THE STEAM ENGINE. 



so absolute as it might at first appear to be. It 

 practicable to keep the water in the condenser 'at a temperature 

 lower than 100, and at that temperature steam is evolved which 

 has a pressure of about one pound per square inch, which, after 

 all, will still react upon the piston. 



In comparing, then, the non-condensing and condensing engine, 

 it is apparent, that while the latter gives a much greater amount of 

 moving power with the same rate of evaporation, and consequently 

 with the same consumption of fuel, the former is vastly more 

 simple in its mechanism, lighter in its weight, more inexpensive 

 in its construction and maintenance, and much more portable. 



42. From what has been explained, it will be understood how 

 the piston-rod is made to move with any desired force alternately 

 in one direction or other, through a space equal to the stroke of th3 

 piston, or, what is the same, to the length of the cylinder. 



The manner in which this force is transmitted to the object to 

 which the engine is applied, is extremely various. In some cases 

 the end of the piston-rod is connected with that of a vibrating 

 beam, to which a motion of oscillation is imparted like that of the 

 handle of a pump. In other cases it is put in connection with a 

 winch or crank, by which a motion of revolution is imparted to an 

 axle or shaft, in the same manner as a man working at a windlass 

 causes a rope to wind upon its axle. In other cases it is connected 

 with a wheel, to which it imparts rotation, as in some forms of 

 the locomotive engine. In short, the expedients by which the 

 alternate force of the piston is applied to the particular work to 

 be performed by the engine are so numerous, and differ so much 

 one from another, that it would be quite impossible to give any 

 general account which would include them. 



43. To convey, however, some idea of one of the most common 

 methods of transmitting the force of the piston, we shall take the 

 case of the steam engine generally used to propel the machinery 

 of the larger class of factories, a view of which is given in fig. 26. 

 The several parts will be easily understood, after what has been 

 stated, without further explanation. 



c is the steam cylinder. 



p, the steam piston. 



v v', the valves for admitting and withdrawing the steam, at 

 each end of the cylinder. 



E, the piston-rod of the air pump. 



L, the piston-rod of the hot-water pump. 



K", the piston-rod of the cold-water pump. 



i, the handle of the cock by which the jet in the condc 

 made to play with more or less force. 



&, d,y, c, a system of jointed rods called the parallel motion, by 

 34 



lenser is 





