PIPES FOR IRRIGATION PURPOSES. Ill 



vitrified clay or cement, are classified as low-pressure 

 pipes ; those made of riveted sheet iron or steel No. 14, 

 or of wooden staves banded with wrought iron, are 

 classified as medium-pressure pipes ; and those made 

 of sheet iron or steel No. 12, or of cast iron, are classi- 

 fied as high-pressure pipes. While these classifications 

 corre(5lly separate the different kinds of pipe into classes 

 on the basis of their ability to resist pressure, still there 

 is also a difference between the various kinds of pipe 

 belonging to each class. In low-pressure pipes sheet- 

 iron or steel is the strongest, vitrified clay the next 

 strongest, and cement the weakest. In medium-pres- 

 sure pipes sheet-iron or steel is the stronger and wooden 

 pipe the weaker, while in high-pressure pipes cast-iron 

 is stronger than sheet-iron or steel pipe. The judg- 

 ment or skill of the person in charge must always be 

 exercised in choosing a pipe suitable for each case, as 

 no inflexible rule can be laid down which does not vary 

 with the conditions met. 



Grades of Iron Pipe. — There is virtually no 

 difference in the prices of iron and steel piping. 

 Wrought iron is more rigid, making a pipe less likely 

 to become dented or flattened by external pressure, and 

 more porous, which allows the particles of asphalt 

 coating both to enter and become assimilated, as it 

 were, with the iron ; on the other hand, it is less 

 strong to resist an internal pressure, and is likely to 

 scale while being bent, which may prevent a perfedl 

 coating. The greater strength of steel can seldom be 

 utilized except under high pressures, on account of its 

 liability to collapse, but its smooth surface without 

 scales or other defedls is an advantage. As toughness 



