64 TOWERS AND TANKS FOR WATER-WORKS. 



amount of the constituent elements that are associated with 

 the iron, and each of which affect more or less the hardness 

 and strength of the material. 



"The principal of these are carbon, manganese, silicon, 

 phosphorus, and sulphur, the first-named being purposely 

 retained as useful or necessary, the others being rejected, as 

 far as practicable, as objectionable when in excess of certain 

 minute proportions. 



" The grade and character of the steel is usually known by 

 the percentage of contained carbon. Steel used in structures 

 usually varies in tensile strength from 55,000 to 70,000 Ibs. 

 per square inch of section, or from .10 to .25 per cent, of 

 carbon. 



"The following table exhibits the physical characteristics 

 of open-hearth basic steel of the various grades, the results 

 derived from an extensive series of tests indicating the ten- 

 dency of a total average of the composition hereafter de- 

 scribed to approximate to the figures given in the table. 



"The predominant elements other than carbon averaged 

 throughout the series as follows : manganese, .40 ; phosphorus, 

 .04; sulphur, .05 per cent. Any increase of these elements 

 is attended with an increase of tensile strength and reduced 

 ductility, and vice versa. The tensile strength of the steel 

 is also affected to some extent by the temperature at which it 

 is finished, and the rate of cooling; these influences being 

 more apparent in the grades containing highest carbon. 

 Therefore the values given have only a general significance, 

 and the results of individual tests may vary widely above or 

 below the figures in the table. 



" For Bessemer or open-hearth acid process steel, the ten- 

 sile strength will ordinarily be greater for the same percent- 

 age of carbon given in this table, for the reason that the pro- 

 portions of phosphorus and sulphur, and sometimes manga- 

 nese, are usually higher than in open-hearth basic steel, each 



