involved in the Construction of Artillery. 



217 



147. The extensions, for each ton of load, up to 15 tons, when wrought-iron 

 begins to lose form, and its elasticity to be permanently impaired, may be 

 compared in the following Table, deduced from Professor Hodgkinson's expe- 

 riments : — 



Tablk VIII. 

 Total Extensions in proportion to L. 



148. It is not a little remarkable that the Yorkshire and Staffordshire cast- 

 irons, which long experience has shown to be so valuable for gun-founding, 

 possess by no means high coefficients of ultimate strength, while they are 

 remarkable for their great extensibility and ductility. So prominent is this in 

 Low Moor cast-iron, that it receives deeply the mark of the hammer, when 

 struck cold, and possesses a certain amount of malleability, like an extremely 

 soft and "over-worked" wrought-iron; and there can be no doubt, that it is 

 the latter property, upon which its great value for cannon rests. The ultimate 

 tensile strength of Low Moor and of Bowling cast-iron, is only 5 667 tons, and 

 6'032 tons respectively; while that of many other "makes" ranges from 7 up 

 to 10'477 tons per square inch, and that of various mixed irons still higher. 



149. It is not, therefore, by looking for foreign cast-irons (however smelted 

 or procured), with great ultimate cohesion alone (to which the views of those 

 " in authority" appear to be limited), that a better material of this class, than we 

 at present possess, can be obtained; but by seeking that, which shall possess swcA 



