Several of the arrangements just described, were not conceived untll a long time after the period at which we have arrived ; but as the explanation may lead to a clearer understanding of the subsequent advances of the engine, it was thought better to step a little out of the regular course of narration. ilesides, the reader will at once perceive that the engine, as thus improved, became an almost self-regulating power, freed from the capital difficulties which had heretofore beset it.
With these grand outlines of an excellent work before him, Mr. Watt, sup ported by the enterprising spirit and pecuniary resources of his partner Bolton, could now turn his exclusive attention to the perfection of the engine. Another most valuable improvement had suggested itself to his mind sg far back as 1769, but it was not carried into !lull effect by him until nine years subsequently and became the subject of a new patent, granted to him in March 1782, for an expansion engine.
With an engine constructed on the •old plan, it was necessary that a careftd estimate should be made of the work to be performed, so that it should fully exe cute it(task without its power exceedhig the load to an extravagant degree ; because such a circumstance would occasion a motion so rapid, that, acting alternately in opposite directions, no building or machinery could withstand the jolts and shocks produced thereby. Many engine. bad been shattered by the pumps drawing air, or by a pump rod breaking, by which accidents the steam piston descended with such rapidity and violence that every thing gave way before it.
Besides the waste of fuel, the difficulty of management, and increase of cost, hi an engine so constructed that it shall be superior to its ordinary tasks, were great drawbacks upon its general efficiency. The expansion engine, however, removes these disadvantages, as it can at all times be precipely proportioned, at least during the working stroke, to the load of work that then happens to be upon it. This is effected by shutting of the further entrance of steam from the boiler when the piston has been pressed down in the cylinder for a certain pro portion of its total descent, (say one-half, one-third, one-fourth, and so on) and then leaving the remainder of the descent to be accomplished by the expan sive force of that steam which is already introduced into the cylinder. By this
means the acting force becomes regulated, and the pins are so placed in the plug frame as that the valve shall be closed at the moment the piston arrives at its prescribed limit in the cylinder. The piston first pressed down by steam from the boiler to this extent, the same body of steam expands; and though diminished in elasticity, it will be sufficient to complete the full descent of the piston.
When the steam is shut of at a portion of the descent in this manner, the pressure on the piston is continually diminishing, as the steam becomes more and more rare ; and consequently the accelerating force which works the engine diminishes. The motion of the descent, therefore, will no longer be uniformly accelerated, but rather retarded ; and by contriving the connecting machinery in such a way that the chains or at the outer end of the beam shall continually exert the same pressure to lift the pump rods, or that the machinery shall vary its force according to the necessity, the force of the piston upon the beam and pump-rods can be regulated at pleasure, and be made to produce an uniform effect.
It is remarkable that the endeavours of Mr. Watt to economize steam, and equalize the descent of the piston, by the mode just described, should have led to an accidental discovery of great value, which, without giving the elaborate calculations that demonstrate the fact, may be thus explained. He found that steam 'admitted into a cylinder to one-fourth of its depth, and exerting a pressure of 6333 pounds, when allowed to expand into the whole capacity of the cylinder, added a pressure of 8781 pounds ; and moreover, that had the cylinder been filled with steam of the same force, and exerting the accumulated pressure (6333 x 4) 25.332 pounds, the steam expended in that case would have been four times greater than when it was stopped at one-fourth ; and yet the accumulated pressure was not twice as great, being nearly five-thirds. One-fourth of the steam performs nearly three-fifths of the work, and an equal quantity performs more than twice as much work when thus admitted during one-fourth of the motion.