The question raised by Professor Vallarta about the longitude effect can probably best be answered by a review of the experimental evidence on this point.
The existence of a longitude effect at sea-level was reported by Clay 1 and independently by Millikan, 2 who estimated that the intensity at Singapore (geomagnetic latitude o ยฐ, geographic longitude Ioo ยฐ E.) was about 4 per cent. below that at Peru (geomagnetic latitude o ยฐ, geographic longitude 75 ยฐ W.). Airplane flights by Millikan, Korff and Neher, a carrying the same electroscopes as used in the sea-level survey 2 to 26,ooo feet in the Philippines and to 3o,ooo feet in Peru, failed to disclose any greater effect up to 26,ooo feet. Recently, Millikan and Neher 4 have made balloon flights in Madras, India, geomagnetic latitude 3 ยฐ N., which may with good approximation be regarded as extensions to a greater altitude of the intensity measurements made in airplane flights in the Philippines (geomagnetic latitude 3 ยฐ N.).
At the same time, Korff, Curtiss and Astin ~ have made balloon flights in Peru, geomagnetic latitude I ยฐ S. While the latter flights were made using single counters, and all the other measurements were made using electroscopes, good agreement exists between the intensities determined by the counters and by the electroscopes up to the highest altitudes reached by the electroscope flights in Peru, which was about 3 meters of 1 j. Clay, Physica, I, 829 (I934).