2,6 FARM MACHINERY 



It is to be noted that in double-acting engines the faces 

 of the piston on which the pressure in the engine cylinder 

 acts differ by the area of the cross section of the piston 

 rod. It is customary to calculate the indicated horse 

 power for each end of the cylinder, and take the sum for 

 the indicated horse power of the engine. 



37. Heat. Work, as measured by the foot-pound, is 

 mechanical energy or the energy of motion. Energy is 

 defined as the power to produce a change of any kind and 

 manifests itself in many forms. It may be transformed 

 from one form to another without affecting the whole 

 amount. Heat represents one form of energy, and it is 

 the purpose of all heat engines to transform this heat 

 energy into mechanical energy. Like work, heat may be 

 measured. The unit used for this purpose is the British 

 thermal unit. 



The British thermal unit (B.T.U.) is the amount of 

 heat required to raise the temperature of I pound of water 

 i F. To make the unit more specific, the change of tem- 

 perature is usually specified as being between 62 and 

 63 F. The work equivalent of the British thermal unit 

 is sometimes called the Joule (J) and is equal to 778 foot- 

 pounds of work. 



Thermal efficiency is a term used in connection with 

 heat engines to represent the ratio between the amount 

 of energy received from the engine in tKe form of work 

 and the amount given to it in the form of heat. The 

 thermal efficiency of a steam engine seldom exceeds 15 

 per cent and of a gas engine 30 per cent. 



38. Electrical energy. By means of a dynamo, mechan- 

 ical energy may be converted into electrical energy or the 

 energy of an electric current. An electric current may 

 be likened to the flow of Avater through a pipe in that it 

 has pressure and volume, In the water pipe the pressure 



