72 TOWERS AND TANKS FOR WATER-WORKS. 



to the author, be considered in the preparation of a set of 

 specifications. In this connection, and quoting from the 

 "Manufacture and Properties of Structural Steel:" "The 

 effects caused by variations in rolling temperatures appear in 

 their most marked degree in the comparison of plates of dif- 

 ferent gauges. It is not customary to test the same heat in 

 several sizes, but by long experience the manufacturer is able 

 to judge the relative properties of each thickness. The heads 

 of two widely known plate mills have given me their estimate 

 that, taking one-half inch as a basis, there will be the follow- 

 ing changes in the physical properties for every increase of 

 one quarter of an inch in thickness: 



(1) A decrease in ultimate strength of 1000 pounds per 

 square inch. 



(2) A decrease in elongation of one per cent., when meas- 

 ured in an 8 in. parallel section. 



(3) A decrease in reduction of area of two per cent. 



It is therefore plain that in writing specifications some 

 allowance must be made for these conditions, since a require- 

 ment which is perfectly proper for a three-eighths inch plate 

 will be unreasonable for a plate of one and a half inches. 



" Moreover the effect is cumulative, since a hard steel must 

 be used in making the thick plate, and this will tend to lessen 

 the difficulty rather than make up for the reduction caused by 

 the larger section. In plates below three-eighths of an inch in 

 thickness it is also necessary to make allowances, since it is 

 almost impossible to finish them at a high temperature, and 

 the test will give a high ultimate strength and a low ductility." 



Whilst it may appear unnecessary to exact as a pre- 

 requisite the percentage of permissible alloys, other, per- 

 haps, than phosphorus and sulphur, it may not be amiss to 

 include in the specifications, certain requirements as to silicon 

 and manganese. 



In the " Manufacture and Properties of Structural Steel " 



