62 TOWERS AND TANKS FOR WATER-WORKS. 



acter of the finished product is of a suitable grade, whose 

 chemical and physical properties having been specified, will be 

 conscientiously made to meet the requirements. This ad- 

 vances two important subjects: first, What chemical and phys- 

 ical requirements are deemed most suitable for stand-pipe 

 work ? and, having determined this, How can certainty in ob- 

 taining what is considered requisite be secured ? 



The temperature at which steel is finished, depending ob- 

 viously upon the mass being worked, has been shown to exert 

 a marked effect upon its physical properties, and to such an 

 extent that concessions are allowed amounting, as will be 

 observed from the manufacturers' standard, to 10,000 pounds 

 to cover the various widths and thicknesses of sections. There 

 seems to be an increasing tendency to test each separate thick- 

 ness, and in view of the fact that tests made from the same 

 melt but upon different thicknesses of metal, finished at differ- 

 ent temperatures show great variability in tensile strength, 

 the practice seems commendable. Considering the physical 

 characteristics of a good structural steel, authorities agree that 

 the metal should be soft, tough, and ductile; disputing, how- 

 ever, as to the exact limits and variation in tensile strength. 

 In this connection Mr. Campbell says: 



"The tendency in the first epoch of steel structures was 

 toward a hard alloy, but the later practice has been a con- 

 tinual progress toward toughness. There was a halt in this 

 movement at a tensile strength of 60,000 pounds, not entirely 

 on account of any magic virtue in the figure, but because the 

 ordinary mild steels gave that result, and a much higher price 

 was charged for a softer metal. The conditions to-day are 

 somewhat different, for the reduced cost of low-phosphorus 

 pig iron, and the introduction of the basic-hearth, have alter- 

 ed the economic situation. 



"A steel with a tensile strength of 50,000 to 58,000 

 pounds per square inch is a most attractive material, possess- 



