258 THE NEW WORLD OF SCIENCE 



throughout of the ductile and rather soft gamma iron. The 

 pressure of the passing wheels soon strains a very thin layer 

 on the top of the rail beyond its elastic limit, and thus shifts 

 it to the hard beta state, which because of its hardness resists 

 the abrasion of the wheels, while its integral union with the 

 ductile gamma body of the rail prevents it from breaking 

 readily. 



A helmet is pressed into shape from a flat sheet. In thus 

 pressing a helmet of manganese steel the incidental plastic 

 deformation transfers enough iron from the gamma to the 

 hard beta state to make the mass hard and rigid, while leav- 

 ing enough ductile gamma iron to prevent shattering under the 

 impact of the bullet, and the wounding of the wearer by fly- 

 ing fragments. Hence this alloy, in spite of its low ballistic 

 resistance when in the form of heavy ship's armor, has great 

 ballistic resistance when pressed into helmets. Many millions 

 of these manganese steel helmets were worn by the soldiers of 

 the American and British Armies. Indeed the manganese steel 

 made for this purpose by the Hadfield firm alone represented 

 nearly four million helmets. They are incomparably more 

 resistant than the French helmets, which strangely enough 

 were made of a soft weak steel. The German helmet was 

 about 12 per cent, thicker and about half heavier than the 

 manganese steel ones, weighing 37 ounces against the former's 

 2 5 l /2- On the other hand it protects the back of the head and 

 neck much better. 



A helmet must neither perforate, splinter, nor indent deeply. 

 Its wearer may be killed by the helmet's indenting so deeply 

 as to fracture his skull sand-bagwise, even though it is not 

 actually perforated by the bullet. 



Inestimable as was the service rendered by this helmet, a 

 fair weighing of its merits and defects against those of the 

 German helmet, and of the material and design developed in 

 the experiments carried out by the American Army Ordnance 

 Department jointly with the Enginering Division of the Na- 

 tional Research Council, remains to be made. 



