they actually become large enough to be seen from the earth. Its distance from us is so great that it has been estimated that it would take its light, traveling at the rate of 186,000 miles a second, a thousand years to reach us. One of the component gases of this mighty nebula is called helium, so named because it was first found in the sun. It will take but a moment to tell you briefly the history of our knowledge of this wonderful gas which partly makes up this splendid nebula and which promises to be of such tremendous importance to the human race. The Story of Helium Gas At the total eclipse of the sun on August 17, 1868, Janssen, the celebrated French astronomer, found a bright yellow line in the spectrum of the solar prominences, near the well-known sodium lines, indicating the presence of a previously unknown gaseous element. This being the first that was known of this element it was, as we have said, called helium. Efforts were made to find this rare substance on the earth but for many years without success. In 1895, however, Sir William (then Professor) Ramsay, in examining with a spectroscope the gas obtained from a rare mineral from Norway, called cleveite, discovered the presence of a bright line in its spectrum which seemed to be identical with that of helium. This gas was found later in other places, especially in some of the mineral springs of Germany. Two German physicists, Runge and Paschen, on investigating its presence found that the line produced by it was double, a bright line and a faint line, while that of helium in the solar spectrum seemed to be single. This fact made it appear doubtful if the new substance really was the same as that found in the sun. Professor Hale, then beginning his career as a young astronomer in Chicago, hearing of the doubt cast upon this discovery, at once examined this line in the spectrum of the sun with the powerful means at his command. Fortunately there was a brilliant prominence or sun-flame (in which helium shows its presence) then visible projected above the sun's surface. Under careful examination he saw that the helium line was really double-a bright and a faint line! Thus was established the identity of the substance found by Ramsay with that in the sun. In reality there are many other but less conspicuous lines due to helium. Later this gas was also found to be present in the nebulae and in some of the stars, which from this fact are called helium stars. As time went on helium was found to be rather abundant on the earth, especially in connection with certain oil wells in Texas and elsewhere. Singularly enough it is also found to be non-inflammable and to have a lightness or lifting power but little less than that of hydrogen. Recognizing the immense importance of this gas for balloon purposes in the great war-for balloons filled with it could not be set on fire by incendiary shells or other means, which is the great weakness of the hydrogen balloon-the United States government erected large plants in some of the oil regions and put its experts to work to produce this wonderful gas in large quantities. So successful were these men that when the armistice was declared there were great stores of this precious substance on the Government docks ready for shipment to France. And to show how the war, while it greatly increased the price of things in general, made at least one thing less expensive, Professor Moore, who was put in charge of this department, states that before the war helium gas could only be produced at an expense of some two thousand dollars a cubic foot but at the close of the war it could be made at a cost of only ten cents a cubic foot. In conclusion I wish to thank you for your patience in listening to me. Yerkes Observatory, April, 1920. 4 Night View of Pasadena and Los Angeles, California, from Photographs of the Rotation of the Earth; 1. Pointed to Equator of the Sky, Exposure 1 h. -E. E. BARNARD 1 |