. Compendium of meteorology. Meteorology. ATMOSPHERIC TIDES AND OSCILLATIONS 521 world.'' Martyn finds at latitudes 35°S and 27°S a lunar variation in the height of the E-region which is opposite in phase to that found by Appleton and Weekes in the higher latitude of England. Martyn also finds lunar variations in both the heights and electron densities of the F-region. Near the magnetic equator these varia- tions are very much larger than the already large vari- ations found in the E-region. At Huancayo (Peru) the total tidal variation at certain hours and seasons amounts to some 60 km in heig
. Compendium of meteorology. Meteorology. ATMOSPHERIC TIDES AND OSCILLATIONS 521 world.'' Martyn finds at latitudes 35°S and 27°S a lunar variation in the height of the E-region which is opposite in phase to that found by Appleton and Weekes in the higher latitude of England. Martyn also finds lunar variations in both the heights and electron densities of the F-region. Near the magnetic equator these varia- tions are very much larger than the already large vari- ations found in the E-region. At Huancayo (Peru) the total tidal variation at certain hours and seasons amounts to some 60 km in height and 20 per cent in electron density. These variations concern the layers of electrons and ions interspersed among the neutral mole- cules of the high atmosphere. Their relation to the tidal oscillations of the main body of air requires considera- tion of electrodynamic as well as of hydrodynamic forces. There is no doubt that when the theory of these variations is fully understood the observational results will provide important and interesting information about the lunar atmospheric tide at high KM Fig. 14.—Harmonic dial [6] for the lunar tide in the E-region of the ionosphere. The circle shows the probable error for any one of the eleven separate dial points, each determined from 12-14 days' data. Lunar Tidal Variations of Cosmic Rays. Cosmic ray observations provide, in a surprising and most interest- ing way, information as to the lunar air tide at a level of eighteen or twenty kilometres above the ground, though a precise interpretation of the data awaits fur- ther study. Among the cosmic rays received at the ground are mesons, supposed to be generated (by the primary rays) at this level; being unstable, a propor- tion of them are transformed on their way to the ground. If the lunar tide raises or lowers the level of the mean air pressure at which the mesons are generated, their path to the ground will be lengthened or shortened, and the number of survivors
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