84 POWER AND THE PLOW 



of these is fixed and the other movable. At the proper 

 point in the cycle the points of these electrodes are sepa- 

 rated by a violent blow and the current leaps across in an 

 arc hot enough to fire the charge. 



The current is commonly produced by either a wet or dry 

 battery, a magneto, or a small dynamo, which is sometimes 

 termed an auto-sparker. On small stationary engines batteries 

 are often used alone, but as these deteriorate rapidly, they are 

 used on larger engines simply for starting, current afterward 

 being supplied by mechanical means. The dynamo is less 

 frequently used than the magneto. Both are usually gear- 

 driven, as it is of the utmost importance that they be kept 

 in exact time with the events in the cycle. 



CYLINDER 



In single-cylinder engines it is customary to place the cylinder 

 horizontal. Double-cylinder engines have cylinders arranged 

 in pairs and set either vertically or horizontally. Some are 

 set on a slight incline from the horizontal. In what is known 

 as the opposed type, the cylinders are placed horizontally on 

 opposite sides of the crankshaft. All of the three cylinder 

 tractors now on the market empty the cylinders vertically. In 

 some cases they are set lengthwise and in other cases crosswise 

 of the frame. The following arrangements are found in four- 

 cylinder engines : 



(1) Cylinders vertical with crankshaft lengthwise of the 

 frame. 



(2) Cylinders vertical and set crosswise. 



(3) Cylinders horizontal with crankshaft crosswise to the 

 frame. 



(4) Cylinders horizontal with one pair opposed to the other, 

 the crankshaft being crosswise to the frame. The first class is 

 by far the more numerous. 



The cylinder is commonly made of cast iron and provided 



