The next summer, Rossi returned to Mount Evans where he performed experiments near Echo Lake at an elevation 3,230 metres (10,600 ft). With the use of anti coincidence techniques, the apparatus made it possible to measure the mean free path before decay of two groups of mesotrons with different average momentum. The results, published with David B. Hall, not only confirmed the proportionality between particle momentum and the mean free path of mesotrons before decay that is expected on the basis of relativity theory, but also presented an improved estimate of the lifetime at rest: (2.4±0.3) microseconds. These results and those of the previous year were not only the first to show definitively that mesotrons are unstable, but also the first experimental confirmation of the time dilation of moving clocks predicted by relativity theory.