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, some of which will be hereafter explained.
When,Watt substituted the elastic force 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 lbs. on the inch, the appre hension of danger from the explosion of boilers in which team of high pressure was generated constituting the chief limit To a further extension of the practice ; at the present day condensing engines are even worked as high as from 25 to 30 lbs. pressure. The nature of those improvements in the construction of boilers will be briefly explained hereafter, by which steam of 200 lbs. on the inch may bo generated, if requisite, with nearly as much security as that of 4 lira. in the earlier boilers ; but at present, simply stating that such is the case, we proceed to explain some important changes which have been con sequently made in the principles of the engine.
When the steam is firtit 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 cylinder, 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 diminish, 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 preisure 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 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'e 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 expansim 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 end of the stroke.