420 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK iv. 



its work there it passes into the condenser. The pressure in the 

 condenser cannot quite be reduced to zero practically, but in good 

 performance the remaining pressure of the steam does not exceed 

 three inches of mercury, or about twenty-seven inches less than that 

 of the atmosphere ; this would be technically known as twenty-seven 

 inches of vacuum. 



IV. THE TRANSMITTING MACHINERY. 



It remains to show how the motion of the piston is transmitted; by 

 what machinery it is transformed, regulated, and kept constant. The 

 problem to be solved is not peculiar to steam-engines, any motive 

 power may give rise to the same question. " Given the to-and-fro 

 motion of the piston-rod, or reciprocating motion, as it is called, to 

 find a method of transmission which shall change it into a continuous 

 circular motion, which may turn, for example, a main shaft, in the 

 motion of which all the partial motions required in the factory may 

 share." 



The oldest, which is still adopted in a great number of cases, are 

 the beam-engines, of which Fig. 296 shows the principle. 



The rod t of the piston, whose lower extremity describes a vertical 

 straight line, is jointed at the other' extremity to a great oscillating bar, 

 qr lever, AB, which is made to move (in a vertical plane) about a 

 fixed axis I. This piece is the beam, to the other extremity of 

 which a connecting-rod is jointed, which works in its turn a crank, 

 attached at o to the axle to be put in motion. Owing to this arrange- 

 ment the alternate rectilinear motion of the piston is transformed 

 into a continuous circular motion of a wheel. Here the beam is 

 above the piston-rod, but it can be also placed below, and we shall see 

 examples of that arrangement in the marine steam-engines. 



By the beam, the connecting-rod, and the crank, the alternating 

 and rectilinear motion of the piston is transformed into a continuous 

 circular motion ; but this transformation is not direct, for the extremi- 

 ties of the beams in oscillating each describe an arc of a circle, first in 

 one direction and then in the other, so that the motion is at first 

 circular and alternating. It is the connecting-rod and the crank 

 which complete the -transformation, and produce the continuity of the 



