PIPES FOR IRRIGATION PURPOSES. 



83 



strongest and the cement the weakest. In the medium 

 pressure pipes the sheet-iron or steel is the stronger and 

 the wooden pipe the weaker, while in the high ^pressure 

 pipes the cast-iron is stronger than the sheet-iron or 

 steel pipe. The judgment 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 dif- 

 ference 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 

 perfect 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 defects is an advantage. As toughness and 

 malleability are more to be desired than great tensile 

 strength, it is customary to specify that the plates in 

 either iron or steel shall be annealed ; in other words, 

 heated to a cherry red in a close oven and then slowly 

 cooled, or what is better, cooled in lime or oil. 



In this country riveted pipes come in sheets three to 

 three and one-half feet in width and of various lengths. 

 These sheets after being sized and punched in multiple 

 punching machines are bent around rollers to the re- 

 quired size, taking care that the grain of the iron or 



FIG. 21. RIVETED IRON PIPE. 



