546 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



axis. The strength of metals in large masses is much less than in small 

 ones, owing to initial tensions. 



INITIAL STRAIN IN WROUGHT-IRON. 



28. "The initial strains of large cylindrical forgings are, to some extent, 

 deranged by a cause that operates so unfavorably in solid cast-iron guns — 

 the cooling of the exterior first, and the consequent stretching of the inte- 

 rior. Mr. Clay acknowledged this difficulty before the defense commis- 

 sioners, and stated that his new process, hollow forging, overcame it. Such 

 a result actually occurred in the case of the Horsfall gun; a breech-plug or 

 false bottom was placed in the chamber to cover a crack arising from this 

 cause." 



29. " One of the illustrations given by Lieut. Col. Clark represents a 12- 

 inch wrought-iron cylinder, |-inch thick and 9 inches deep, after being 

 heated to redness, and cooled by immersing its lower half in cold water — 

 these operations having been repeated twenty times. The upper edge of 

 the cylinder (in the air) did not alter; the lower edge (in the water) con- 

 tracted 3-inch in the circumference, and at about one inch above the water- 

 line the circumference was reduced 5.5 inches. 



30. " The general effects noticed in the paper are a maximum contraction 

 of the metal about one inch above the water-line ; and this is the same 

 whether the metal be immersed one-half or two-thirds its depth, or whether 

 it be 9, 6, oi 3 inch deep. With wrought-iron the heatings and coolings 

 could be repeated from 15 to 20 times before the metal showed any signs 

 of separation ; but with cast-iron, after the fifth testing, the metal was 

 cracked, and the hollow cylinder separated all round just below the water- 

 line after the second heating. Cast-steel stood 20 heatings, but was very- 

 much cracked all over its surface. (See fig. 2.) 



31. " As respects the change of form of cast-iron and steel, the result 

 was similar to that in wrought-iron, but not nearly so large in amount. 

 Tin showed no change of form, there being, apparently, no intermediate 

 state between the melting point and absolute solidity. Brass, gun-metal, 

 and zinc showed the effect slightly; but instead of a contraction just above 

 the water-line there was an expansion or bulging." In this example the 

 hollow cylinder was cooled from the inside and outside surfaces. The fol- 

 lowing example shows the effect of heating and cooling from one surface 

 only. 



32. If we produce a homogenous ball of wrought-iron, and subject it to 

 repeated heating and cooling, it becomes larger in dimensions, and may, 

 by continuing the process, be made distinctly hollow. Upon heating it the 

 outside receives the heat first, enlarging the interior — the dimensions of 

 masses of metals may be enlarged or separated by the application of suffi- 

 cient force, indefinitely; while the dimensions of a mass cannot be mate- 

 rially reduced, no matter what force is applied — at a later time when the 

 interior is heated, the outside is enlarged, or receives what has been termed 

 a '^permanent set." Again, in cooling the outside loses its heat first, being 

 unable to compress the inner metal, (for the resistance to first increment of 

 compression is about six times as great as the resistance to first increment 

 of distention to give a permanent set;) it is by repeating the process ano- 

 ther enlargement occurs, and so on. And this explains the cause of the 



