THE STEAM ENGINE. 



ration, by reason of the remarkable geometrical intuition which it 

 manifested in one who was uninstructed in the advanced prin- 

 ciples of geometrical analysis upon which the perfection of its 

 action depends. Although this beautiful arrangement has been 

 very generally superseded by others of greater simplicity, and of 

 sufficient, though less, precision of action, it will not be unin- 

 teresting here to attempt a brief and popular explanation of the 

 principles upon which its performance depends. 



The end of the beam with which the top of the piston-rod is 

 connected vibrating upon its centre, necessarily plays in a circular 

 arc, the convexity of which is presented to the right in fig. 26. 

 Now it is clear, that if the end y of the piston-rod were imme- 

 diately jointed to this end of the beam, it would be bent towards 

 the right through the convexity of the arc, while the beam moves 

 from its highest or lowest position to the middle of its play, and 

 that while it moves from the latter to the former position it will 

 be deflected back towards the left. Now, the efficient performance 

 of the engine absolutely requires that the piston-rod should not be 

 exposed to any such alternate strain, but that it should be guided 

 in a perfectly straight line in the direction of the axis of the cylinder ; 

 and this is precisely what the parallel motion accomplishes. 



As we have just explained, the point h plays in an arc whose 

 convexity is presented to the right. Now, the joint c d, or link, as 

 it is called, moves upon a fixed centre, c. and consequently plays 

 in an arc whose convexity is presented to the left, that is, contrary 

 to the former. "While the point h throws the upper end of the 

 link g h to the right, by reason of the convexity of its play being 

 on that side, the point d throws the lower end g to the left, by 

 reason of its convexity being on the contrary side. 



Now, the proportion of the lengths of the rods is so nicely 

 adjusted, that the effect of the rod c d in throwing the point g to 

 the left is exactly equal to the effect of the beam in throwing it 

 to the right ; and the consequence of this mutual compensation is, 

 that the point g, to which the end of the piston-rod is jointed, is 

 thrown neither to the right nor to the left, but is moved upwards 

 and downwards in a straight line. 



48. To be enabled to verify the efficiency of the engine and 

 enforce a due economy of fuel, it is necessary to be provided with 

 indicators, by which at all times the effective force of the piston can 

 be ascertained. Now this effective force depends conjointly upon 

 the pressure of the steam which moves the piston and the reaction 

 of the tmcondensed steam, and of the gases which the air pump 

 may fail to withdraw from the condenser. Two mercurial gauges 

 are accordingly provided for this purpose in all large stationary 

 engines which are constructed on the condensing principle. 



42 







