392 



STEAM ENGINE. 



is also usual to work the hot water pump which 

 supplies the boiler, off the working beam, but in 

 this engine it is wrought off the parallel motion, 

 the rod from which is seen close to the main centre 

 columns. These several pumps, together with the 

 condenser and hot well, are all enclosed within the 

 cistern on which the cylinder and main centre 

 columns rest, and are shown in fig. 3. It will be 

 seen that the slide valves in this, as in the high pres- 

 sure engine, are wrought by an eccentric. 



As the end of the woiking beam moves in an arc 

 of a circle, it is necessary to have some means of 

 guiding the piston rod, so that it may move up and 

 down in a straight line. This is accomplished by 

 that beautiful contrivance of Watt, the parallel 

 motion. There are various modifications of this 

 motion, such as the one of the engine we are now 

 describing, but we shall confine ourselves to the 

 common form, which the accompanying diagram 

 will illustrate. The top of the piston rod and the 



top of the pump rod are connected by joints at B 

 and C to the rods D B and E C, which rods are 

 called straps or links. The links are connected by 

 joints, B and C, to the working beam, and at their 

 other extremities to a cross bar D E. The links 

 are of the same length, and the bar D E being 

 equal in length to the distance of the centres B and 

 C, B C E D will be a parallelogram ; hence the bar 

 D E, being parallel to the working beam, is called 

 the parallel bar. Another bar, F E, is connected 

 to the parallelogram by a joint at E, and is move- 

 able round a fixed centre, F. This bar is called 

 the radius bar. By the alternate motion of the 

 beam, up and down, it is plain that the joint C will 

 describe an arc of a circle whose centre is the cen- 

 tre A of the working beam; and the joint E will 

 describe an arc whose centre is the other extremity 

 F of the radius bar ; the centres A and F being fix- 

 ed. Now the centres E and C are connected by 

 the link E C ; and by the alternate motion of the 

 beam the joint C will draw the upper end of the 

 link towards the left hand side of the figure, and 

 the other end E will advance towards the right, 

 and the link will become inclined ; but there will 

 be some point, as G, in the link which is bent 

 neither to the right nor to the left, and if to this 

 point the top of the air pump rod be attached, it 

 will rise and fall in a vertical line. And as the 

 length of the rods are so adjusted that the centre 

 D keeps parallel to the point G, therefore the pis- 



ton rod attached to this point will also move in a 

 straight vertical direction. 



In the transverse elevation, fig. 2, is seen the 

 governor. Its purpose is for regulating the speed 

 of the engine. It consists of two balls suspended 

 from the upright spindle by two rods or arms move- 

 able on the point of suspension. The spindle gets 

 its motion from the crank shaft by means of the 

 bevel wheels. When the engine is working, the 

 spindle revolves, carrying the balls round about along 

 with it. The centrifugal force that the balls thus 

 acquire, will cause them to recede from the spin- 

 dle or centre of motion to a distance proportionate 

 to the speed of the engine. In the steam pipe, near 

 to the nozels, is a throtle valve, as it is termed, 

 for admitting more or less steam to act on the pis- 

 ton. It is so contrived that this valve is operated 

 on by the governor balls as they expand or collapse. 

 For instance, should it happen that the engine was 

 going too quick, the balls would immediately fly 

 farther asunder, and would act on the throtle valve 

 so as to allow less steam to enter, and thus check 

 the engine from attaining too great a velocity. On 

 the contrary, if the engine was happening to go too 

 slow, the balls would come nearer each other, 

 which would open the throtle valve, and admit 

 more steam to the engine, and thus enable it to 

 keep up to its required speed. 



In a steam engine planned so that the communi- 

 cation between the boiler and the cylinder is closed 

 before the piston reaches the end of its stroke, the 

 steam in the cylinder expands, and carries the pis- 

 ton through the part of the stroke in which the 

 cylinder has no communication with the boiler. 

 This is by far the best method of working the 

 steam engine, as all the power that is got out of 

 the steam after the communication from the boiler 

 to the cylinder is closed, is power gained over what 

 would be had if the same quantity of steam was 

 used in an engine working full pressure the whole 

 length of its stroke ; and although the steam used is 

 of a high pressure, there is only a small quantity 

 admitted into the cylinder at each stroke, and on 

 this account an air pump and condenser may be 

 used to great advantage in a high pressure engine. 

 There are various methods in use of cutting off 

 the steam ; the plans now to be described seem well 

 adapted for the purpose : they were invented by Mr 

 James Whitelaw, and are modifications of some of 

 his plans given in No. 13 of the Repertory of 

 Patent Inventions. 



The following cut shows a plan of expan- 

 sion gear adapted to the cylinder and nozles of a 

 steam boat engine of the ordinary form. In this 

 fig. it will be seen that the cut off or expansion 

 valves work upon the back of the face part of the 

 large slide valve of the engine ; the top cut off valve 

 is shown covering its port in the face of the slide 

 valve and the port for the bottom cut off valve is 

 shown open. The small rocking shaft a, is sup- 

 ported upon brackets which may be fixed at a con- 

 venient distance above the rocking shaft b, which 

 works the large slide valve. It will be seen that 

 the rocking shaft b is cranked in order to let the 

 end c of the lever d c, be so near the centre of mo- 

 tion of the shaft b, as that the motions which this 

 shaft gives to the large slide valve will not shift 

 the cut off valves from any position on the large 

 slide-valve into which they may be put by means 

 of the wiper and other apparatus which work them. 

 The lever d c which gives motion to the cut off valves 

 is fixed upon a centre, an end of which works into 



