Popular Science Monthly 



47 



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. 



