternating can be converted into 

 a continuous circular motion ; it must therefore be employed, notwith- 

 standing the variation in the power transmitted by means of it, conse- 

 quent on that of the angle formed by the rod and crank with each 

 other. Thus, for example, when the rod and crank are in the same 

 direction, which occurs twice at every rotation, no force whatever is 

 transmitted by it, and the primary one is entirely suspended or held 

 in equilibrium by the resistance of the axed centres on which the 

 crank and rod turn. 



In the steam-engine the rod and crank are so adjusted that these 

 two neutral positions occur when the piston is at one or the other end 

 of the cylinder, and the valves are so arranged that, both steam passages 

 being closed, all communication between the engine and the boiler is 

 cut off; otherwise the steam, which could not under these circum- 

 stances move the piston, would exert its force to the detriment of the 

 machine ; as soon, however, as the momentum of the fly-wheel has 

 carried the crank past these positions, the motion reciprocally imparted 

 through it to the piston and valves admits the entrance of the steam 

 from the boiler into the cylinder again. It is one of the important 

 details in the construction of the engine, that the piston should be in 

 that point of iU course when the steam exerts its maximum of effect 

 on it, at the time when, the rod and crank being at right angles 



to each other, the maximum of force may be exerted to turn the 

 fly-wheel. 



Since the diameter of the circle described by the crank must be 

 equal to the length of the stroke, or to the distance through which the 

 piston moves, it might be thought advantageous to increase the length 

 of the stroke, as admitting of a longer crank ; but there are limits 

 to this length, determined by a variety of circumstances, Borne of which 

 will be hereafter explained. 



When^Watt substituted the elastic foroe of steam for the pressure 

 of the atmosphere, he introduced a source of power which might be 

 increased to an indefinite extent, provided it were found advantageous 

 to employ it ; and the question naturally suggests itself, what is the 

 elastic force or pressure at which the maximum of useful effect can be 

 produced with a minimum expenditure of fuel? Unfortunately no 

 direct answer can be given ; in mathematical language, the unknown 

 quantity is a function of too many variables to be capable of deter- 

 mination, except by repeated experiments for every specific engine, 

 this quantity varying with the principle of its construction, even with 

 the details. The results of such experiments seem to show, that 

 generally it is more advantageous to employ steam of a comparatively 

 high elastic force ; accordingly the pressure was increased, in engines 

 constructed by Watt, from 4 to 8 or even 12 Ibs. on the inch, the appre- 

 hension of danger from the explosion of boilers in which steam of high 

 pressure was generated constituting the chief limit to a further 

 extension of the practice ; at the present day condensiug engines are 

 even worked as high as from 25 U> 30 Ibs. pressure. The nature of 

 those improvements in the construction of boilers will be briefly 

 explained hereafter, by which steam of 200 Ibs. on the inch may be 

 generated, if requisite, with nearly as much security as that of 4 Ibs. in 

 the earlier boilers ; but at present, simply stating that such is the case, 

 we proceed to explain some important changes which have beeu con- 

 sequently made in the principles of the engine. 



When the steam is firp t admitted into the cylinder, the total space 

 filled by the steam is immediately augmented by that through which 

 the piston moves ; and if the capacity of the boiler were not several 

 times greater than that of the cy Under, the consequence would be a 

 gradual diminution of the pressure, supposing the total quantity to 

 remain the same : but the moment the pressure in the boiler tends to 

 dimmish, an additional quantity of water passes into the state of 

 vapour, of the same tension as that previously generated, provided the 

 temperature be maintained ; hence the pressure on the piston may be 

 regarded as sensibly the same throughout the whole of its stroke, 

 provided that pressure be somewhat greater than that of the atmo- 

 sphere, and the [communication with the boiler remain open. It must 

 not, however, be supposed that the pressure on the piston is the same 

 with that of the steam in the boiler ; all that is here asserted is that 

 the pressure on the former will be uniform. 



But if the pressure be considerably greater than that of the 

 atmosphere, the steam, even when separated from the water, while 

 expanding in the enlarging space formed by the motion of the piston, 

 will exert sufficient force to continue that motion, till at last the 

 pressure diminishing inversely as the space increases, and directly as 

 the temperature, according to Mariotte's and Gay-Lussac's laws, that 

 pressure will finally be not in equilibrium with the resistance, and all 

 motion will cease. This is the important principle of working engines, 

 originally proposed by Watt, though not employed by him, but 

 which now, from the improvements in boilers above alluded to, is 

 becoming general under the name of that of expansion. In the 

 common engine, if the pressure on the piston continue uniform 

 during the stroke, as it would do if the communication with the 

 boiler remained open, the piston would move with, an accelerating 

 velocity till it arrived at the end of the cylinder, when the motion in 

 that direction being suddenly stopped, the momentum must be ex- 

 pended on some of the fixed points of the machine, to its manifest 

 injury, and with the useless expenditure of so much power; accord- 

 ingly the communication with the boiler is now always cut off when 

 the piston has arrived at a certain point, and with a momentum 

 sufficient to carry it to the end of its stroke without any useless ex- 

 penditure of force, while the steam behind it, which was originally of 

 but a few pounds pressure above that of the atmosphere, thus limited 

 in quantity, rapidly declines in force, and ceases to urge the piston on. 

 But on the " expansion principle," when the steam possesses con- 

 siderable elastic force, the communication with the boiler may be cut 

 off much sooner, and the piston is urged forward by the expansive 

 force of the steam, which, although decreasing as the space increases, 

 is yet sufficient to carry the piston to the eud of the stroke. 



If it be asked how it is advantageous to use half the quantity of 

 steam at twice the pressure, when it takes perhaps twice the quantity 

 of fuel to raise the steam to the double pressure, the answer is, that it 

 can be shown analytically that the total force exerted by steam acting 

 expansively is greater than that which would be exerted by steam of a 

 constant pressure, equal to the mean of those exerted, first, at the 

 moment the steam-valve is closed, and, secondly, when the piston 

 arrives at the end of its stroke ; consequently, as less steam may be 

 used to produce the required effect, a saving of fuel is the result, or in 

 other words, the quantity of steam may be much less than half, at 

 double the pressure, or the pressure much less than doubled, to pro- 

 duce the same effect. 



