524 



Farm Mechanics. 



2. Compressing the charge preparatory to explosion. 



3. Igniting the compressed charge. 



4. Expansion of the charge after its explosion. 



5. Expulsion of the waste products of the explosion. 



657. Arrangements to Prevent Over-heating. The con- 

 tinual repetition of the explosions in the cylinder of the 

 engine results in so much heating of the parts, where any 

 considerable work is done, that it is found necessary to 

 provide means for absorbing the heat not changed into me- 

 chanical motion. This is usually done by providing the 

 working parts which come in contact with the heat with 

 water jackets in which water or oil is kept circulating to 

 absorb the heat imparted to them. 



Where water is used to cool with it is necessary in freez- 

 ing weather to draw it off when the engine is shut down to 

 avoid injury, but where a lubricating oil is used as the cir- 

 culating medium there is no danger of this sort. 



FIG. 255. Horizontal gasoline engine . 



658. Types of Gasoline Engines. Gasoline engines, like 

 the boilers of steam engines, are spoken of as vertical or 

 horizontal according as the cylinder is upright or horizon- 

 tal. It is possible to make the floor space occupied by the 

 upright engines less than with the horizontal forms, but 

 with few exceptions all the larger engines "belong to the 



