To comprehend the necessity for a rapid condensation, it must be remembered that the effective power of the engine depends on the pressure on the piston minus any resistance it encounters and on the space through which it moves. If the steam could be instantly con verted into water, and so entirely removed; a perfect vacuum would be formed beneath the piston, in which case, there being no resistance from this source to overcome, a maximum of power would be obtained; but if the condensation be slow, or only partial, since the piston will begin to move the instant there is any inequality in the pressures exerted on its opposite surfaces, its motion will be retarded, or the power diminished, by the resistance to compression offered by the uncondensed steam ; and although that resistance would tend to diminish as the condensation proceeded, yet the space occupied by the steam diminishing in consequence of the descent of the piston in nearly the same proportion, the resistance would be nearly constant through the whole of that descent.
On the other band, to maintain the temperature of the cylinder as high as possible, Watt, at first, cased it in wood to retard the radiation, and subsequently surrounded it by a second iron cylinder, admitting steam from the boiler between the two. This casing, or "jacket," as it is termed, is not used in most modern engines made since Watt's time, for reasons which will hereafter appear ; and the effects of radiation from the surface of the cylinder are now chiefly guarded against as much as possible by keeping that surface bright and smooth.
The second of Watt's improvements on Newcomen's engine consisted in closing the cylinder at top, the piston-rod being made to pass through a cylindrical neck in that top, termed a stuffing-box, from the passage being rendered steam-tight by a stuffing of tow saturated with grease, which by its lubrication diminished the additional friction resulting from this arrangement. The object of this alteration was to admit of the elastic force of the steam being employed to impel the piston downwards, instead of simple atmospheric pressure. For this pur pose, the steam wax admitted from the boiler above the piston at the same moment the condensation took place in the condenser; the steam-passage being made double for the purpose, so that the communi cation with the condenser could be cut off when that with the cylinder was opened,alternately. When the piston had descended to the bottom of the cylinder, the counterpoise at the pomp-rod raised it again, as in Newcomen's engine ; but to allow of this upward motion, it was neces sary to remove the steam which was above the piston, and this was done by allowing it to pass under the piston, and into the condenser through a passage opened at the proper instant for this purpose. Such is the general principle of Mr. Watt's single-acting engine, which hence became a steam-engine, and was no longer an atmospheric one.
By a further improvement, the counterpoise at the pump-rod was done away with, which obviously had been so much added to the unproductive work of the engine, since this weight had to be raised in addition to that of the water. The upward stroke of the piston was now produced by admitting the steam below it, to act by its elasticity, as it had previously done above when causing the piston to descend : thus the engine became double-acting, and assumed that essential general principle which it has ever since maintained, although all the details of its construction have been improved upon by successive engineers.
The changes in the engine introduced by Watt created the necessity for two pumps, and commonly three, which are worked by rods attached to the beam : the first of these is the hot-water or air pump, intended to remove the air, condensed water, and steam from the condenser, in which they would otherwise accumulate, and finally stop the action; for this water cannot flow away into an open cistern, as it had done in Neweemen's engine, since by Watt's principle it is essential that the condenser should be as steam-tight and as perfectly closed as the cylinder, or else the steam could not exert a pressure greater than that of the atmosphere, as it is intended to do in order to increase the effective force of the engine. The second is a force-pump, required to return the water drawn from the condenser by the first back to the boiler, as will be hereafter explained; and the third, termed the cold water pump, is that alluded to in a preceding paragraph as supplying the cold-water cistern which contains the condenser.
Having thus explained the general principle of the engine, some of the details of its construction must now be considered, and the piston [Hvaaanaies] may claim our first notice, both from its paramount importance and the practical difficulties to be overcome in its forma tion. In hydraulic machines, all vessels, pipes, valves, &c., must be made water-tight : in Bramah's pump, for example, the efficiency of the engine entirely depends on the accurate fitting of parts moving in contact, which must be perfectly water-tight, though subjected to a pressure of many hundred pounds on each square inch of surface ;— the utmost perfection of skill in workmanship is requisite to ensure this object, and the difficulty is obviously considerably increased when steam or gases are the fluids to be dealt with. Now the piston of a steam-engine must be steam-tight, and yet move with a minimum of friction in the cylinder ; and as this latter, from defective workman ship, can never be a perfectly true one, the cylindrical periphery of the piston must be so contrived as to be capable of adapting itself to every inequality in the surface against which it slides : this is effected in common pistons by their being made two inches or more less in diameter than the cylinder, leaving a projecting flange at the bottom, which, together with a top plate bolted to them, accurately fits the cylinder. Tow or soft rope, saturated with grease, is carefully wound round the cylindrical body of the piston, between the upper plate and lower flange; the former is then drawn up by screws, compressing the intervening packing till it perfectly fits the cylinder, and yet by its elasticity allows of its adapting itself to the inequalities in the surface. The first (fig. 2) of the annexed figures will explain the details of the construction of the ordinary piston.