THE STEAM-ENGINE. 



The strap receives from the eccentric a circular 

 motion exactly similar to that given by the con- 

 necting-rod to the crank pin, and this motion the 

 rod transforms into a reciprocating one, anc 

 transmits to the valve. The process is exactly 

 the reverse of that by which the reciprocating 

 motion of the piston-rod is changed into a circular 

 one, and transmitted to the crank by the con- 

 necting-rod. The varieties of valve-gear and 

 expansion-gear are as endless as the varieties of 

 valves. Many of them are highly ingenious, and 

 many absurdly complicated, while not a few 

 possess both these qualifications. Any detailed 

 description of them, however, is unnecessary here. 



The Fly-wheel is a large wheel fixed on the 

 crank-shaft, and having a very heavy rim. Its 

 use is to equalise the motion of the shaft, and to 

 enable, by its momentum, the engine to pass what 

 are called the ' dead points ' these will be found 

 described in the article MECHANICS. 



The Condenser is simply a cast-iron box of any 

 convenient shape. The water for condensing the 

 steam is introduced into it in a jet in such a way 

 that its particles mix with the steam at once on 

 entering, and condense it almost instantaneously. 

 The 'surface condenser,' in which the steam is 

 condensed by contact with the surface of a number 

 of small tubes, through which a current of cold 

 water continually flows, is one of the most im- 

 portant of modern improvements, and will be 

 found described further on. It is, however, only 

 of use when the injection-water contains unvapor- 

 isable matter such as the salt in sea-water 

 which would otherwise be deposited in the boiler 

 as a crust, and do much harm. 



The Governor, shewn in fig. 11, is an ingenious 

 application by Watt of mechanism long used in 

 water-mills. Its object is to make the engine to a 

 great extent regulate its own speed, so that it shall 

 neither be pulled up altogether by a sudden in- 

 crease of load, nor 'race' when any part of its 

 load is suddenly removed. It consists essentially 

 of a spindle or upright rod, with a pulley, by which 



Fig. n. 



it is caused to revolve, fixed on it. Two bent 

 levers are pivoted on a pin near the top of the 

 spindle, and at the lower end of each is fixed a 

 heavy cast-iron ball When the engine is running 

 at its proper speed, the balls revolve with the 

 spindle in the position shewn ; but if that speed be 

 increased, the centrifugal force causes them to fly 

 outward, and consequently upward; and conversely, 

 if it be decreased, they fall downward towards the 

 centre. At the upper end of the spindle is a 

 system of levers, by which it will be seen that the 

 raising of the balls tends to close, and their lower- 



ing to open, the throttle-valve at the right of the 

 engraving. This valve is simply a disc of metal 

 placed in the steam-pipe near the cylinder. The 

 further, therefore, it is opened, the greater the 

 amount of steam admitted to the cylinder, and 

 vice versd, and so the tendency of the engine to 

 alter its speed arising from causes extraneous to 

 itself, is just balanced by the alteration made in 

 the amount of steam admitted through the throttle- 

 valve. 



In all marine and locomotive engines, as well as 

 in many stationary ones, it is essential that the 

 crank-shaft should be able to revolve in both 

 directions, and for this purpose they are fitted 

 with reversing-gear. The direction in which the 

 shaft of an engine revolves depends entirely on 

 the position of the eccentric which works the 

 valves in relation to the crank. The reversing- 

 gear which is now almost universally used is 

 called the 'link-motion,' and its construction is 

 shewn in fig. 12. In it two eccentrics are used, 



Fig. 12. 



called ' fonvard ' and ' backward,' according to the 

 direction in which their action on the valve causes 

 the shaft to turn, a is the rod of the fonvard 

 eccentric, and b the rod of the backward eccentric. 

 These are respectively jointed to pins at the top 

 and bottom ends of a curved link, in the centre of 

 which is a slot or opening. The valve-rod c has 

 a cross-head which slides between two parallel 

 faces, d, d, and is thus compelled to move in a 

 straight line. It has also a small block, e, at its 

 extremity, which fits the slot of the link. By 

 means of a rod, f, the link can be lifted or 

 lowered, and the position of the block of the valve- 

 rod in the slot can thus be altered. Thus the 

 .ink may be raised so as to make the block occupy 

 :he lowest part of the slot, as at C, or it may be 



owered so as to make it occupy the highest part, 

 as at B. A shews a position midway between the 

 other two. When the link occupies the position 

 B, the valve-rod receives the full motion of the 

 brward eccentric, and in consequence, the engine 

 turns in its usual direction. When, however, its 

 sosition is altered to C, the valve-rod receives the 

 iill motion of the backward eccentric, and the 

 engine consequently moves in the reverse direction. 

 [f the link is placed in any intermediate position, 

 the valve obviously receives only a part of the full 

 stroke of the eccentric. This (which is called 



linking-up') has been found to be equivalent 

 to increasing the lap, and the link-motion thus 

 serves (up to a certain point) the purpose of a 

 variable expansion-gear. 



437 



