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( 30 ) In forging wrought iron at a welding heat, you may, if there be no foreign substance interposed, weld together soundly the sides of a cavity, but you cannot effect this in forging steel. Do not, however, suppose I am desirous of introducing welding processes; far from it. In fact, between wrought iron and steel you are in this dilemma. In wrought iron every weld is a matter of uncertainty, and in making a steel ingot you never can be sure there may not exist some latent flaw.

That this is no imaginary case the following instance will show. A 6-inch gun (to which I shall also allude hereafter), one of the second kind that were made, and therefore technically known as a Mark 2 gun, after having fired 278 rounds, burst, and it was found that in the front part of the powder-chamber there was a flaw, the two sides of which had been forged close together, but had never been in metallic union; this cell lay embedded in the thickness of the wall of the chamber, and the boring tool had just not broken into it, there was a thin skin of sound metal left. The erosion, however, had at last worn a way into this flattened cell—a mere crack, for its sides were so close together—the powder gases entered, and pressing on the sides of the cell-crack, gradually spread it, as a wedge might have done, along that which had previously been solid metal, until the flaw was so far increased that the next round caused the burst. The marks of the internal extension, that had been going on, were very obvious after the explosion, and were very interesting as telling the history of that which resulted in the burst.

It would be a great boon indeed to all engaged in metallurgical operations, if the electrician could succeed in indicating the existence of latent flaws.

Experiments are being carried out now, to see whether it is possible, to try the whole tube (before it is built up into the gun), by internal shock pressure tests, and I think that these experiments may result in success; but if they do they will after all not be absolutely conclusive, any more than the proof of the finished gun is conclusive. I have but little doubt, if a shock