390 



STEAM ENGINE. 



shaft and tbe centre of the wheel do not coincide. 

 There is a ring or strap fitted loose upon the cir- 

 cumference of the eccentric wheel, and the rod v is 

 fixed to the strap ; by which arrangement, if the 

 crank shaft revolves, the rod v will move on end 

 from and towards the shaft once during a revolu- 

 tion. The rod v communicates its motions to the 

 lever w, fixed upon the rocking shaft (so called 

 from the nature of its motions) x x. The rocking 

 shaft x x, works into brackets fixed to the frame 

 work, and it has another lever fixed upon it, which 

 conveys motion to the rod in connexion with the 

 valve. The above remarks will be rendered more 

 plain by the following figure, in which O repre- 

 sents the centre of the shaft, P the centre of a 



wheel fixed on that shaft, so that the centre or axis 

 of the shaft shall be at a given distance from the 

 centre of the wheel. A brass ring is placed loosely, 

 but not so as to shake, in the circumference of the 

 wheel, to which ring the rods E A, F A are firmly 

 attached. These arms unite at A, being formed 

 into an eye through which a pin passes, connecting 

 them to the lever A B. It is plain that as the 

 shaft O turns upon its axis the wheel whose centre 

 is P will be turned round with it, and the rods 

 attached to the ring upon its circumference will be 

 drawn backwards and forwards so as to move the 

 end A and give an alternating motion to the line 

 AB. 



Suppose a steam engine with its parts in the 

 positions as shown in fig. 4, and that the steam in 

 the boiler was raised to such a pressure as that its 

 force on the piston would always be more than suf- 

 ficient to overcome the friction of the machinery 

 and the resistance of the atmosphere. Then as the 

 valve is in such a position as that the steam can 

 only act on the side a of the piston next the latter, 

 the piston will go in the direction of the arrow till 

 it reaches the end of the cylinder. But as the 

 valve is always in motion, and the eccentric is fixed 

 on the crank shaft, in such a way, that when the 

 piston reaches the other end y of the cylinder, it 

 has shifted the valve so as just to be beginning to 

 open the passage for the steam to get into this end 

 y of the cylinder, and also to open the passage for it 

 to escape from the end a into the atmosphere. On 

 this account the motion of the piston will be 

 reversed, and it will now move to the end a of 

 the cylinder, and when it has reached this end, the 

 valve will be so placed that the steam will send the 

 piston again towards y, and in this way the piston 

 acquires a continuous alternating rectilinear mo- 

 tion, which by intervening contrivances communi- 

 cates a continued rotatory motion to the crank 

 shaft. It will be understood that as the steam has 

 less tendency to turn the crank shaft when the 

 crank is near the position shown in fig. 5, than when 

 it is in the position shown in fig. 4, the motion of 

 this shaft will not be uniform. But the fly wheel 

 is used to cause the motion of the crank shaft to be 

 nearly uniform at every part of the stroke of the 

 piston, by its taking in part of the power when 

 the steam acts with its greatest force to turn the 

 shaft, and again giving out this power when the 



steam has little tendency, from the position of the 

 crank, to turn the engine. The fly wheel is, in 

 short, a regulator of the engine ; for as has been ob- 

 served, in (in elementary work on mechanics, " the 

 effect of the fly wheel in accumulating force, has 

 led some to suppose that there is, positively, a 

 creation of force in the fly ; but this is a mistake, 

 for it is only, as it were, a magazine of power, 

 where there is no force but what has been delivered 

 to it. The great use of the fly wheel is thus to 

 deliver out, at proper intervals, that force which 

 has been previously communicated to it ; and al- 

 though there is absolutely a small loss of power by 

 the use of the fly, yet this is more than compensated 

 for by its utility as a regulator." 



The piston is described above as being packed 

 with hemp ; but it is now common to use what are 

 called metallic pistons, which are superior in many 

 respects ; they are attended with less trouble and 

 expense, and they glide up and down the cylinder 

 with less friction ; indeed hemp packing for high 

 pressure engines is not at all answerable, as the 

 high temperature of the steam soon destroys it. 



The metallic piston of Mr Barton, has been much 

 approved of; its structure will be understood from 

 the following figures : a a a a are four metal 

 segments ; b b b b four right-angled wedges in- 

 terposed between the segments, their points form- 

 ing a portion of the periphery of the circle ; .c c c c 

 is a thin steel spring, formed into a single broad 

 hoop, and pressed into the undulated form repre- 

 sented, by which it is found to act with uniform 

 energy upon the wedges, until they and the seg- 

 ments become so much worn in the course of 



time, that the steel spring recovers itself into its 

 original circular figure ; d is the frame-work, cast 

 in one piece, with the lower plate of the piston ; e 

 is the piston rod ; the open spaces shown on the 

 plan within the circular frame d, are cavities to les- 

 sen the weight of metal; the other spaces are 

 cavities to allow of the free action of the circular 

 spring. To prevent the segments from falling out 



