"Where is the source of this energy? Both Madame Curie and myself are unable to go beyond hypotheses; one of these consists in supposing the atoms of radium evolving and transforming into another simple body, and, despite the extreme slowness of that transformation, which cannot be located during a year, the amount of energy involved in that transformation is tremendous. [Illustration: M. and Mme. Curie Finishing the Preparation of some Radium.] "The second hypothesis consists in the supposition that radium is capable of capturing and utilising some radiations of unknown nature which cross space without our knowledge." CHAPTER II FLYING MACHINES[A] _Santos-Dumont's Steerable Balloons_ Among the inventors engaged in building flying machines the most famous, perhaps, is M. Santos-Dumont, whose thrilling adventures and noteworthy successes have given him world-wide fame. He was the first, indeed, to build a balloon that was really steerable with any degree of certainty, winning a prize of $20,000 for driving his great air-ship over a certain specified course in Paris and bringing it back to the starting-point within a specified time. Another experimenter who has had some degree of success is the German, Count Zeppelin, who guided a huge air-ship over Lake Geneva, Switzerland, in 1901. [A] In the first "Boys' Book of Inventions," the author devoted a chapter entitled "Through the Air" to the interesting work of the inventors of flying machines who have experimented with aëroplanes; that is, soaring machines modelled after the wings of a bird. The work of Professor S. P. Langley with his marvellous Aërodrome, and that of Hiram Maxim and of Otto Lilienthal, were given especial consideration. In the present chapter attention is directed to an entirely different class of flying machines--the steerable balloons. Carl E. Myers, an American, an expert balloonist, has also built balloons of small size which he has been able to steer. And mention must also be made of M. Severo, the Frenchman, whose ship, Pax, exploded in the air on its first trip, dropping the inventor and his assistant hundreds of feet downward to their death on the pavements of Paris. It will be most interesting and instructive to consider especially the work of Santos-Dumont, for he has been not only the most successful in making actual flights of any of the inventors who have taken up this great problem of air navigation, but his adventures have been most romantic and thrilling. In five years' time he has built and operated no fewer than ten great air-ships which he has sailed in various parts of Europe and in America. He has even crowned his experiences with more than one shipwreck in the air, an adventure by the side of which an ordinary sea-wreck is tame indeed, and he has escaped with his life as a result not only of good fortune but of real daring and presence of mind in the face of danger. [Illustration: M. Alberto Santos-Dumont.]