The Austrian physicist Victor Francis Hess hypothesized that some of the ionization was caused by radiation from the sky. In order to defend this hypothesis, Hess designed instruments capable of operating at high altitudes and performed observations on ionization up to an altitude of 5.3 km. From 1911 to 1913, Hess made ten flights to meticulously measure ionization levels. Through prior calculations, he did not expect there to be any ionization above an altitude of 500 m if terrestrial sources were the sole cause of radiation. His measurements however, revealed that although the ionization levels initially decreased with altitude, they began to sharply rise at some point. At the peaks of his flights, he found that the ionization levels were much greater than at the surface. Hess was then able to conclude that "a radiation of very high penetrating power enters our atmosphere from above". Furthermore, one of Hess's flights was during a near-total eclipse of the Sun. Since he did not observe a dip in ionization levels, Hess reasoned that the source had to be further away in space. For this discovery, Hess was one of the people awarded the Nobel Prize in Physics in 1936. In 1925, Robert Millikan confirmed Hess's findings and subsequently coined the term ' cosmic rays '.