A sensitive photoelectric tube can catch these impacts. Such a station is called a water-Cherenkov Detector or 'tank'. The Auger Observatory has both types of detectors covering the same area, which allows for very precise measurements. When an air shower hits multiple Cherenkov Detectors on the ground, the direction of the ray can be calculated using basic geometrics. The longitudinal axis point can be determined from the "densities" in each affected ground station. Depending on the "time difference" of impact places, the angle of the axis can be determined. Only when the axis would be vertical, all ground detectors register at the very same moment in time, and any tilting of the axis will cause a time difference between earliest and latest touchdown. Cosmic rays were discovered in 1912 by Victor Hess. He measured a difference in ionisation at different heights (using the Eiffel tower and a Hess-manned hot air balloon), an indication of the atmospheric thinning (so "spreading") of a single ray. Influence of the Sun was ruled out by measuring during an eclipse. Many scientists researched the phenomenon, sometimes independently, and in 1937 Pierre Auger could conclude in detail that it was a single ray that interacted with air nuclei, causing an electron and photon air shower. At the same time, the third particle muon was discovered (behaving like a very heavy electron). In 1967 University of Leeds had developed a water-Cherenkov detector (or "surface station"; a small water basin, 1.2 m deep; also called "tank") and created a 12 km2 detection area Haverah Park using 200 such tanks. They were arranged in groups of four in a triangular (Y) ground pattern, the triangles in different sizes. The observatory worked for 20 years, and produced the main design parameters for the ground detection system at Auger Observatory. It was Alan Watson who in the later years led the research team and subsequently co-initiated Auger Observatory Collaboration. Meanwhile, from the Volcano Ranch (New Mexico, 1959–1978), the Fly's Eye (Dugway, Utah) and its successor the High Resolution Fly's Eye Cosmic Ray Detector called "HiRes" or "Fly's Eye" (University of Utah), the technique of the fluorescence detector was developed. These are optical telescopes, adjusted to picture UV light rays when looking over a surface area. It uses faceted observation (hence the fly's eye reference), to produce pixeled pictures at high speed.