SCIENCE OF THRESHING. 151 



between the points or projections of two contacting 

 metals, the pieces may be moved with much less re- 

 sistance or friction. Lubricating oils or greases are 

 used to supply these particles. The globules of the 

 lubricating material tend to hold the surfaces of 

 the metals apart, permitting the small projections to 

 pass each other without touching, thus reducing the 

 friction and wear. The materials best adapted for 

 lubricating purposes are those whose globules retain 

 their natural shape under the greatest pressure. This 

 property of lubricating oils and greases is called 

 viscosity. The lubricant in a revolving journal will 

 lose its viscosity after a time, and permit the surfaces 

 to come in contact; for this reason it is necessary that 

 a fresh supply of oil be fed to the journal to replenish 

 the old oil. 



The cylinder rings and valve seats require special 

 lubricating oil, capable of resisting the heat to which 

 the parts are exposed. Cylinder oil should resist a 

 temperature much higher than the steam temperature 

 before its particles become dissociated or evaporate. 

 If cylinder oil is so light or volatile that as soon as 

 it is exposed to the high temperature of the steam the 

 liquid portion evaporates, leaving a dry, sooty like 

 residue, it is valueless as a lubricant and a positive 

 injury to the face of a valve or cylinder ring. 



Cylinder oil may be easily tested by putting a small 

 quantity on a steam pipe under boiler pressure, whose 

 heat is therefore equal to the engine cylinder. If 

 the oil boils, smokes or evaporates quickly, it is unfit 

 for cylinder lubrication. Put a little oil such as 



