502 Farm Mechanics. 



portant that the line of draft be as nearly as possible at 

 right angles to the sweep, for it is this an^le which renders 

 the highest per cent, of the draft available. It will be 

 clear from the upper portion of the figure, representing a 

 plan of a 14-horse sweep, that the line of draft there can- 

 not be at right angles to the sweeps and that it is impossi- 

 ble for it to be so in any sweep power. On this account, 

 there is a considerable portion of the draft lost in produc- 

 ing pressure on the bull-wheel and this is greater the 

 shorter the sweeps are and the longer the hitch is between 

 the horses and the sweep. If the line of draft made an 

 angle of 45 degrees with the sweep, one-half of the power 

 would be lost in pressure on the bull-wheel and in increas- 

 ing the friction. 



STEAM ENGINES. 



The steam engine is one of the earliest of man's inven- 

 tions designed to utilize or transform molecular motion, 

 in the form of heat, into useful work. The intense vibra- 

 tions which are caused by the burning fuel in the combus- 

 tion chamber are imparted to the water, converting it into 

 steam capable of exerting greater or less pressure, accord- 

 ing as its temperature is high or low. 



626. Principle of Action in the Steam Engine. It was 

 shown in (43) that 96G.6 heat units are required to convert 

 one pound of water at 212 F. into steam at 212 under a 

 pressure of one atmosphere; and in (41) it is shown that 

 these heat units are equivalent to 752,305 foot-pounds of 

 work. 



The fuel value of one pound of coal is 14,000 heat units 

 which ; expressed in foot-pounds, is 



14,000 X 778.3 = 10,896,200 foot-pounds. 



The steam engine aims to utilize the power of coal or other 

 fuel by transforming its enormous potential energy into 



