Welch goes on to state that the "first real thermal soaring" occurred in 1930 by A. Haller and Wolf Hirth, with Hirth using a variometer in his Musterle. Frank Irving states that Arthur Kantrowitz first mentioned total energy in 1940. However, as early as 1901, Wilbur Wright wrote about thermals, "when gliding operators have attained greater skill, they can, with comparative safety, maintain themselves in the air for hours at a time in this way, and thus by constant practice so increase their knowledge and skill that they can rise into the higher air and search out the currents which enable the soaring birds to transport themselves to any desired point, by first rising in a circle, and then sailing off at a descending angle."[2][3]