The .Pittsburgh Commercial gives the 'ollowing interesting account of the cp.stng of a tweuty iuch gun at the Fort l'itt Works : One of the most important events in :he history of the rebelliou, and which inaugurates a new era in the manufacture of heavy guns, took place on Thursday, between 12 and 1 o'clock, at the Fort Pitt Works. A gun with a calibre of twenty inches diameter was cast on the Rodman principie, in some twentytwo minutes from the time of tapping the lurnaees The arrangements for casting this mammoth piece of ordnance were most perfect, and during the process everything worked like clock-work. The an}ount of metal used for this huge gun was 85 tons, or 170,000 pounds. The metal ■jVgh jaelted in threo furnces, which were charged at five o'clock iu tho evening. At twenty minutes past 12, everything being in. ïsdiness, the three furnaces were simultaneously tapped, and the molten metal was convoyed by aeans of three conductors to a large reservoir close to the mould. The rnould was set perpendicularly in a mammoth pit, and was held in position by huge chains suspended from two large craues of a lifting capacity of forty tons each, and worked by steam. As soon as the reservoir was sufficiently charged, the metal flowed from it into the mould, and in tweniytvro minutes from the time of tapping the furnaecs, the mould was fillecr to within a few inches of the top. As the metal settled, the mould was filled by the wprkmen with metal from furnaces held in reserve in case of ncoident. The casting was perfected without the slightest confusión, the whole being done under the immediate superintondence of Mr. Joseph Cayo, foremau of the works. The following statistipo will give some idea of the magnitude of the casting : Weight of gun in rough 160,000 libs. Weight wjièn fimshed, 1 12,005 Bis. Total lenglh wIisü ünïshed.. 20 ft. 3 in. Length of' liore' ■■í'í '"■ Maxinmm diameter '5 ft. 2 ip. Minimum diameter 2 ft, lp ii;. The gun, it is estimated, will se a spherical shot of half a ton weight, and a sliell of seven hundred and fifty pounds. If the gun was rifled, it is thought it would throw a solid shot of one ton in weight. Tho amount uf powder to be used in charging the gun is estimated from eighty to one hundred pounds. ïhe gun is said to be the largest in the world, and to Pittsburgh belongs the honor of adding such a -heavy pieoe of ordnance to the scienee of war. TLe casting of the gun was on the hollow core principia, invented by Major Kodman, U. S. A. Tho eoie censista of an iron clipfler,' haji an iuoh 4n (liipkness, through which a stream of water ia kept coutinuously running during the process of casting. This cylinder was wrapped with rope which is coverod bjr sand, and outside of --flits for a siiort di'stance frora the mouth of the mould, is a sheet iron cylindei. During the casting the water disehargcd from the core indi cated a temperatura of from forty to forty-two degrees; The gun will romain in tho ;-,it r.nderground, during the pooling pTÓ0C3S, sorae ten days or two weeks, aftev which 'it will lip haulcd out by means cf tho largp cranes referred to, and remoVod en a i;ailway truc]; to t];o finishing department of the works, where it will be placctl n 3 latho and finished.