444 



THE NATIONAL GEOGRAPHIC MAGAZINE 



during the wind resistance, the speed and 

 cruising radius of the craft may be ma- 

 terially increased. 



MACHINE-GUNS CAN BE} MOUNTED ON TOP 

 OF HEUUM AIRSHIPS 



In military airships machine-guns may 

 be mounted directly on the envelope in- 

 stead of being tucked away in the gon- 

 dolas, as far as possible from the dan- 

 gerous gas ; and, on the other hand, all 

 danger of attack with incendiary bullets 

 disappears. This is not simply theory; 

 tests have been conducted on model bal- 

 loons filled with helium, and all efforts 

 to explode them or bring them down with 

 incendiary bullets failed. 



There is always a chance in a thousand 

 that an anti-aircraft shell might explode 

 directly within the envelope, and it might 

 also be possible to bring down a helium- 

 filled balloon by driving an airplane 

 through it bodily, but otherwise it would 

 seem to be invulnerable. 



Assistant Secretary Roosevelt, of the 

 Navy, has summed up these advantages 

 in his statement that "with the fire risk 

 eliminated, the rigid airship, or Zeppelin, 

 will be one of the most powerful weapons 

 known." 



The only apparent disadvantage of 

 helium is the fact that it is about twice 

 as heavy as hydrogen, ioo cubic feet of 

 helium weighing 17.8 ounces and the 

 same volume of hydrogen only 9 ounces. 

 Both gases, however, are so exceedingly 

 light in comparison with air (which 

 weighs 8 pounds per 100 cubic feet) that 

 this is of little practical importance, the 

 buoyancy, or lifting power, of helium be- 

 ing 93 per cent that of hydrogen. More- 

 over, this greater weight has its compen- 

 sations, for hydrogen is so light that it 

 passes through the walls of the gas bag 

 and escapes at a far more rapid rate than 

 helium. 



THE SUN GAVE US THE CUUE TO HEUUM 



Helium is one of nature's own prod- 

 ucts, being a true chemical element — a 

 body that cannot be broken up or decom- 

 posed into other simpler substances — and 

 is not to be confused with materials like 

 mustard gas, which are manufactured 

 compounds. Helium, moreover, is one of 

 the most interesting of all the elements. 



Prior to about i860 the chemist could 

 deal only with substances that he could 

 actually hold in his hands and weigh on 

 the chemical balance ; but the invention 

 of the spectroscope opened up a new 

 field, for it permitted him to study at a 

 distance the gases or vapors given off by 

 all substances when heated. 



The spectroscope depends on the fa- 

 miliar principle of the prism, which 

 breaks up sunlight into the colors of the 

 rainbow or spectrum; but it is arranged 

 to take advantage of the fact that the 

 light given out by a white-hot mass of 

 iron, for example, produces certain char- 

 acteristic lines rather than the continu- 

 ous band of rainbow colors. Each ele- 

 ment has its characteristic spectrum by 

 which it may be identified, whether it 

 happens to be on the laboratory table or 

 in some distant star. 



In 1868 an eclipse of tne sun was visible 

 in India, and for the first time the spectro- 

 scope was used to examine the colored 

 atmosphere which envelops the sun. 



Many of our familiar earthly elements, 

 like sodium, iron, hydrogen, etc., were 

 identified, but the British astronomer, 

 Lockyer, observed a bright yellow line in 

 the spectrum which did not correspond 

 with that of any known substance. He 

 concluded, therefore, that he had discov- 

 ered a new element, and named it helium, 

 after the Greek word for the sun, rjXios. 



HOW HELIUM WAS POUND ON EARTH 



It was not until 1895, or twenty-seven 

 years later, that helium was actually 

 found on the earth, and the circum- 

 stances attending its discovery are inter- 

 esting. 



In 1888 Dr. Hillebrand, of the U. S. 

 Geological Survey, found that the heavy, 

 black mineral uraninite, when treated 

 with acid, gave off an inactive gas, and 

 having proved that this gas was in part 

 nitrogen, he concluded, as no other such 

 gases were then known, that it was all 

 nitrogen. Four years later, however, 

 Lord Rayleigh and Sir William Ramsay 

 discovered a new element in the atmos- 

 phere — a heavy, inert gas which they 

 named argon. 



In. 1 895 Ramsay heard of Hillebrand's 

 work on the inert gas given off by uran- 

 inite and at once suspected that this gas 



