Page:Popular Science Monthly Volume 91.djvu/63

 Popular Science Monthly

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��Making the first cuttings from a large round forged steel block which is to become part of a big gun. The work is being done on a large lathe in the machine shop plant. Another steel ingot or block can be seen just at the edge of the picture. Powerful cutting tools are necessary

��the crystals fine or to "refine the grain" as it is called. To do this the steel gun in one stage of its manufacture is heated until red hot throughout. In this condition it is suddenly cooled by lifting it high in the air and lowering it with a big electric crane down into a big tank of oil. It is thus suddenly cooled and the crystals made very small.

But the gun in this condition is so hard that it has to be tempered or annealed. This is done by heating up the steel again to a lower temperature and cooling it slowly. This operation greatly toughens and strengthens the steel.

There is something wonderful and also mysterious about the flight of very heavy shells and the energy that makes them go. It is, of course, the charge of powder in the back end of the gun that does the work. An electric spark explodes the charge, which is usually a nitro-glycerine com- pound. If gunpowder increased in the requisite quantity, the gun itself might burst before the shell was driven out; modern smokeless powder burns more

��gradually. You can light a cigar with it, so slowly does it burn. The principle of the phenomenon is that the explosive is, when ignited, at once changed to a gas which, confined in a tiny steel chamber, must find its way out. As it does, the shell is forced out by the only opening left.

The life of a big gun is surprisingly short. The powerful explosive, such as cordite for example, creates an intense heat on the walls of the cannon. This gradually melts at each shot a little of the inner surface, constantly wearing away a thin layer of the steel. It is sometimes called the erosion of a gun. This erosion, or wearing away, is so persistent and gradual that the very big guns can be fired only a limited number of times. It is said the actual life — that is the sum total of the time consumed in the firing of the shots as long as a gun lasts — is really in some cases not more than a second or two. After that a new steel lining is put in.

Many things have been tried to reduce erosion, for the cost of one large gun runs into many thousands of dollars.

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