412 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



Their relative conducting powers are: 



Lead 180 



Steel 360 



Bronze 1150 



The gun is exhibited in the following section. 



The bronze casting A^ is a hollow sphere, seven feet in diameter, cast 

 upon a core, leaving six inches thickness of metal. A cj'lindrical center, 

 A, on one axis, has also six inches thickness of metal, and stays, A^, 

 four inches in diameter, radiate from the center of the external form, con- 

 necting the cylindrical center A with the outer shell A^. Thsee stays, 

 A'^, I term conductors of heat. The outer shell A"^, and the hollow cylin- 

 der A, that envelops the steel lining, being of uniform thickness, and 



cooled from all surfaces, will be a casting without strain, having been 

 uniformly cooled. The core that forms the chamber between A and A^ is 

 supported in its proper place by distance cores, through the outer shell A^, 

 resting upon the outer case of the mold. Through these distance cores E 

 are vents by which the gases and heat from the chamber core escape as 

 the metal flows into the mold in casting. 



The first operation after removing the gun from the mold is to remove 

 the sand, of which the chamber-core is composed, through the holes left 

 by the distance-cores, and then to clear out the chamber with acid, in order 

 to remove any sand that may adhere to its surface. After cleaning the 

 metal casting thoroughly, the whole external surface of A~ is hammered, 

 for the purpose of compressing the metal to its greatest density, and it is 

 designed to affect the density of the metal to about half its thickness. 

 The interior of A, to be afterward lined with steel, is enlarged by pressure 

 obtained by small rollers revolved by a sleeve, with a shaft for its center. 

 The rollers are pressed outward against the surface as they revolve, by 

 wedges, until the whole surface has been passed over many times, thus 



