200 AGRICULTURAL ENGINEERING 



the furrow slice of tough sod gradually; second, the general- 

 purpose plow, to be used for general plowing in stubble and 

 light sod; and, third, the stubble plow, with an abrupt mold- 

 board for pulverizing the soil, used only in old ground. 

 Among these three classes there are numberless shapes of 

 plows difficult to classify. 



Fig. 108. The three principal types of plows, showing in order the 

 stubble, the general purpose, and the prairie breaker plows. 



Construction. The moldboard may be made of soft- 

 center steel or chilled iron; but the latter is used but very 

 little in the Middle West, where the soil is of such a character 

 that the hard-tempered surface of the soft-centered steel is 

 required to scour properly. Certain localities are furnished 

 with plows with common cast-steel moldboards; but they 

 can not be used where many rocks are encountered, in 

 which case a soft share, at least, must be provided. The 

 wearing properties of the soft-centered steel share is secured 

 through its hardness; but to secure hardness a certain 



amount of brittleness 

 must remain, even with a 

 soft center to the metal. 

 Adjusting the Walk- 

 ing Plow. The walk- 

 Fig. 109. A steel beanT^alking plow of ing plow HlUSt have its 



point turned down 



slightly in order to cause the plow to take to the ground. 

 This gives what is called "suction" to the plow, and is 

 resisted by the upward pull of the draft. It is imperative 

 that the suction be sufficient, and quite as important that 



