735 



STEAM AND STEAM-ENGINE. 



STEAM AND STEAM-ENGINE. 



7S1 



that through which the piston moved : the injected water and con- 

 densed steam-water flowed off into the cistern L through K, as the air 

 had previously done. The cock I was now closed, and F opened, and 

 the action was repeated, and when this engine was first introduced, it 

 was the duty of an attendant to open and shut these cocks alternately ; 

 but subsequently lever handles to open and shut the cocks were acted 

 on by pins or cams, carried by a rod suspended from a beam ; and the 

 engine became self-acting. This improvement was rudely made in the 

 first instance by a boy named Potter, for the purpose of saving himself 

 trouble ; it was subsequently perfected by an engineer named Beigh- 

 ton in 1718. 



Newcornen's engine was successively improved upon by Smeaton, 

 Brindley, and other engineers, previous to Watt's time, and from its 

 intrinsic merits it remained in general use under the appropriate name 

 of the " atmospheric engine " during the greater part of the last cen- 

 tury, but was only used for pumping water. 



The first and most important of Watt's improvements on the engine 

 consisted in effecting the condensation in a separate vessel, termed the 

 condfnter, which communicated with the cylinder. This condenser 

 being filled with steam from the boiler at the same time with the 

 cylinder, the jet of cold water, admitted into the former only, effected 

 the condensation of the whole volume of steam, both of that in the 

 cylinder as well as of that in the condenser, in conformity with the 

 well-known principle in physics, that an action originated in any 

 part of a homogeneous fluid is almost instantaneously communicated 

 throughout its mass. 



To effect still further the object of this separate condensation, Watt 

 placed his condenser in a cistern, the temperature of which was kept 

 constant by a fresh supply of cold water, brought from a well by a 

 pump, to be presently mentioned ; for otherwise, the heat given out 

 by the condensing steam would, by heating the vessel and the water 

 surrounding it, have prevented the rapid or almost instantaneous 

 condensation necessary to the efficient action of the engine. 



To comprehend the necessity for a rapid condensation, it must be 

 remembered that the effective power of the engine depends on the 

 pr ensure on the piston minus any resistance it encounters and on the 

 (pace 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 hand, 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 

 (team from the boiler between the two. This casing, or " jacket," as 

 it is termed, Li 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 aa 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 itu[Rg-box, from the passage 

 being rendered steam-tight by a stuffing of tow saturated with grease, 

 which by ita 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 was 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 

 if tin; cylinder, the counterpoise at the pump-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 iteam-enyine, 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 Mow it, to act by its elasticity, 

 as it had previously done abort when causing the piston to descend : 



* One cubic Inch of water occupies 1711 cubic inches of space, in tbe form of 

 tieam at tlf ; consequently, tbe space occupied by the water after the con- 

 densation may be neglected in the computation. 



ARTS AND SCI. DIV. VOL. TIL 



thus the eugine 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, iu 

 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 

 Newcomen'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, aa it is intended to do iu 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 

 [HYDRAULICS] 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 (Jig. 2) of the annexed figures will explain the details of the 

 construction of the ordinary piston. 



fig. 4. 



But as the friction of this common piston is necessarily very con- 

 siderable, the better class of engines have usually what are called 

 metallic pistons, of which there are different kinds, invented by Cart- 

 wright, Jessop, Barton, and others. The body of these pistons is 

 metal, made in pieces or segments, acted on by springs radiating from 

 a centre ; so that while the friction is diminished by both surfaces 

 being metallic, the piston, owing to its construction, can adapt itself to 

 thu irregularities of the cylinder. It is found in practice that these 

 metallic pistons wear for a long time, and do not of course require the 

 frequent repacking necessary to those with tow or hempen stuffing. 

 Figs. 3 and 4 represent a plan and elevation of an improved form of 

 Barton's piston, to explain the principle. 



In Newcomen's engine the steam was admitted to the cylinder, and 

 the communication again cut off by means of a cock of the common 

 construction ; but a more efficient contrivance is requisite when the 

 steam is to be admitted alternately both above and below the piston, 

 and to the condenser, as it is in engines since the introduction of 



Si 



