The east-west and longitude effects

In measuring the difference between the numbers of charged particles effecting a line of Geiger-Muller counters when it is tilted toward the west and then toward the east, the common practice is to rotate the line of counters about an axis normal to the equipotential gravitational surface at the location in which the experiment is being conducted. Now it has been shown in sea-level surveys by Clay(1) and by Millikan and Neher(2) that the variations in the surface magnetic field for the same geomagnetic latitude extend to sufficient heights to influence the charged particles coming in at the equator, giving rise to the "longitude effect."