A cosmic-ray telescope is described having an angular resolution of approximately ±15°. The distance between the two outermost trays, each containing 8 Geiger counters, is 1 meter. The area of each tray is approximately 24×24 ${\mathrm{cm}}^2$. Triple coincidences modulate a transmitter and the signals, including those giving the air pressure and temperature of the instrument, are recorded on the ground. The counting rate is such that at the peak of the curve the relative probable error during a 4 minute interval is about 1.5 percent.

Accidental counts are found to be nearly negligible at all altitudes and latitudes, but some correction needs to be made for loss in efficiency because of the inherent dead time of the counters. An absolute determination of cosmic-ray intensity at the vertical at Pasadena was made in order to express the results as nearly as possible independently of the apparatus used.

DOI:https://doi.org/10.1103/RevModPhys.20.353