2 6o THE NEW WORLD OF SCIENCE 



practice, and they should be no more likely to explode in a 

 gun aimed at a German than in one aimed at a board. 



Light High-Conductivity Alloys for Air-Craft Engines. 

 Because the weight of an engine which is to develop a given 

 quantity of energy is inversely as its piston-speed, the little 

 aircraft engines make an extremely great number of revolu- 

 tions per minute. This, or more generally the lightness of the 

 engines for the energy they develop, leads to an extremely 

 great internal heat development per unit of weight, and hence 

 of thermal capacity, and hence finally to a tendency of the 

 engines to overheat. To meet this tendency these engines 

 need material of great thermal conductivity as well as strength, 

 so that the heat developed in them may escape readily, and 

 not heat them so hot as to crack the lubricating oil, and thus 

 choke them with carbon. Hence the use of the light high- 

 conductivity copper-aluminum alloys of the duralumin class. 

 Their chief value here lies in their combination of great thermal 

 conductivity with immunity towards the embrittlement which 

 steel suffers at about 300 C, rather than in their lightness, for 

 none of them is as strong as steel per unit of weight. 



These alloys were improved greatly during the war. The 

 discovery of the best conditions for melting, alloying, and 

 casting made it possible to increase the tensile strength required 

 in the reception tests by nearly forty per cent. The regula- 

 tion of the manufacture at the works of the Aluminum Cast- 

 ings Company was so close that only thirty castings in about 

 ninety thousand were rejected, or at the rate of one in three 

 thousand. 



Stainless Steel for the Valves of Aircraft Engines. The 

 tendency to overheat which we have just considered is ex- 

 aggerated in the valves, because they are surrounded on all 

 sides by the hot gases, and have so little chance to get rid of 

 their heat by conduction that their temperature is said to reach 

 1000 degrees Centigrade (1832 F.), or above the melting point 

 of these copper aluminum alloys. At this temperature most 

 alloys of iron oxidize very rapidly. To meet these very try- 



